text:
Causality

Causality is an influence by which one event, process, state, or object (a cause) contributes to the production of another event, process, state, or object (an effect) where the cause is partly responsible for the effect, and the effect is partly dependent on the cause. In general, a process has many causes, which are also said to be causal factors for it, and all lie in its past. An effect can in turn be a cause of, or causal factor for, many other effects, which all lie in its future. Some writers have held that causality is metaphysically prior to notions of time and space.

Causality is an abstraction that indicates how the world progresses. As such it is a basic concept; it is more apt to be an explanation of other concepts of progression than something to be explained by other more fundamental concepts. The concept is like those of agency and efficacy. For this reason, a leap of intuition may be needed to grasp it. Accordingly, causality is implicit in the structure of ordinary language, as well as explicit in the language of scientific causal notation.

In English studies of Aristotelian philosophy, the word "cause" is used as a specialized technical term, the translation of Aristotle's term αἰτία, by which Aristotle meant "explanation" or "answer to a 'why' question". Aristotle categorized the four types of answers as material, formal, efficient, and final "causes". In this case, the "cause" is the explanans for the explanandum, and failure to recognize that different kinds of "cause" are being considered can lead to futile debate. Of Aristotle's four explanatory modes, the one nearest to the concerns of the present article is the "efficient" one.

David Hume, as part of his opposition to rationalism, argued that pure reason alone cannot prove the reality of efficient causality; instead, he appealed to custom and mental habit, observing that all human knowledge derives solely from experience.

The topic of causality remains a staple in contemporary philosophy.

Metaphysics
The nature of cause and effect is a concern of the subject known as metaphysics. Kant thought that time and space were notions prior to human understanding of the progress or evolution of the world, and he also recognized the priority of causality. But he did not have the understanding that came with knowledge of Minkowski geometry and the special theory of relativity, that the notion of causality can be used as a prior foundation from which to construct notions of time and space.

Ontology
A general metaphysical question about cause and effect is: "what kind of entity can be a cause, and what kind of entity can be an effect?"

One viewpoint on this question is that cause and effect are of one and the same kind of entity, causality being an asymmetric relation between them. That is to say, it would make good sense grammatically to say either "A is the cause and B the effect" or "B is the cause and A the effect", though only one of those two can be actually true. In this view, one opinion, proposed as a metaphysical principle in process philosophy, is that every cause and every effect is respectively some process, event, becoming, or happening. An example is 'his tripping over the step was the cause, and his breaking his ankle the effect'. Another view is that causes and effects are 'states of affairs', with the exact natures of those entities being more loosely defined than in process philosophy.

Another viewpoint on this question is the more classical one, that a cause and its effect can be of different kinds of entity. For example, in Aristotle's efficient causal explanation, an action can be a cause while an enduring object is its effect. For example, the generative actions of his parents can be regarded as the efficient cause, with Socrates being the effect, Socrates being regarded as an enduring object, in philosophical tradition called a 'substance', as distinct from an action.

Epistemology
Since causality is a subtle metaphysical notion, considerable intellectual effort, along with exhibition of evidence, is needed to establish knowledge of it in particular empirical circumstances. According to David Hume, the human mind is unable to perceive causal relations directly. On this ground, the scholar distinguished between the regularity view of causality and the counterfactual notion. According to the counterfactual view, X causes Y if and only if, without X, Y would not exist. Hume interpreted the latter as an ontological view, i.e., as a description of the nature of causality but, given the limitations of the human mind, advised using the former (stating, roughly, that X causes Y if and only if the two events are spatiotemporally conjoined, and X precedes Y) as an epistemic definition of causality. We need an epistemic concept of causality in order to distinguish between causal and noncausal relations. The contemporary philosophical literature on causality can be divided into five big approaches to causality. These include the (mentioned above) regularity, probabilistic, counterfactual, mechanistic, and manipulationist views. The five approaches can be shown to be reductive, i.e., define causality in terms of relations of other types. According to this reading, they define causality in terms of, respectively, empirical regularities (constant conjunctions of events), changes in conditional probabilities, counterfactual conditions, mechanisms underlying causal relations, and invariance under intervention.

Geometrical significance
Causality has the properties of antecedence and contiguity. These are topological, and are ingredients for space-time geometry. As developed by Alfred Robb, these properties allow the derivation of the notions of time and space. Max Jammer writes "the Einstein postulate ... opens the way to a straightforward construction of the causal topology ... of Minkowski space." Causal efficacy propagates no faster than light.

Thus, the notion of causality is metaphysically prior to the notions of time and space. In practical terms, this is because use of the relation of causality is necessary for the interpretation of empirical experiments. Interpretation of experiments is needed to establish the physical and geometrical notions of time and space.

Volition
The deterministic world-view holds that the history of the universe can be exhaustively represented as a progression of events following one after the other as cause and effect. Incompatibilism holds that determinism is incompatible with free will, so if determinism is true, "free will" does not exist. Compatibilism, on the other hand, holds that determinism is compatible with, or even necessary for, free will.

Necessary and sufficient causes
Causes may sometimes be distinguished into two types: necessary and sufficient. A third type of causation, which requires neither necessity nor sufficiency, but which contributes to the effect, is called a "contributory cause".

 * Necessary causes: If x is a necessary cause of y, then the presence of y necessarily implies the prior occurrence of x. The presence of x, however, does not imply that y will occur.
 * Sufficient causes: If x is a sufficient cause of y, then the presence of x necessarily implies the subsequent occurrence of y. However, another cause z may alternatively cause y. Thus the presence of y does not imply the prior occurrence of x.
 * Contributory causes: For some specific effect, in a singular case, a factor that is a contributory cause is one among several co-occurrent causes. It is implicit that all of them are contributory. For the specific effect, in general, there is no implication that a contributory cause is necessary, though it may be so. In general, a factor that is a contributory cause is not sufficient, because it is by definition accompanied by other causes, which would not count as causes if it were sufficient. For the specific effect, a factor that is on some occasions a contributory cause might on some other occasions be sufficient, but on those other occasions it would not be merely contributory.

J. L. Mackie argues that usual talk of "cause" in fact refers to INUS conditions (insufficient but non-redundant parts of a condition which is itself unnecessary but sufficient for the occurrence of the effect). An example is a short circuit as a cause for a house burning down. Consider the collection of events: the short circuit, the proximity of flammable material, and the absence of firefighters. Together these are unnecessary but sufficient to the house's burning down (since many other collections of events certainly could have led to the house burning down, for example shooting the house with a flamethrower in the presence of oxygen and so forth). Within this collection, the short circuit is an insufficient (since the short circuit by itself would not have caused the fire) but non-redundant (because the fire would not have happened without it, everything else being equal) part of a condition which is itself unnecessary but sufficient for the occurrence of the effect. So, the short circuit is an INUS condition for the occurrence of the house burning down.

Contrasted with conditionals
Conditional statements are not statements of causality. An important distinction is that statements of causality require the antecedent to precede or coincide with the consequent in time, whereas conditional statements do not require this temporal order. Confusion commonly arises since many different statements in English may be presented using "If ..., then ..." form (and, arguably, because this form is far more commonly used to make a statement of causality). The two types of statements are distinct, however.

For example, all of the following statements are true when interpreting "If ..., then ..." as the material conditional:

 * 1) If Barack Obama is president of the United States in 2011, then Germany is in Europe.
 * 2) If George Washington is president of the United States in 2011, then $⟨arbitrary statement⟩$.

The first is true since both the antecedent and the consequent are true. The second is true in sentential logic and indeterminate in natural language, regardless of the consequent statement that follows, because the antecedent is false.

The ordinary indicative conditional has somewhat more structure than the material conditional. For instance, although the first is the closest, neither of the preceding two statements seems true as an ordinary indicative reading. But the sentence: intuitively seems to be true, even though there is no straightforward causal relation in this hypothetical situation between Shakespeare's not writing Macbeth and someone else's actually writing it.
 * If Shakespeare of Stratford-on-Avon did not write Macbeth, then someone else did.

Another sort of conditional, the counterfactual conditional, has a stronger connection with causality, yet even counterfactual statements are not all examples of causality. Consider the following two statements:

 * 1) If A were a triangle, then A would have three sides.
 * 2) If switch S were thrown, then bulb B would light.

In the first case, it would be incorrect to say that A's being a triangle caused it to have three sides, since the relationship between triangularity and three-sidedness is that of definition. The property of having three sides actually determines A's state as a triangle. Nonetheless, even when interpreted counterfactually, the first statement is true. An early version of Aristotle's "four cause" theory is described as recognizing "essential cause". In this version of the theory, that the closed polygon has three sides is said to be the "essential cause" of its being a triangle. This use of the word 'cause' is of course now far obsolete. Nevertheless, it is within the scope of ordinary language to say that it is essential to a triangle that it has three sides.

A full grasp of the concept of conditionals is important to understanding the literature on causality. In everyday language, loose conditional statements are often enough made, and need to be interpreted carefully.

Questionable cause
Fallacies of questionable cause, also known as causal fallacies, non-causa pro causa (Latin for "non-cause for cause"), or false cause, are informal fallacies where a cause is incorrectly identified.

Counterfactual theories
Counterfactual theories define causation in terms of a counterfactual relation, and can often be seen as "floating" their account of causality on top of an account of the logic of counterfactual conditionals. Counterfactual theories reduce facts about causation to facts about what would have been true under counterfactual circumstances. The idea is that causal relations can be framed in the form of "Had C not occurred, E would not have occurred." This approach can be traced back to David Hume's definition of the causal relation as that "where, if the first object had not been, the second never had existed." More full-fledged analysis of causation in terms of counterfactual conditionals only came in the 20th century after development of the possible world semantics for the evaluation of counterfactual conditionals. In his 1973 paper "Causation," David Lewis proposed the following definition of the notion of causal dependence:

 * An event E causally depends on C if, and only if, (i) if C had occurred, then E would have occurred, and (ii) if C had not occurred, then E would not have occurred.

Causation is then analyzed in terms of counterfactual dependence. That is, C causes E if and only if there exists a sequence of events C, D1, D2, ... Dk, E such that each event in the sequence counterfactually depends on the previous. This chain of causal dependence may be called a mechanism.

Note that the analysis does not purport to explain how we make causal judgements or how we reason about causation, but rather to give a metaphysical account of what it is for there to be a causal relation between some pair of events. If correct, the analysis has the power to explain certain features of causation. Knowing that causation is a matter of counterfactual dependence, we may reflect on the nature of counterfactual dependence to account for the nature of causation. For example, in his paper "Counterfactual Dependence and Time's Arrow," Lewis sought to account for the time-directedness of counterfactual dependence in terms of the semantics of the counterfactual conditional. If correct, this theory can serve to explain a fundamental part of our experience, which is that we can causally affect the future but not the past.

One challenge for the counterfactual account is overdetermination, whereby an effect has multiple causes. For instance, suppose Alice and Bob both throw bricks at a window and it breaks. If Alice hadn't thrown the brick, then it still would have broken, suggesting that Alice wasn't a cause; however, intuitively, Alice did cause the window to break. The Halpern-Pearl definitions of causality take account of examples like these. The first and third Halpern-Pearl conditions are easiest to understand: AC1 requires that Alice threw the brick and the window broke in the actual work. AC3 requires that Alice throwing the brick is a minimal cause (cf. blowing a kiss and throwing a brick). Taking the "updated" version of AC2(a), the basic idea is that we have to find a set of variables and settings thereof such that preventing Alice from throwing a brick also stops the window from breaking. One way to do this is to stop Bob from throwing the brick. Finally, for AC2(b), we have to hold things as per AC2(a) and show that Alice throwing the brick breaks the window. (The full definition is a little more involved, involving checking all subsets of variables.)

Probabilistic causation
Interpreting causation as a deterministic relation means that if A causes B, then A must always be followed by B. In this sense, war does not cause deaths, nor does smoking cause cancer or emphysema. As a result, many turn to a notion of probabilistic causation. Informally, A ("The person is a smoker") probabilistically causes B ("The person has now or will have cancer at some time in the future"), if the information that A occurred increases the likelihood of Bs occurrence. Formally, P{B|A}≥ P{B} where P{B|A} is the conditional probability that B will occur given the information that A occurred, and P{B} is the probability that B will occur having no knowledge whether A did or did not occur. This intuitive condition is not adequate as a definition for probabilistic causation because of its being too general and thus not meeting our intuitive notion of cause and effect. For example, if A denotes the event "The person is a smoker," B denotes the event "The person now has or will have cancer at some time in the future" and C denotes the event "The person now has or will have emphysema some time in the future," then the following three relationships hold: P{B|A} ≥ P{B}, P{C|A} ≥ P{C} and P{B|C} ≥ P{B}. The last relationship states that knowing that the person has emphysema increases the likelihood that he will have cancer. The reason for this is that having the information that the person has emphysema increases the likelihood that the person is a smoker, thus indirectly increasing the likelihood that the person will have cancer. However, we would not want to conclude that having emphysema causes cancer. Thus, we need additional conditions such as temporal relationship of A to B and a rational explanation as to the mechanism of action. It is hard to quantify this last requirement and thus different authors prefer somewhat different definitions.

Causal calculus
When experimental interventions are infeasible or illegal, the derivation of a cause-and-effect relationship from observational studies must rest on some qualitative theoretical assumptions, for example, that symptoms do not cause diseases, usually expressed in the form of missing arrows in causal graphs such as Bayesian networks or path diagrams. The theory underlying these derivations relies on the distinction between conditional probabilities, as in $$P(cancer|smoking)$$, and interventional probabilities, as in $$P(cancer|do(smoking))$$. The former reads: "the probability of finding cancer in a person known to smoke, having started, unforced by the experimenter, to do so at an unspecified time in the past", while the latter reads: "the probability of finding cancer in a person forced by the experimenter to smoke at a specified time in the past". The former is a statistical notion that can be estimated by observation with negligible intervention by the experimenter, while the latter is a causal notion which is estimated in an experiment with an important controlled randomized intervention. It is specifically characteristic of quantal phenomena that observations defined by incompatible variables always involve important intervention by the experimenter, as described quantitatively by the observer effect. In classical thermodynamics, processes are initiated by interventions called thermodynamic operations. In other branches of science, for example astronomy, the experimenter can often observe with negligible intervention.

The theory of "causal calculus" (also known as do-calculus, Judea Pearl's Causal Calculus, Calculus of Actions) permits one to infer interventional probabilities from conditional probabilities in causal Bayesian networks with unmeasured variables. One very practical result of this theory is the characterization of confounding variables, namely, a sufficient set of variables that, if adjusted for, would yield the correct causal effect between variables of interest. It can be shown that a sufficient set for estimating the causal effect of $$X$$ on $$Y$$ is any set of non-descendants of $$X$$ that $$d$$-separate $$X$$ from $$Y$$ after removing all arrows emanating from $$X$$. This criterion, called "backdoor", provides a mathematical definition of "confounding" and helps researchers identify accessible sets of variables worthy of measurement.

Structure learning
While derivations in causal calculus rely on the structure of the causal graph, parts of the causal structure can, under certain assumptions, be learned from statistical data. The basic idea goes back to Sewall Wright's 1921 work on path analysis. A "recovery" algorithm was developed by Rebane and Pearl (1987) which rests on Wright's distinction between the three possible types of causal substructures allowed in a directed acyclic graph (DAG):

 * 1) $$X \rightarrow Y \rightarrow Z$$
 * 2) $$X \leftarrow Y \rightarrow Z$$
 * 3) $$X \rightarrow Y \leftarrow Z$$

Type 1 and type 2 represent the same statistical dependencies (i.e., $$X$$ and $$Z$$ are independent given $$Y$$) and are, therefore, indistinguishable within purely cross-sectional data. Type 3, however, can be uniquely identified, since $$X$$ and $$Z$$ are marginally independent and all other pairs are dependent. Thus, while the skeletons (the graphs stripped of arrows) of these three triplets are identical, the directionality of the arrows is partially identifiable. The same distinction applies when $$X$$ and $$Z$$ have common ancestors, except that one must first condition on those ancestors. Algorithms have been developed to systematically determine the skeleton of the underlying graph and, then, orient all arrows whose directionality is dictated by the conditional independencies observed.

Alternative methods of structure learning search through the many possible causal structures among the variables, and remove ones which are strongly incompatible with the observed correlations. In general this leaves a set of possible causal relations, which should then be tested by analyzing time series data or, preferably, designing appropriately controlled experiments. In contrast with Bayesian Networks, path analysis (and its generalization, structural equation modeling), serve better to estimate a known causal effect or to test a causal model than to generate causal hypotheses.

For nonexperimental data, causal direction can often be inferred if information about time is available. This is because (according to many, though not all, theories) causes must precede their effects temporally. This can be determined by statistical time series models, for instance, or with a statistical test based on the idea of Granger causality, or by direct experimental manipulation. The use of temporal data can permit statistical tests of a pre-existing theory of causal direction. For instance, our degree of confidence in the direction and nature of causality is much greater when supported by cross-correlations, ARIMA models, or cross-spectral analysis using vector time series data than by cross-sectional data.

Derivation theories
Nobel laureate Herbert A. Simon and philosopher Nicholas Rescher claim that the asymmetry of the causal relation is unrelated to the asymmetry of any mode of implication that contraposes. Rather, a causal relation is not a relation between values of variables, but a function of one variable (the cause) on to another (the effect). So, given a system of equations, and a set of variables appearing in these equations, we can introduce an asymmetric relation among individual equations and variables that corresponds perfectly to our commonsense notion of a causal ordering. The system of equations must have certain properties, most importantly, if some values are chosen arbitrarily, the remaining values will be determined uniquely through a path of serial discovery that is perfectly causal. They postulate the inherent serialization of such a system of equations may correctly capture causation in all empirical fields, including physics and economics.

Manipulation theories
Some theorists have equated causality with manipulability. Under these theories, x causes y only in the case that one can change x in order to change y. This coincides with commonsense notions of causations, since often we ask causal questions in order to change some feature of the world. For instance, we are interested in knowing the causes of crime so that we might find ways of reducing it.

These theories have been criticized on two primary grounds. First, theorists complain that these accounts are circular. Attempting to reduce causal claims to manipulation requires that manipulation is more basic than causal interaction. But describing manipulations in non-causal terms has provided a substantial difficulty.

The second criticism centers around concerns of anthropocentrism. It seems to many people that causality is some existing relationship in the world that we can harness for our desires. If causality is identified with our manipulation, then this intuition is lost. In this sense, it makes humans overly central to interactions in the world.

Some attempts to defend manipulability theories are recent accounts that do not claim to reduce causality to manipulation. These accounts use manipulation as a sign or feature in causation without claiming that manipulation is more fundamental than causation.

Process theories
Some theorists are interested in distinguishing between causal processes and non-causal processes (Russell 1948; Salmon 1984). These theorists often want to distinguish between a process and a pseudo-process. As an example, a ball moving through the air (a process) is contrasted with the motion of a shadow (a pseudo-process). The former is causal in nature while the latter is not.

Salmon (1984) claims that causal processes can be identified by their ability to transmit an alteration over space and time. An alteration of the ball (a mark by a pen, perhaps) is carried with it as the ball goes through the air. On the other hand, an alteration of the shadow (insofar as it is possible) will not be transmitted by the shadow as it moves along.

These theorists claim that the important concept for understanding causality is not causal relationships or causal interactions, but rather identifying causal processes. The former notions can then be defined in terms of causal processes.

A subgroup of the process theories is the mechanistic view on causality. It states that causal relations supervene on mechanisms. While the notion of mechanism is understood differently, the definition put forward by the group of philosophers referred to as the 'New Mechanists' dominate the literature.

Science
For the scientific investigation of efficient causality, the cause and effect are each best conceived of as temporally transient processes.

Within the conceptual frame of the scientific method, an investigator sets up several distinct and contrasting temporally transient material processes that have the structure of experiments, and records candidate material responses, normally intending to determine causality in the physical world. For instance, one may want to know whether a high intake of carrots causes humans to develop the bubonic plague. The quantity of carrot intake is a process that is varied from occasion to occasion. The occurrence or non-occurrence of subsequent bubonic plague is recorded. To establish causality, the experiment must fulfill certain criteria, only one example of which is mentioned here. For example, instances of the hypothesized cause must be set up to occur at a time when the hypothesized effect is relatively unlikely in the absence of the hypothesized cause; such unlikelihood is to be established by empirical evidence. A mere observation of a correlation is not nearly adequate to establish causality. In nearly all cases, establishment of causality relies on repetition of experiments and probabilistic reasoning. Hardly ever is causality established more firmly than as more or less probable. It is most convenient for establishment of causality if the contrasting material states of affairs are precisely matched, except for only one variable factor, perhaps measured by a real number.

Physics
One has to be careful in the use of the word cause in physics. Properly speaking, the hypothesized cause and the hypothesized effect are each temporally transient processes. For example, force is a useful concept for the explanation of acceleration, but force is not by itself a cause. More is needed. For example, a temporally transient process might be characterized by a definite change of force at a definite time. Such a process can be regarded as a cause. Causality is not inherently implied in equations of motion, but postulated as an additional constraint that needs to be satisfied (i.e. a cause always precedes its effect). This constraint has mathematical implications such as the Kramers-Kronig relations.

Causality is one of the most fundamental and essential notions of physics. Causal efficacy cannot 'propagate' faster than light. Otherwise, reference coordinate systems could be constructed (using the Lorentz transform of special relativity) in which an observer would see an effect precede its cause (i.e. the postulate of causality would be violated).

Causal notions appear in the context of the flow of mass-energy. Any actual process has causal efficacy that can propagate no faster than light. In contrast, an abstraction has no causal efficacy. Its mathematical expression does not propagate in the ordinary sense of the word, though it may refer to virtual or nominal 'velocities' with magnitudes greater than that of light. For example, wave packets are mathematical objects that have group velocity and phase velocity. The energy of a wave packet travels at the group velocity (under normal circumstances); since energy has causal efficacy, the group velocity cannot be faster than the speed of light. The phase of a wave packet travels at the phase velocity; since phase is not causal, the phase velocity of a wave packet can be faster than light.

Causal notions are important in general relativity to the extent that the existence of an arrow of time demands that the universe's semi-Riemannian manifold be orientable, so that "future" and "past" are globally definable quantities.

Engineering
A causal system is a system with output and internal states that depends only on the current and previous input values. A system that has some dependence on input values from the future (in addition to possible past or current input values) is termed an acausal system, and a system that depends solely on future input values is an anticausal system. Acausal filters, for example, can only exist as postprocessing filters, because these filters can extract future values from a memory buffer or a file.

We have to be very careful with causality in physics and engineering. Cellier, Elmqvist, and Otter describe causality forming the basis of physics as a misconception, because physics is essentially acausal. In their article they cite a simple example: "The relationship between voltage across and current through an electrical resistor can be described by Ohm's law: V = IR, yet, whether it is the current flowing through the resistor that causes a voltage drop, or whether it is the difference between the electrical potentials on the two wires that causes current to flow is, from a physical perspective, a meaningless question". In fact, if we explain cause-effect using the law, we need two explanations to describe an electrical resistor: as a voltage-drop-causer or as a current-flow-causer. There is no physical experiment in the world that can distinguish between action and reaction.

Biology, medicine and epidemiology
Austin Bradford Hill built upon the work of Hume and Popper and suggested in his paper "The Environment and Disease: Association or Causation?" that aspects of an association such as strength, consistency, specificity, and temporality be considered in attempting to distinguish causal from noncausal associations in the epidemiological situation. (See Bradford-Hill criteria.) He did not note however, that temporality is the only necessary criterion among those aspects. Directed acyclic graphs (DAGs) are increasingly used in epidemiology to help enlighten causal thinking.

Psychology
Psychologists take an empirical approach to causality, investigating how people and non-human animals detect or infer causation from sensory information, prior experience and innate knowledge.

Attribution: Attribution theory is the theory concerning how people explain individual occurrences of causation. Attribution can be external (assigning causality to an outside agent or force—claiming that some outside thing motivated the event) or internal (assigning causality to factors within the person—taking personal responsibility or accountability for one's actions and claiming that the person was directly responsible for the event). Taking causation one step further, the type of attribution a person provides influences their future behavior.

The intention behind the cause or the effect can be covered by the subject of action. See also accident; blame; intent; and responsibility.

Whereas David Hume argued that causes are inferred from non-causal observations, Immanuel Kant claimed that people have innate assumptions about causes. Within psychology, Patricia Cheng attempted to reconcile the Humean and Kantian views. According to her power PC theory, people filter observations of events through an intuition that causes have the power to generate (or prevent) their effects, thereby inferring specific cause-effect relations.
 * Causal powers

Our view of causation depends on what we consider to be the relevant events. Another way to view the statement, "Lightning causes thunder" is to see both lightning and thunder as two perceptions of the same event, viz., an electric discharge that we perceive first visually and then aurally.
 * Causation and salience

David Sobel and Alison Gopnik from the Psychology Department of UC Berkeley designed a device known as the blicket detector which would turn on when an object was placed on it. Their research suggests that "even young children will easily and swiftly learn about a new causal power of an object and spontaneously use that information in classifying and naming the object."
 * Naming and causality

Some researchers such as Anjan Chatterjee at the University of Pennsylvania and Jonathan Fugelsang at the University of Waterloo are using neuroscience techniques to investigate the neural and psychological underpinnings of causal launching events in which one object causes another object to move. Both temporal and spatial factors can be manipulated.
 * Perception of launching events

See Causal Reasoning (Psychology) for more information.

Statistics and economics
Statistics and economics usually employ pre-existing data or experimental data to infer causality by regression methods. The body of statistical techniques involves substantial use of regression analysis. Typically a linear relationship such as

 * $$y_i = a_0 + a_1x_{1,i} + a_2x_{2,i} + \dots + a_kx_{k,i} + e_i$$

is postulated, in which $$y_i$$ is the ith observation of the dependent variable (hypothesized to be the caused variable), $$x_{j,i}$$ for j=1,...,k is the ith observation on the jth independent variable (hypothesized to be a causative variable), and $$e_i$$ is the error term for the ith observation (containing the combined effects of all other causative variables, which must be uncorrelated with the included independent variables). If there is reason to believe that none of the $$x_j$$s is caused by y, then estimates of the coefficients $$a_j$$ are obtained. If the null hypothesis that $$a_j=0$$ is rejected, then the alternative hypothesis that $$a_{j} \ne 0 $$ and equivalently that $$x_j$$ causes y cannot be rejected. On the other hand, if the null hypothesis that $$a_j=0$$ cannot be rejected, then equivalently the hypothesis of no causal effect of $$x_j$$ on y cannot be rejected. Here the notion of causality is one of contributory causality as discussed above: If the true value $$a_j \ne 0$$, then a change in $$x_j$$ will result in a change in y unless some other causative variable(s), either included in the regression or implicit in the error term, change in such a way as to exactly offset its effect; thus a change in $$x_j$$ is not sufficient to change y. Likewise, a change in $$x_j$$ is not necessary to change y, because a change in y could be caused by something implicit in the error term (or by some other causative explanatory variable included in the model).

The above way of testing for causality requires belief that there is no reverse causation, in which y would cause $$x_j$$. This belief can be established in one of several ways. First, the variable $$x_j$$ may be a non-economic variable: for example, if rainfall amount $$x_j$$ is hypothesized to affect the futures price y of some agricultural commodity, it is impossible that in fact the futures price affects rainfall amount (provided that cloud seeding is never attempted). Second, the instrumental variables technique may be employed to remove any reverse causation by introducing a role for other variables (instruments) that are known to be unaffected by the dependent variable. Third, the principle that effects cannot precede causes can be invoked, by including on the right side of the regression only variables that precede in time the dependent variable; this principle is invoked, for example, in testing for Granger causality and in its multivariate analog, vector autoregression, both of which control for lagged values of the dependent variable while testing for causal effects of lagged independent variables.

Regression analysis controls for other relevant variables by including them as regressors (explanatory variables). This helps to avoid false inferences of causality due to the presence of a third, underlying, variable that influences both the potentially causative variable and the potentially caused variable: its effect on the potentially caused variable is captured by directly including it in the regression, so that effect will not be picked up as an indirect effect through the potentially causative variable of interest. Given the above procedures, coincidental (as opposed to causal) correlation can be probabilistically rejected if data samples are large and if regression results pass cross-validation tests showing that the correlations hold even for data that were not used in the regression. Asserting with certitude that a common-cause is absent and the regression represents the true causal structure is in principle impossible.

The problem of omitted variable bias, however, has to be balanced against the risk of inserting Causal colliders, in which the addition of a new variable $$x_{j+1}$$ induces a correlation between $$x_j$$ and $$y$$ via Berkson's paradox.

Apart from constructing statistical models of observational and experimental data, economists use axiomatic (mathematical) models to infer and represent causal mechanisms. Highly abstract theoretical models that isolate and idealize one mechanism dominate microeconomics. In macroeconomics, economists use broad mathematical models that are calibrated on historical data. A subgroup of calibrated models, dynamic stochastic general equilibrium (DSGE) models are employed to represent (in a simplified way) the whole economy and simulate changes in fiscal and monetary policy.

Management
For quality control in manufacturing in the 1960s, Kaoru Ishikawa developed a cause and effect diagram, known as an Ishikawa diagram or fishbone diagram. The diagram categorizes causes, such as into the six main categories shown here. These categories are then sub-divided. Ishikawa's method identifies "causes" in brainstorming sessions conducted among various groups involved in the manufacturing process. These groups can then be labeled as categories in the diagrams. The use of these diagrams has now spread beyond quality control, and they are used in other areas of management and in design and engineering. Ishikawa diagrams have been criticized for failing to make the distinction between necessary conditions and sufficient conditions. It seems that Ishikawa was not even aware of this distinction.

History
In the discussion of history, events are sometimes considered as if in some way being agents that can then bring about other historical events. Thus, the combination of poor harvests, the hardships of the peasants, high taxes, lack of representation of the people, and kingly ineptitude are among the causes of the French Revolution. This is a somewhat Platonic and Hegelian view that reifies causes as ontological entities. In Aristotelian terminology, this use approximates to the case of the efficient cause.

Some philosophers of history such as Arthur Danto have claimed that "explanations in history and elsewhere" describe "not simply an event—something that happens—but a change". Like many practicing historians, they treat causes as intersecting actions and sets of actions which bring about "larger changes", in Danto's words: to decide "what are the elements which persist through a change" is "rather simple" when treating an individual's "shift in attitude", but "it is considerably more complex and metaphysically challenging when we are interested in such a change as, say, the break-up of feudalism or the emergence of nationalism".

Much of the historical debate about causes has focused on the relationship between communicative and other actions, between singular and repeated ones, and between actions, structures of action or group and institutional contexts and wider sets of conditions. John Gaddis has distinguished between exceptional and general causes (following Marc Bloch) and between "routine" and "distinctive links" in causal relationships: "in accounting for what happened at Hiroshima on August 6, 1945, we attach greater importance to the fact that President Truman ordered the dropping of an atomic bomb than to the decision of the Army Air Force to carry out his orders." He has also pointed to the difference between immediate, intermediate and distant causes. For his part, Christopher Lloyd puts forward four "general concepts of causation" used in history: the "metaphysical idealist concept, which asserts that the phenomena of the universe are products of or emanations from an omnipotent being or such final cause"; "the empiricist (or Humean) regularity concept, which is based on the idea of causation being a matter of constant conjunctions of events"; "the functional/teleological/consequential concept", which is "goal-directed, so that goals are causes"; and the "realist, structurist and dispositional approach, which sees relational structures and internal dispositions as the causes of phenomena".

Law
According to law and jurisprudence, legal cause must be demonstrated to hold a defendant liable for a crime or a tort (i.e. a civil wrong such as negligence or trespass). It must be proven that causality, or a "sufficient causal link" relates the defendant's actions to the criminal event or damage in question. Causation is also an essential legal element that must be proven to qualify for remedy measures under international trade law.

Hindu philosophy
Vedic period (c. 1750–500 BCE) literature has karma's Eastern origins. Karma is the belief held by Sanatana Dharma and major religions that a person's actions cause certain effects in the current life and/or in future life, positively or negatively. The various philosophical schools (darshanas) provide different accounts of the subject. The doctrine of satkaryavada affirms that the effect inheres in the cause in some way. The effect is thus either a real or apparent modification of the cause. The doctrine of asatkaryavada affirms that the effect does not inhere in the cause, but is a new arising. See Nyaya for some details of the theory of causation in the Nyaya school. In Brahma Samhita, Brahma describes Krishna as the prime cause of all causes.

Bhagavad-gītā 18.14 identifies five causes for any action (knowing which it can be perfected): the body, the individual soul, the senses, the efforts and the supersoul.

According to Monier-Williams, in the Nyāya causation theory from Sutra I.2.I,2 in the Vaisheshika philosophy, from causal non-existence is effectual non-existence; but, not effectual non-existence from causal non-existence. A cause precedes an effect. With a threads and cloth metaphors, three causes are:

 * 1) Co-inherence cause: resulting from substantial contact, 'substantial causes', threads are substantial to cloth, corresponding to Aristotle's material cause.
 * 2) Non-substantial cause: Methods putting threads into cloth, corresponding to Aristotle's formal cause.
 * 3) Instrumental cause: Tools to make the cloth, corresponding to Aristotle's efficient cause.

Monier-Williams also proposed that Aristotle's and the Nyaya's causality are considered conditional aggregates necessary to man's productive work.

Buddhist philosophy
Karma is the causality principle focusing on 1) causes, 2) actions, 3) effects, where it is the mind's phenomena that guide the actions that the actor performs. Buddhism trains the actor's actions for continued and uncontrived virtuous outcomes aimed at reducing suffering. This follows the Subject–verb–object structure.

The general or universal definition of pratityasamutpada (or "dependent origination" or "dependent arising" or "interdependent co-arising") is that everything arises in dependence upon multiple causes and conditions; nothing exists as a singular, independent entity. A traditional example in Buddhist texts is of three sticks standing upright and leaning against each other and supporting each other. If one stick is taken away, the other two will fall to the ground.

Causality in the Chittamatrin Buddhist school approach, Asanga's (c. 400 CE) mind-only Buddhist school, asserts that objects cause consciousness in the mind's image. Because causes precede effects, which must be different entities, then subject and object are different. For this school, there are no objects which are entities external to a perceiving consciousness. The Chittamatrin and the Yogachara Svatantrika schools accept that there are no objects external to the observer's causality. This largely follows the Nikayas approach.

The Vaibhashika (c. 500 CE) is an early Buddhist school which favors direct object contact and accepts simultaneous cause and effects. This is based in the consciousness example which says, intentions and feelings are mutually accompanying mental factors that support each other like poles in tripod. In contrast, simultaneous cause and effect rejectors say that if the effect already exists, then it cannot effect the same way again. How past, present and future are accepted is a basis for various Buddhist school's causality viewpoints.

All the classic Buddhist schools teach karma. "The law of karma is a special instance of the law of cause and effect, according to which all our actions of body, speech, and mind are causes and all our experiences are their effects."

Aristotelian
Aristotle identified four kinds of answer or explanatory mode to various "Why?" questions. He thought that, for any given topic, all four kinds of explanatory mode were important, each in its own right. As a result of traditional specialized philosophical peculiarities of language, with translations between ancient Greek, Latin, and English, the word 'cause' is nowadays in specialized philosophical writings used to label Aristotle's four kinds. In ordinary language, the word 'cause' has a variety of meanings, the most common of which refers to efficient causation, which is the topic of the present article.
 * Material cause, the material whence a thing has come or that which persists while it changes, as for example, one's mother or the bronze of a statue (see also substance theory).
 * Formal cause, whereby a thing's dynamic form or static shape determines the thing's properties and function, as a human differs from a statue of a human or as a statue differs from a lump of bronze.
 * Efficient cause, which imparts the first relevant movement, as a human lifts a rock or raises a statue. This is the main topic of the present article.
 * Final cause, the criterion of completion, or the end; it may refer to an action or to an inanimate process. Examples: Socrates takes a walk after dinner for the sake of his health; earth falls to the lowest level because that is its nature.

Of Aristotle's four kinds or explanatory modes, only one, the 'efficient cause' is a cause as defined in the leading paragraph of this present article. The other three explanatory modes might be rendered material composition, structure and dynamics, and, again, criterion of completion. The word that Aristotle used was $αἰτία$. For the present purpose, that Greek word would be better translated as "explanation" than as "cause" as those words are most often used in current English. Another translation of Aristotle is that he meant "the four Becauses" as four kinds of answer to "why" questions.

Aristotle assumed efficient causality as referring to a basic fact of experience, not explicable by, or reducible to, anything more fundamental or basic.

In some works of Aristotle, the four causes are listed as (1) the essential cause, (2) the logical ground, (3) the moving cause, and (4) the final cause. In this listing, a statement of essential cause is a demonstration that an indicated object conforms to a definition of the word that refers to it. A statement of logical ground is an argument as to why an object statement is true. These are further examples of the idea that a "cause" in general in the context of Aristotle's usage is an "explanation".

The word "efficient" used here can also be translated from Aristotle as "moving" or "initiating".

Efficient causation was connected with Aristotelian physics, which recognized the four elements (earth, air, fire, water), and added the fifth element (aether). Water and earth by their intrinsic property gravitas or heaviness intrinsically fall toward, whereas air and fire by their intrinsic property levitas or lightness intrinsically rise away from, Earth's center—the motionless center of the universe—in a straight line while accelerating during the substance's approach to its natural place.

As air remained on Earth, however, and did not escape Earth while eventually achieving infinite speed—an absurdity—Aristotle inferred that the universe is finite in size and contains an invisible substance that holds planet Earth and its atmosphere, the sublunary sphere, centered in the universe. And since celestial bodies exhibit perpetual, unaccelerated motion orbiting planet Earth in unchanging relations, Aristotle inferred that the fifth element, aither, that fills space and composes celestial bodies intrinsically moves in perpetual circles, the only constant motion between two points. (An object traveling a straight line from point A to B and back must stop at either point before returning to the other.)

Left to itself, a thing exhibits natural motion, but can—according to Aristotelian metaphysics—exhibit enforced motion imparted by an efficient cause. The form of plants endows plants with the processes nutrition and reproduction, the form of animals adds locomotion, and the form of humankind adds reason atop these. A rock normally exhibits natural motion—explained by the rock's material cause of being composed of the element earth—but a living thing can lift the rock, an enforced motion diverting the rock from its natural place and natural motion. As a further kind of explanation, Aristotle identified the final cause, specifying a purpose or criterion of completion in light of which something should be understood.

Aristotle himself explained,

"Cause means

(a) in one sense, that as the result of whose presence something comes into being—e.g., the bronze of a statue and the silver of a cup, and the classes which contain these [i.e., the material cause];

(b) in another sense, the form or pattern; that is, the essential formula and the classes which contain it—e.g. the ratio 2:1 and number in general is the cause of the octave—and the parts of the formula [i.e., the formal cause].

(c) The source of the first beginning of change or rest; e.g. the man who plans is a cause, and the father is the cause of the child, and in general that which produces is the cause of that which is produced, and that which changes of that which is changed [i.e., the efficient cause].

(d) The same as 'end'; i.e. the final cause; e.g., as the 'end' of walking is health. For why does a man walk? 'To be healthy', we say, and by saying this we consider that we have supplied the cause [the final cause].

(e) All those means towards the end which arise at the instigation of something else, as, e.g., fat-reducing, purging, drugs, and instruments are causes of health; for they all have the end as their object, although they differ from each other as being some instruments, others actions [i.e., necessary conditions]."

Aristotle further discerned two modes of causation: proper (prior) causation and accidental (chance) causation. All causes, proper and accidental, can be spoken as potential or as actual, particular or generic. The same language refers to the effects of causes, so that generic effects are assigned to generic causes, particular effects to particular causes, and actual effects to operating causes.

Averting infinite regress, Aristotle inferred the first mover—an unmoved mover. The first mover's motion, too, must have been caused, but, being an unmoved mover, must have moved only toward a particular goal or desire.

Pyrrhonism
While the plausibility of causality was accepted in Pyrrhonism, it was equally accepted that it was plausible that nothing was the cause of anything.

Middle Ages
In line with Aristotelian cosmology, Thomas Aquinas posed a hierarchy prioritizing Aristotle's four causes: "final > efficient > material > formal". Aquinas sought to identify the first efficient cause—now simply first cause—as everyone would agree, said Aquinas, to call it God. Later in the Middle Ages, many scholars conceded that the first cause was God, but explained that many earthly events occur within God's design or plan, and thereby scholars sought freedom to investigate the numerous secondary causes.

After the Middle Ages
For Aristotelian philosophy before Aquinas, the word cause had a broad meaning. It meant 'answer to a why question' or 'explanation', and Aristotelian scholars recognized four kinds of such answers. With the end of the Middle Ages, in many philosophical usages, the meaning of the word 'cause' narrowed. It often lost that broad meaning, and was restricted to just one of the four kinds. For authors such as Niccolò Machiavelli, in the field of political thinking, and Francis Bacon, concerning science more generally, Aristotle's moving cause was the focus of their interest. A widely used modern definition of causality in this newly narrowed sense was assumed by David Hume. He undertook an epistemological and metaphysical investigation of the notion of moving cause. He denied that we can ever perceive cause and effect, except by developing a habit or custom of mind where we come to associate two types of object or event, always contiguous and occurring one after the other. In Part III, section XV of his book A Treatise of Human Nature, Hume expanded this to a list of eight ways of judging whether two things might be cause and effect. The first three:

And then additionally there are three connected criteria which come from our experience and which are "the source of most of our philosophical reasonings": 1. "The same cause always produces the same effect, and the same effect never arises but from the same cause. This principle we derive from experience, and is the source of most of our philosophical reasonings."
 * 1) "The cause and effect must be contiguous in space and time."
 * 2) "The cause must be prior to the effect."
 * 3) "There must be a constant union betwixt the cause and effect. 'Tis chiefly this quality, that constitutes the relation."

2. Hanging upon the above, Hume says that "where several different objects produce the same effect, it must be by means of some quality, which we discover to be common amongst them."

3. And "founded on the same reason": "The difference in the effects of two resembling objects must proceed from that particular, in which they differ." And then two more: 1. "When any object increases or diminishes with the increase or diminution of its cause, 'tis to be regarded as a compounded effect, deriv'd from the union of the several different effects, which arise from the several different parts of the cause."

2. An "object, which exists for any time in its full perfection without any effect, is not the sole cause of that effect, but requires to be assisted by some other principle, which may forward its influence and operation."

In 1949, physicist Max Born distinguished determination from causality. For him, determination meant that actual events are so linked by laws of nature that certainly reliable predictions and retrodictions can be made from sufficient present data about them. He describes two kinds of causation: nomic or generic causation and singular causation. Nomic causality means that cause and effect are linked by more or less certain or probabilistic general laws covering many possible or potential instances; this can be recognized as a probabilized version of Hume's criterion 3. An occasion of singular causation is a particular occurrence of a definite complex of events that are physically linked by antecedence and contiguity, which may be recognized as criteria 1 and 2.

title: Causality


text:
Isotope separation

Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" nuclide are used to figure out reaction mechanisms). By tonnage, separating natural uranium into enriched uranium and depleted uranium is the largest application. In the following text, mainly uranium enrichment is considered. This process is crucial in the manufacture of uranium fuel for nuclear power plants and is also required for the creation of uranium-based nuclear weapons. Plutonium-based weapons use plutonium produced in a nuclear reactor, which must be operated in such a way as to produce plutonium already of suitable isotopic mix or grade.

While chemical elements can be purified through chemical processes, isotopes of the same element have nearly identical chemical properties which makes this type of separation impractical, except for separation of deuterium.

Techniques
There are three types of isotope separation techniques:
 * Those based directly on the atomic weight of the isotope.
 * Those based on the small differences in chemical reaction rates produced by different atomic weights.
 * Those based on properties not directly connected to atomic weight, such as nuclear resonances.

The third type of separation is still experimental; practical separation techniques all depend in some way on the atomic mass. It is therefore generally easier to separate isotopes with a larger relative mass difference. For example, deuterium has twice the mass of ordinary (light) hydrogen and it is generally easier to purify it than to separate uranium-235 from the more common uranium-238. On the other extreme, separation of fissile plutonium-239 from the common impurity plutonium-240, while desirable in that it would allow the creation of gun-type fission weapons from plutonium, is generally agreed to be impractical.

Enrichment cascades
All large-scale isotope separation schemes employ a number of similar stages which produce successively higher concentrations of the desired isotope. Each stage enriches the product of the previous step further before being sent to the next stage. Similarly, the tailings from each stage are returned to the previous stage for further processing. This creates a sequential enriching system called a cascade. There are two important factors that characterize the performance of a cascade. The first is the separation factor, which is a number greater than 1. The second is the number of required stages to get the desired purity.

Commercial materials
To date, large-scale commercial isotope separation of only three elements has occurred. In each case, the rarer of the two most common isotopes of an element has been concentrated for use in nuclear technology:
 * Uranium isotopes have been separated to prepare enriched uranium for use as nuclear reactor fuel and in nuclear weapons.
 * Hydrogen isotopes have been separated to prepare heavy water for use as a moderator in nuclear reactors.
 * Tritium is both a nuisance in the coolant / moderator of water moderated reactors and a valuable product; it is thus sometimes separated from the coolant.
 * Lithium-6 has been concentrated for use in thermonuclear weapons. Tritium is commonly produced from lithium-6 which is often enriched for this purpose.

Some isotopically purified elements are used in smaller quantities for specialist applications, especially in the semiconductor industry, where purified silicon is used to improve crystal structure and thermal conductivity, and carbon with greater isotopic purity to make diamonds with greater thermal conductivity.

Isotope separation is an important process for both peaceful and military nuclear technology, and therefore the capability that a nation has for isotope separation is of extreme interest to the intelligence community.

Alternatives
The only alternative to isotope separation is to manufacture the required isotope in its pure form. This may be done by irradiation of a suitable target, but care is needed in target selection and other factors to ensure that only the required isotope of the element of interest is produced. Isotopes of other elements are not so great a problem as they can be removed by chemical means.

This is particularly relevant in the preparation of high-grade plutonium-239 for use in weapons. It is not practical to separate Pu-239 from Pu-240 or Pu-241. Fissile Pu-239 is produced following neutron capture by uranium-238, but further neutron capture will produce Pu-240 which is less fissile and worse, is a fairly strong neutron emitter, and Pu-241 which decays to Am-241, a strong alpha emitter that poses self-heating and radiotoxicity problems. Therefore, the uranium targets used to produce military plutonium must be irradiated for only a short time, to minimise the production of these unwanted isotopes. Conversely, blending plutonium with Pu-240 renders it less suitable for nuclear weapons.

If the desired goal is not an atom bomb but running a nuclear power plant, the alternative to enrichment of uranium for use in a light-water reactor is the use of a neutron moderator with a lower neutron absorption cross section than protium. Options include heavy water as used in CANDU type reactors or graphite as used in magnox or RBMK reactors. Obtaining heavy water however also requires isotope separation, in this case of hydrogen isotopes, which is easier due to the bigger variation in atomic weight. Both magnox and RBMK reactors had undesirable properties when run with natural uranium, which ultimately led to the replacement of this fuel with low enriched uranium, negating the advantage of foregoing enrichment. Pressurized heavy-water reactors such as the CANDU are still in active use and India which has limited domestic uranium resources and been under a partial nuclear embargo ever since it became an atom bomb state in particular relies on heavy water moderated reactors for its nuclear power. A big downside of heavy water reactors is the enormous upfront cost of the heavy water.

Diffusion
Often done with gases, but also with liquids, the diffusion method relies on the fact that in thermal equilibrium, two isotopes with the same energy will have different average velocities. The lighter atoms (or the molecules containing them) will travel more quickly through a membrane, whose pore diameters are not larger than the mean free path length (Knudsen flow). The speed ratio is equal to the inverse square root of the mass ratio, so the amount of separation is small. For example for 235UF6 versus 238UF6 it is 1.0043. Hence many cascaded stages are needed to obtain high purity. This method is expensive due to the work needed to push gas through a membrane and the many stages necessary, each requiring recompression of the gas.

The first large-scale separation of uranium isotopes was achieved by the United States in large gaseous diffusion separation plants at Clinton Engineering Works, which were established as part of the Manhattan Project. These used uranium hexafluoride gas as the process fluid. Nickel powder and electro-deposited nickel mesh diffusion barriers were pioneered by Edward Adler and Edward Norris. Due to the high energy consumption, enrichment of uranium by diffusion was gradually replaced by more efficient methods.

The last diffusion plant closed in 2013. The Paducah Gaseous Diffusion Plant was a US government effort to generate highly enriched uranium to power military reactors and create nuclear bombs which led to the establishment of the facility in 1952. Paducah's enrichment was initially kept to low levels, and the facility operated as a "feed facility" for other defence facilities that processed the enriched uranium at Oak Ridge National Laboratory in Oak Ridge, Tennessee, and Portsmouth Gaseous Diffusion Plant in Piketon, Ohio. The goal of Paducah and its sister facility in Piketon was adjusted in the 1960s when they started to enrich uranium for use in commercial nuclear reactors to produce energy.

Centrifugal

Centrifugal schemes rapidly rotate the material allowing the heavier isotopes to go closer to an outer radial wall. This is often done in gaseous form using a Zippe-type centrifuge. Centrifuging plasma can separate isotopes as well as separating ranges of elements for radioactive waste reduction, nuclear reprocessing, and other purposes. The process is called "plasma mass separation"; the devices are called "plasma mass filter" or "plasma centrifuge" (not to be confused with medical centrifuges).

The centrifugal separation of isotopes was first suggested by Aston and Lindemann in 1919 and the first successful experiments were reported by Beams and Haynes on isotopes of chlorine in 1936. However attempts to use the technology during the Manhattan Project were unproductive. In modern times it is the main method used throughout the world to enrich uranium and as a result remains a fairly secretive process, hindering a more widespread uptake of the technology. In general a feed of UF6 gas is connected to a cylinder that is rotated at high speed. Near the outer edge of the cylinder heavier gas molecules containing U-238 collect, while molecules containing U-235 concentrate at the centre and are then fed to another cascade stage.

Use of gaseous centrifugal technology to enrich isotopes is desirable as power consumption is greatly reduced when compared to more conventional techniques such as diffusion plants since fewer cascade steps are required to reach similar degrees of separation. As well as requiring less energy to achieve the same separation, far smaller scale plants are possible, making them an economic possibility for a small nation attempting to produce a nuclear weapon. Pakistan is believed to have used this method in developing its nuclear weapons.

Vortex tubes were used by South Africa in their Helikon vortex separation process. The gas is injected tangentially into a chamber with special geometry that further increases its rotation to a very high rate, causing the isotopes to separate. The method is simple because vortex tubes have no moving parts, but energy intensive, about 50 times greater than gas centrifuges. A similar process, known as jet nozzle was created in Germany, with a demonstration plant built in Brazil, and they went as far as developing a site to fuel the country's nuclear plants.

Electromagnetic
Electromagnetic separation is mass spectrometry on a large scale, so it is sometimes referred to as mass spectrometry. It uses the fact that charged particles are deflected in a magnetic field and the amount of deflection depends upon the particle's mass. It is very expensive for the quantity produced, as it has an extremely low throughput, but it can allow very high purities to be achieved. This method is often used for processing small amounts of pure isotopes for research or specific use (such as isotopic tracers) but is impractical for industrial use.

At Oak Ridge National Laboratory and at the University of California, Berkeley, Ernest O. Lawrence developed electromagnetic separation for much of the uranium used in the first atomic bombs. Devices using his principle are named calutrons. After the war the method was largely abandoned as impractical. It had only been undertaken (along with diffusion and other technologies) to guarantee there would be enough material for use, whatever the cost. Its main eventual contribution to the war effort was to further concentrate material from the gaseous diffusion plants to higher levels of purity.

Laser
In this method a laser is tuned to a wavelength which excites only one isotope of the material and ionizes those atoms preferentially. For atoms, the resonant absorption of light for an isotope depends on

 * the nuclear mass (noticeable mainly with light elements)
 * the nuclear volume (causing a deviation from the Coulomb potential, noticeable for heavier elements)
 * hyperfine splitting of electronic transitions, if the nucleus has a spin,

allowing finely tuned lasers to interact with only one isotope. After the atom is ionized it can be removed from the sample by applying an electric field. This method is often abbreviated as AVLIS (atomic vapor laser isotope separation). This method has only been developed as laser technology has improved in the 1970s to 1980s. Attempts to develop it to an industrial scale for uranium enrichment were successively given up in the 1990s "due to never ending technical difficulties" and because centrifuges have reached technical maturity in the meantime. However, it is a major concern to those in the field of nuclear proliferation, because it may be cheaper and more easily hidden than other methods of isotope separation. Tunable lasers used in AVLIS include the dye laser and more recently diode lasers.

A second method of laser separation is known as molecular laser isotope separation (MLIS). In this method, an infrared laser is directed at uranium hexafluoride gas (if enrichment of uranium is desired), exciting molecules that contain a U-235 atom. A second laser, either also in the IR (infrared multiphoton dissociation) or in the UV, frees a fluorine atom, leaving uranium pentafluoride which then precipitates out of the gas. Cascading the MLIS stages is more difficult than with other methods because the UF5 must be fluorinated back to UF6 before being introduced into the next MLIS stage. But with light elements, the isotope selectivity is usually good enough that cascading is not required.

Several alternative MLIS schemes have been developed. For example, one uses a first laser in the near-infrared or visible region, where a selectivity of over 20:1 can be obtained in a single stage. This method is called OP-IRMPD (Overtone Pre-excitation—IR Multiple Photon Dissociation). But due to the small absorption probability in the overtones, too many photons remain unused, so that the method did not reach industrial feasibility. Also some other MLIS methods suffer from wasting of the expensive photons.

Finally, the 'Separation of isotopes by laser excitation' (SILEX) process, developed by Silex Systems in Australia, has been licensed to General Electric for the development of a pilot enrichment plant. For uranium, it uses a cold molecular beam with UF6 in a carrier gas, in which the 235UF6 is selectively excited by an infrared laser near 16 μm. In contrast to the excited molecules, the nonexcited heavier isotopic molecules tends to form clusters with the carrier gas, and these clusters stay closer to the axis of the molecular beam, so that they can pass a skimmer and are thus separated from the excited lighter isotope.

Quite recently yet another scheme has been proposed for the deuterium separation using Trojan wavepackets in circularly polarized electromagnetic field. The process of Trojan wave packet formation by the adiabatic-rapid passage depends in ultra-sensitive way on the reduced electron and nucleus mass which with the same field frequency further leads to excitation of Trojan or anti-Trojan wavepacket depending on the kind of the isotope. Those and their giant, rotating electric dipole moments are then $$\pi$$-shifted in phase and the beam of such atoms splits in the gradient of the electric field in the analogy to Stern–Gerlach experiment.

Chemical methods
Although isotopes of a single element are normally described as having the same chemical properties, this is not strictly true. In particular, reaction rates are very slightly affected by atomic mass.

Techniques using this are most effective for light atoms such as hydrogen. Lighter isotopes tend to react or evaporate more quickly than heavy isotopes, allowing them to be separated. This is how heavy water is produced commercially, see Girdler sulfide process for details. Lighter isotopes also disassociate more rapidly under an electric field. This process in a large cascade was used at the heavy water production plant at Rjukan.

One candidate for the largest kinetic isotopic effect ever measured at room temperature, 305, may eventually be used for the separation of tritium (T). The effects for the oxidation of tritiated formate anions to HTO were measured as:

 * k(HCO2−) = 9.54 M−1s−1
 * k(H)/k(D) = 38
 * k(DCO2−) = 9.54 M−1s−1
 * k(D)/k(T) = 8.1
 * k(TCO2−) = 9.54 M−1s−1
 * k(H)/k(T) = 305
 * }
 * k(TCO2−) = 9.54 M−1s−1
 * k(H)/k(T) = 305
 * }

Distillation
Isotopes of hydrogen, carbon, oxygen, and nitrogen can be enriched by distilling suitable light compounds over long columns. The separation factor is the ratio of vapor pressures of two isotopic molecules. In equilibrium such a separation results at each theoretical plate of the column and is multiplied by the same factor in the next step (at the next plate). Because the elementary separation factor is small, a large number of such plates is needed. This requires total column heights of 20 to 300 m.

The lower vapor pressure of the heavier molecule is due to its higher energy of vaporization, which in turn results from its lower energy of zero-point vibration in the intermolecular potential. As expected from formulas for vapor pressure, the ratio becomes more favorable at lower temperatures (lower pressures). The vapor pressure ratio for H2O to D2O is 1.055 at 50 °C (123 mbar) and 1.026 at 100 °C (1013 mbar). For 12CO to 13CO it is 1.007 near the normal boiling point (81.6 K), and 1.003 for 12CH4 to 13CH4 near 111.7 K (boiling point).

The 13C enrichment by (cryogenic) distillation was developed in the late 1960s by scientists at Los Alamos National Laboratory. It is still the preferred method for13C enrichment. Deuterium enrichment by water distillation is only done, if it was preenriched by a process (chemical exchange) with lower energy demand. Beginning with the low natural abundance (0.015% D) would require evaporation of too large quantities of water.

Separative work unit
Separative work unit (SWU) is a complex unit which is a function of the amount of uranium processed and the degree to which it is enriched, i.e. the extent of increase in the concentration of the U-235 isotope relative to the remainder.

The unit is strictly: kilogram separative work unit, and it measures the quantity of separative work (indicative of energy used in enrichment) when feed and product quantities are expressed in kilograms. The effort expended in separating a mass F of feed of assay xf into a mass P of product assay xp and waste of mass W and assay xw is expressed in terms of the number of separative work units needed, given by the expression SWU = WV(xw) + PV(xp) - FV(xf), where V(x) is the "value function," defined as V(x) = (1 - 2x) ln ((1 - x) /x).

Separative work is expressed in SWUs, kg SW, or kg UTA (from the German Urantrennarbeit )
 * 1 SWU = 1 kg SW = 1 kg UTA
 * 1 kSWU = 1.0 t SW = 1 t UTA
 * 1 MSWU = 1 kt SW = 1 kt UTA

If, for example, for 100 kilograms (220 pounds) of natural uranium, it takes about 60 SWU to produce 10 kilograms (22 pounds) of uranium enriched in U-235 content to 4.5%.

Isotope separators for research
Radioactive beams of specific isotopes are widely used in the fields of experimental physics, biology and materials science. The production and formation of these radioactive atoms into an ionic beam for study is an entire field of research carried out at many laboratories throughout the world. The first isotope separator was developed at the Copenhagen Cyclotron by Bohr and coworkers using the principle of electromagnetic separation. Today, there are many laboratories around the world that supply beams of radioactive ions for use. Arguably the principal Isotope Separator On Line (ISOL) is ISOLDE at CERN, which is a joint European facility spread across the Franco-Swiss border near the city of Geneva. This laboratory uses mainly proton spallation of uranium carbide targets to produce a wide range of radioactive fission fragments that are not found naturally on earth. During spallation (bombardment with high energy protons), a uranium carbide target is heated to several thousand degrees so that radioactive atoms produced in the nuclear reaction are released. Once out of the target, the vapour of radioactive atoms travels to an ionizer cavity. This ionizer cavity is a thin tube made of a refractory metal with a high work function allowing for collisions with the walls to liberate a single electron from a free atom (surface ionization effect). Once ionized, the radioactive species are accelerated by an electrostatic field and injected into an electromagnetic separator. As ions entering the separator are of approximately equal energy, those ions with a smaller mass will be deflected by the magnetic field by a greater amount than those with a heavier mass. This differing radius of curvature allows for isobaric purification to take place. Once purified isobarically, the ion beam is then sent to the individual experiments. In order to increase the purity of the isobaric beam, laser ionization can take place inside the ionizer cavity to selectively ionize a single element chain of interest. At CERN, this device is called the Resonance Ionization Laser Ion Source (RILIS). Currently over 60% of all experiments opt to use the RILIS to increase the purity of radioactive beams.

Beam production capability
As the production of radioactive atoms by the ISOL technique depends on the free atom chemistry of the element to be studied, there are certain beams which cannot be produced by simple proton bombardment of thick actinide targets. Refractory metals such as tungsten and rhenium do not emerge from the target even at high temperatures due to their low vapour pressure. In order to produce these types of beams, a thin target is required. The Ion Guide Isotope Separator On Line (IGISOL) technique was developed in 1981 at the University of Jyväskylä cyclotron laboratory in Finland. In this technique, a thin uranium target is bombarded with protons and nuclear reaction products recoil out of the target in a charged state. The recoils are stopped in a gas cell and then exit through a small hole in the side of the cell where they are accelerated electrostatically and injected into a mass separator. This method of production and extraction takes place on a shorter timescale compared to the standard ISOL technique and isotopes with short half-lives (sub millisecond) can be studied using an IGISOL. An IGISOL has also been combined with a laser ion source at the Leuven Isotope Separator On Line (LISOL) in Belgium. Thin target sources generally provide significantly lower quantities of radioactive ions than thick target sources and this is their main drawback.

As experimental nuclear physics progresses, it is becoming more and more important to study the most exotic of radioactive nuclei. In order to do so, more inventive techniques are required to create nuclei with extreme proton/neutron ratios. An alternative to the ISOL techniques described here is that of fragmentation beams, where the radioactive ions are produced by fragmentation reactions on a fast beam of stable ions impinging on a thin target (usually of beryllium atoms). This technique is used, for example, at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University and at the Radioactive Isotope Beam Factory (RIBF) at RIKEN, in Japan.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Isotope separation


text:
Talk:Turkish language

Aşık Veysel Translation
I'd like to offer the following tweaked translation of the Veysel poem at the end of the article to replace the existing one, which I believe more accurately reflects the poet's choice of words, especially with the use of "dear friends" (dost), rather than "friends" (arkadaş):

I depart, my name remains May dear friends remember me Weddings happen, holidays arrive May dear friends remember me

The soul does not stay caged; it flies away The world is an inn; its visitors depart The moon wanders, the years pass by May dear friends remember me

The soul will depart the body The chimney won't smoke, the hearth won't burn Armfuls of greetings upon you all May dear friends remember me

Myriad flowers bloom and fade Many have laughed, many will laugh Wishes are lies, Death is real May dear friends remember me

Morning, afternoon turn to night Oh, the things that happen to us! Veysel departs, his name remains May dear friends remember me

— Preceding unsigned comment added by 74.67.219.194 (talk • contribs) 5:46, 20 October 2015 (UTC)

Extended-confirmed-protected edit request on 31 July 2022
The graph purportedly showing the origin of the words in Turkish vocabulary should be removed for not complying with WP:OR and for utilizing a flawed methodology.

Also, this sentence from the text should be removed too: "The 2005 edition of Güncel Türkçe Sözlük, the official dictionary of the Turkish language published by Turkish Language Association, contains 104,481 words, of which about 86% are Turkish and 14% are of foreign origin."

The cited source does not say that 86% of words are Turkish.

See my discussion here: https://commons.wikimedia.org/wiki/File_talk:TurkishVocabulary.png#Flawed%20methodology 3512495a (talk) 09:05, 31 July 2022 (UTC)
 * ✅ There does appear to be glaring issues with the pie chart. First, it's a synthesis of two sources: https://web.archive.org/web/20070301064559/http://www.tdk.gov.tr/TR/BelgeGoster.aspx?F6E10F8892433CFFAAF6AA849816B2EF1A46C5FBFA979D0C and https://web.archive.org/web/20070321023726/http://www.tdk.gov.tr/TR/BelgeGoster.aspx?F6E10F8892433CFFAAF6AA849816B2EFB40CE59E171C629F. Neither state or imply that they are related to each other. Second, languages are far more complicated than this. A proper linguistic paper analysis is required to include something similar. The Turkish Language Association is not sufficient for me.

 * I am going to remove this image and related content from articles, pointing to this thread. SWinxy (talk) 04:13, 2 August 2022 (UTC)
 * @SWinxy thanks for doing the hard part for me! I tagged the pie chart last night but I must have fallen asleep and forgot to clean up everything else. And thanks for the tip @3512495a. SamuelRiv (talk) 04:44, 2 August 2022 (UTC)

Which is meant: “ol(mek)”, “-ol(mek)”, or “(-)imek”?
The word-accent section states in a table that both “-ol (as a separate word)” (sic!) and “-ol (as a suffix)” force word stress to fall on the preceding syllable. Yet the examples show the forms “idi” and “-dı” respectively.

First, I propose that separate words not be preceded by “-“.

Second, I wonder whether “(-)ol” and “idi” and “-dı” are from the same verb. I have read on en.wiktionary.org that “imek” is a defective verb, but not that it belongs to “olmek”.

Can someone knowledgeable commenton this?Redav (talk) 12:33, 16 October 2022 (UTC)

Türkiye Türkçesi
I know "Turkey Turkish" or "Turkish of Turkey" isn't frequently used in English. I don't how to to place this name to the lead. Should we say "also referred to as 'Turkey Turkish'/'Turkish of Turkey' (Türkiye Türkçesi) by Turks. That's how Turkish Language Association calls the language, Türkiye Türkçesi. We can not neglect that. Beshogur (talk) 16:27, 2 November 2022 (UTC)
 * I'm mot sure if it's ledeworthy, but definitely deserves a mention here. I can confirm that at least in German scholarship, the term Türkeitürkisch is occasionally used—and not necessarily by people who subscribe to the pan-Turkic myth that the Turkic languages are just dialects of a single Turkish language (e.g. by the former resident Turcologist at my alma mater). –Austronesier (talk) 19:44, 2 November 2022 (UTC)
 * Nonsense. The Turkish language is one of the languages from the Turkic family. It's absurd that calling Turkish as Turkey Turkish, because Turkish is just for Turkey. The term Turkic would be more correct. Moonpulsar (talk) 19:50, 1 March 2023 (UTC)
 * That's incorrect. Turkic is a recent term in English. There are other "Turkish" like Azeri/Azerbaijani Turkish, Khorasani Turkish, etc. Beshogur (talk) 17:12, 2 March 2023 (UTC)
 * The word "Türkçe" in Turkey is used in two senses. The first is "Turkish language", the second is "Turkic language". "Türkiye Türkçesi" actually means Turkey Turkic language. Another example is "Azerbaycan Türkçesi" Azeri Turkic language. 88.243.159.186 (talk) 13:52, 2 March 2023 (UTC)

Why did you undo my citation needed tag? Sure, sources exist, but they need to be cited in the article. Please add a citation. I would do it myself but I don't know enough about Turkish to assess any source. For the meantime, I'm reinstating the tag. — W.andrea (talk) 16:26, 5 April 2023 (UTC)

 * you clearly see it here. There is no reason for citation for an alternative name. Also I opened this "discussion" to further clarify what it is. Türkiye Türkçesi as it is, is the official name by the Turkish Language Association. Did you even read above? Beshogur (talk) 17:04, 5 April 2023 (UTC)
 * There is no reason for citation for an alternative name. The reason I tagged it in the first place is because I wanted to know more about the terminology. For me, as someone who doesn't know much about Turkish, it seems strange to call it "Turkish of Turkey", because what other "Turkish" is there? But after some reading, it seems like some people consider closely related languages like Azeri to be dialects of Turkish. So, I was looking for an authoritative source on the terminology instead of just my reading between the lines. In any case, as the page I linked says:
 * I'd consider this to be like a quotation since it's a particular term.
 * Lastly, please don't be rude; I did read the above. I'm not questioning that the term is used, just asking for a source -- which has now been added by Kansas Bear, great!
 * — W.andrea (talk) 17:36, 5 April 2023 (UTC)
 * For me, as someone who doesn't know much about Turkish, it seems strange to call it "Turkish of Turkey" why dyor? And I explained on talk. I agree that it's strange in English, but had to be included. Beshogur (talk) 17:42, 5 April 2023 (UTC)
 * We should keep the Turkish term and its English equivalent strictly apart when considering due weight. Türkiye Türkçesi is undoubtedly often used in Turkish, and "Turkish of Turkey" is the correct translation of it. But in English, "Turkish of Turkey" is not really often used. Yes, there are attestations, but a) not many (390 in Google Scholar) and b) a good part of these (139 in Google Scholar) have the article "the". As for the latter, "the Turkish of Turkey" can either be understood in the pan-Turkic (no ideological undertones implied!) sense (thus entirely equivalent to Türkiye Türkçesi), but also in contrast to "the Turkish of Albania", "the Turkish of Bulgaria" etc. thus referring to regional varieties that fall into the scope of "Turkish" in the modern English (i.e. narrow) sense.
 * For these reasons, I think that having "Turkish of Turkey" in bold face might be undue for the lede definition. I'd suggest:
 * Turkish (Türkçe, Türk dili; also Türkiye Türkçesi 'Turkish of Turkey')...
 * Thoughts? –Austronesier (talk) 14:46, 6 April 2023 (UTC)
 * Thoughts? –Austronesier (talk) 14:46, 6 April 2023 (UTC)

Why are so many users keep reverting the version every time I add the more detailed map
Like please, stop reverting back to the old map that is shaded by country, the map I added is based on linguistic disturbution, which is more accurate than a language shaded by country 2600:1700:6730:E380:5D9A:83A9:6014:BA26 (talk) 22:26, 26 March 2023 (UTC)

Denmark Is the '''Danish Capital. Tokyo is the Japanese of Japan ''' 2607:FB91:1793:63CF:C56A:910F:2CB7:84B2 (talk) 16:16, 26 July 2023 (UTC)

Is this modern Turkish?
"Where are you going?" is translated as "Nereye gidersin bre?" I lived 15 years in Turkey and I can't remember that I ever heard this "bre". I asked Turkish friends and their opinion was that it may be used by people coming from the Balkan and that it meant "Where are you going, brother?" That means that the word came from an Indo-European language. That may be or not. In your article it is connected with "be" and "bire". "be" may be used like a exclamation mark at least in modern Turkish (not often used) and bire looks like "for one". The word is not mentioned in Osmanlica Türkce by Mustafa Nihat Özön, 5th edition, Istanbul 1995. The normal word for brother in Turkish is kardes and in Ottoman Turkish "birader" was sometimes used, probably a loan from Persian. But at least "nereye gidersin bre?" is not normal in modern Turkish. Keetjan (talk) 21:43, 24 October 2023 (UTC)

Classification section
In the classification section, there is a short video clip whose caption reads: "A Turkish speaker from Kosovo." We urgently need the video's premise, e.g. whether it's a news broadcast or something else, and a synopsis of what is being said. That clip cannot remain up the way it is now. It could be proclaiming anything. -The Gnome (talk) 19:32, 9 January 2024 (UTC)
 * Removed. In case anyone finds its inclusion necessary, they would have to provide full & exact translation. Wikipedia does not abide random information. -The Gnome (talk) 20:32, 14 January 2024 (UTC)

Attributive verbs table - help
In the table where it gives the "case of relative pronoun", the pronoun itself, and example sentences in Turkish, with English literal and actual translations, I am confused about whether it is meant that some fields are empty. Is it meant for the section under "case of relative pronoun" is empty for most of the rows? F.ex. there are a number of empty boxes under the "genitive" box. If these are meant to be attributed to the genitive case, then these boxes should be combined so that the genitive case is logically applied to the following rows as well. If not, some explanation should be given. I'm not well read at all vis-a-vis Turkish nor its grammar, so this table is a tad confusing. Thanks in advance! St.Marie-made (talk) 08:32, 3 May 2024 (UTC)

Proposed merger
I propose merging Longest word in Turkish into Turkish language. I think that the content in the former most naturally fits within the context of the latter, and it seems irregular to me that an article exists solely for what amounts to a hypothetical (even if actually printed, though apparently only for the purpose of proving it can be done) extension of the subject matter of what I consider the master article. Specifically, I would place the "longest word" section within the "Vocabulary" heading, and condense any duplicative or extraneous material. Comments? Al Begamut (talk) 21:37, 30 May 2024 (UTC)

title: Talk:Turkish language


text:
Talk:Anorexia nervosa

Anorexia Nervosa and gender
Data in this section are not in line with current research on topic Srobodao84 (talk) 00:18, 11 August 2021 (UTC)

 * I have deleted it. From the two citations, one was of inadequate quality, and the other study indicates overeating is the only ED symptom more prevalent in men - not relevant to this article. I was not able to find any supporting citations.
 * I did move the second reference since it is relevant to indicate AN prevalence in support of the epidemiology opening claim.Darcyisverycute (talk) 09:15, 24 April 2022 (UTC)

Student Editors for New Atypical Anorexia Page
Three students (including @kgt11 & @VelenWu) will be building out the redlinked article "Atypical Anorexia" as part of a Georgetown WikiEdu course over the next 3-4 weeks.

 * What is the status of that project? I am interested in contributing to it. Darcyisverycute (talk) 09:14, 24 April 2022 (UTC)

Wiki Education assignment: Psychology Capstone
— Assignment last updated by Savannaj13 (talk) 03:10, 19 June 2022 (UTC)

Wiki Education assignment: Psychology Capstone
— Assignment last updated by Snqadri (talk) 01:59, 3 October 2022 (UTC)

Abbreviation disease
May I suggest that it is not good writing to use abbreviations like AN in an article for a general audience like Wikipedia? This bad habit is found in scientific journal articles, and is best thought of as jargon (and undesirable) in even that writing genre. When publishing in a biology journal, it makes sense to shorten deoxyribonucleic acid to dna - every reader knows the polysyllabic word, and dna has a long history of use. Here, the average Wiki reader thinks in terms of 'anorexia,' and has never seen 'AN' before. 'AN' makes the reader stop and think 'what the heck is that?' before clicking that it's an abbreviation. And AN doesn't even have the saving grace of dna once had that it saved characters back when authors were charged by the character to publish their articles. To sum up, if you mean anorexia - or anorexia nervosa - you should say so. There is no possible benefit to the writer or reader to abbreviate (unless it's the pleasure of the writer to show that he/she knows the insider jargon). Plain, explicit language is always best. MarkinBoston (talk) 22:14, 21 May 2023 (UTC)
 * Wikipedia's Manual of Style does not forbid such abbreviations as long as the abbreviation is defined earlier in the article. That's quite common in most styles. In this case, I added it to the first sentence. That's sufficient. That being said, there's nothing stopping you from spelling it out when you see it, as long as it doesn't create clunky sentences. Sundayclose (talk) 23:15, 21 May 2023 (UTC)

Definition
Not all medical definitions agree on whether or not the trouble can be present at any weight.

The Mayo Clinic gives this definition:

Anorexia (an-o-REK-see-uh) nervosa — often simply called anorexia — is an eating disorder characterized by an abnormally low body weight, an intense fear of gaining weight and a distorted perception of weight.

Medline Plus:

Anorexia is an eating disorder that causes people to weigh less than is considered healthy for their age and height, usually by excessive weight loss.

So let's not assert on the Wiki page that the definition doesn't necessarily relate to actual weight. This is just objectively false. Some definitions do, not all of them, there is not a consensus. Jeanvaljean001 (talk) 20:11, 28 February 2024 (UTC)

title: Talk:Anorexia nervosa