Arthur Massot

A genealogy of Freud's death drive hypothesis

Keywords: Freud, metapsychology, energetics, scarcity, history of psychoanalysis

In his 1920 essay Beyond the Pleasure Principle, Freud (1920/1955, p. 38) introduced the death drive hypothesis, according to which "the aim of all life is death". I shall not discuss the truth value of this hypothesis here; instead, I trace its genealogy in order to understand it as a moment in the history of modern Western societies. First, I present Freud's metapsychology, and in particular its "economic" dimension, the death drive being central to this dimension. Secondly, I retrace the history of the concept of energy and of the formulation of the laws of thermodynamics in the nineteenth century. Energetics and thermodynamics are shown to have been important to the Freudian economic dimension. Further, I show that for nineteenth-century scientists, the concern for energy reflected a socio-economic preoccupation with the matter of scarcity. Lastly, I argue that Freud's relationship to energy, as expressed in the death drive hypothesis, also reflected a certain relationship of Western countries to scarcity in the era of the second industrial revolution.

central part of this dimension. 2) Then, I retrace the energetic conceptions developed in the nineteenth-century natural sciences, in direct relation to thermodynamics. The importance of those theories in Freud's conceptions of the death drive and the economic dimension has been amply documented [START_REF] Assoun | Introduction à l'épistémologie freudienne [An Introduction to Freudian Epistemology[END_REF][START_REF] Tran The | The Epistemological Foundations of Freud's Energetics Model[END_REF][START_REF] Tran The | From the Principle of Inertia to the Death Drive: The Influence of the Second Law of Thermodynamics on the Freudian Theory of the Psychical Apparatus[END_REF]. I highlight the stakes related to the industrial revolution and the economic matter of scarcity that determined the formulation of the thermodynamic principles, and in particular that of entropy. The significance of this industrial context for the genesis of the concept of energy has also already been underlined [START_REF] Blay | L'ordre économico-cosmique énergétiste [The Energetic Economic-Cosmic Order[END_REF][START_REF] Kuhn | Energy Conservation as an Example of Simultaneous Discovery[END_REF][START_REF] Kuhn | Energy Conservation as an Example of Simultaneous Discovery[END_REF][START_REF] Rabinbach | The Human Motor: Energy, Fatigue and the Origins of Modernity[END_REF]. 3) If the links between metapsychology and thermodynamics on one hand, and between thermodynamics and the industrial context on the other hand, are both well-documented, less has been written on the indirect relationship between Freud's metapsychology and the industrial ideology, although [START_REF] Raitt | Psychic waste: Freud, Fechner and the principle of constancy[END_REF] provided valuable insights on this issue. In a third step, I therefore explore the consequences of Freud's borrowing of the concept of energy. While there are parallels between the two laws of thermodynamics and the two principles of Freudian economics, the death drive hypothesis reflects a relationship to energy that is different from that found in the work of mid-nineteenth century scientists. I argue that this difference is rooted in the mutation of industrial societies between the midnineteenth century (when the two principles of thermodynamics were formulated) and 1920 (when Beyond the Pleasure Principle was published).

The economic dimension of metapsychology

The Freudian metapsychology Energetic theories were introduced into psychoanalysis via Freud's metapsychology, and especially its "economic dimension". Metapsychology can be understood as an attempt to formalize the theoretical axioms of psychoanalysis [START_REF] Assoun | Introduction à l'épistémologie freudienne [An Introduction to Freudian Epistemology[END_REF](Assoun, , p. 73, 2013, p. 13, p. 24), p. 13, p. 24).

For [START_REF] Freud | Papers on metapsychology. The Unconscious[END_REF]Freud ( /1957, p. 181), p. 181), this approach consists in appreciating psychical processes according to three dimensions: the "topographical", the "dynamic" and the "economic".

1) The topographical dimension understands the mental apparatus as being composed of different interconnected "locations", each with its own mode of functioning [START_REF] Freud | Psycho-analysis[END_REF](Freud, /1959, p. 266), p. 266). On this point, Freud's formulation owes much to neuroanatomy, and more precisely to the theories of specialized brain areas developed in the second half of the nineteenth century (Assoun, 1981, pp. 124-130).

2) The dynamic dimension perceives psychic processes as the interplay of mutually opposed or reinforcing forces [START_REF] Freud | Psycho-analysis[END_REF](Freud, /1959, p. 265), p. 265). This formulation borrows from the vocabulary of Newton's classical mechanics (Tran The et al., 2020, pp. 3-4).

3) The economic dimension understands psychic processes to be invested with variable quantities of energy that flow within the mental apparatus [START_REF] Freud | Psycho-analysis[END_REF](Freud, /1959, pp. 265-266), pp. 265-266). This latter dimension was particularly instrumental to the importation of energetic theories into psychoanalysis.

The second section of this paper will retrace the genealogy of these theories. For now, let us focus on what Freud says about the economic dimension of his metapsychology in particular.

The pleasure principle and the principle of constancy

The influence of the energetic doctrine on Freud can be first spotted in the Project for a Scientific Psychology, a manuscript he wrote in 1895 but never published during his lifetime.

The explicit project of this essay was "to furnish a psychology that shall be a natural science: that is, to represent psychical processes as quantitatively determinate states" [START_REF] Freud | Project for a Scientific Psychology[END_REF](Freud, /1966, p. 295), p. 295). This idea of quantifiable psychical processes is the first presentation of what [START_REF] Freud | Papers on metapsychology. The Unconscious[END_REF]Freud ( /1957, p. 181) , p. 181) would later call the "economic" dimension of his metapsychology. The concept of quantity stems, he writes in the Project, from "pathological clinical observation", and can be understood through the "principle of neuronal inertia" according to which "neurons tend to divest themselves of Q [quantity of energy]" [START_REF] Freud | Project for a Scientific Psychology[END_REF](Freud, /1966, p. 295-296), p. 295-296). This tendency allows for avoiding "unpleasure", which "would have to be regarded as coinciding with a raising of the level of Qή [endogenous quantity of energy] or an increasing quantitative pressure", as opposed to pleasure, which "would be the sensation of discharge" [START_REF] Freud | Project for a Scientific Psychology[END_REF](Freud, /1966, p. 312), p. 312).

This "pleasure principle" is central to the economic dimension. Freud did not actually invent this concept, but borrowed it from Fechner, nowadays known as the founder of psychophysics (Assoun, 1981, pp. 150-153;Laplanche & Pontalis, 1988, p. 342;[START_REF] Sulloway | Freud, Biologist of the Mind: Beyond the Psychoanalytic Legend[END_REF]Sulloway, /1992, pp. 66-67), pp. 66-67). In 1848, Fechner described a supposedly universal Lustprinzip, according to which the quantity of energy in the mental apparatus remained overall constant. However, Fechner himself did not create this concept out of thin air: he extrapolated it from the principle of energy conservation formulated by Helmholtz in the 1840s (Raitt, 2002, p. 64;Wegener, 2005, pp. 272-273). This principle, also known as the first law of thermodynamics, states that the overall quantity of energy contained in the universe stays overall constant. What Fechner did is to apply this principle to the mind [START_REF] Raitt | Psychic waste: Freud, Fechner and the principle of constancy[END_REF]:

Building on Hermann von Helmholtz's 1847 formulation of the first law of thermodynamics, […] Fechner argued that the overall level of energy in the mind also remained constant: "As far as its course is bound to the course of psychophysical processes and these in turn are bound by the law of the conservation of energy, the mind will itself be bound by that law. " (p. 64) We can therefore draw a genealogical line from Helmholtz's principle of energy conservation to Freud's pleasure principle via Fechner. I will later come back to the significance of this genealogy; let us return for the moment to the Freudian principle of pleasure.

Twenty-five years after the unpublished Project, Freud came back to this principle in Beyond the Pleasure Principle. He defines it as follows [START_REF] Freud | Beyond the Pleasure Principle[END_REF](Freud, /1955):

[…] the course taken by mental events is automatically regulated by the pleasure principle. […] the course of those events is invariably set in motion by an unpleasurable tension, and […] its final outcome coincides with a lowering of that tensionthat is, with an avoidance of unpleasurable or a production of pleasure. (p. 7)

The pleasure principle is still defined as a tendency of the mental apparatus to discharge its psychic energy. On other occasions, [START_REF] Freud | Beyond the Pleasure Principle[END_REF]Freud ( /1955) ) instead uses the phrase "principle of constancy", but indicates that he holds the two concepts as equivalent: "the mental apparatus endeavours to keep the quantity of excitation present in it as low as possible or at least to keep it constant. This latter hypothesis is only another way of stating the pleasure principle" (p. 9). This explication is actually far from obvious: the idea of "constancy" (or "stability") rather suggests that of an average homeostatic equilibrium, while the pleasure principle is defined by Freud as a continuous (ideally total) tendency to spend energy (Assoun, 1981, p. 153;David-Ménard, 2017, pp. 247-249;Laplanche & Pontalis, 1988, pp. 342-344;[START_REF] Tran The | From the Principle of Inertia to the Death Drive: The Influence of the Second Law of Thermodynamics on the Freudian Theory of the Psychical Apparatus[END_REF]2). To resolve this tension, one must allow a gap between 1) the "original tendency to total discharge" and 2) "constancy" as "a state stabilized at zero plus" that would be opposed, in a second step, to the first tendency (Assoun, 1981, p. 153). This is the solution adopted by [START_REF] Freud | Beyond the Pleasure Principle[END_REF]Freud ( /1955) ) when he writes that "there exists in the mind a strong tendency towards the pleasure principle, but [...] that tendency is opposed by certain other forces or circumstances, so that the final outcome cannot always be in harmony with the tendency towards pleasure" (pp. 9-10). In the first place, it is the "reality principle" which, "under the influence of the ego's instincts of self-preservation" [START_REF] Freud | Beyond the Pleasure Principle[END_REF](Freud, /1955, p. 10), p. 10), tends to divert pleasure from its tendency to direct and total discharge. 1 Therefore, if pleasure and stability are first supposed to be identical (and de facto non-contradictory), there is an actual difference between a "theoretical" (or "ideal") principle and an "applied" (or "concrete") one (the various terms used by Freud and their respective functions are summarized in Table 1). To recap [START_REF] Laplanche | The Language of Psychoanalysis[END_REF]:

Considerations of conceptual clarity […] urge that a distinction be preserved between a tendency to reduce the quantity of excitation to zero on the one hand, and a tendency to keep this quantity at a constant level on the other; to meet this demand, the pleasure principle must be seen as correlative with the former trend, and the maintenance of constancy treated as a corollary of the action of the reality principle. (p. 345)

As shown above, Freud gives primacy to the pleasure principle (i.e., the tendency to discharge) over the principle of constancy (and consequently over the reality principle too). To support this idea, he commits to a "speculation, often far-fetched speculation, which the reader will consider or dismiss according to his individual predilection" [START_REF] Freud | Beyond the Pleasure Principle[END_REF](Freud, /1955, p.24, p.24). This speculation is in equal parts biological and philosophical reflection on the nonorganic origin of living organisms. [START_REF] Freud | Beyond the Pleasure Principle[END_REF]Freud ( /1955) ) assumes that one can deduce, from this origin, "a universal attribute of instincts and perhaps of organic life": "an instinct is an urge inherent in organic life to restore an earlier state of things which the living entity has been obliged to abandon under the pressure of external disturbing forces" (p. 36). From there, only one step was left to formulate the concept of death drive, according to which "the aim of all life is death" [START_REF] Freud | Beyond the Pleasure Principle[END_REF](Freud, /1955, p. 38), p. 38). To summarize, there is, according to Freud, a tension between a tendency of the organism to totally discharge its energy and a tendency to keep it constant. He gives primacy to the tendency to discharge (pleasure principle), and therefore to death over life. Freud's energetic conceptions presented above raise two questions. Firstly, how did the concept of energy came to such a pessimistic conclusion, that is, that life is an accident while death is an essence?

The economic-energetic ideology

The "physicalist oath"

Between 1830 and 1847, often independently, more than a dozen scholars 2 developed a series of theories that led to the formulation of the principle of energy conservation (or first law of thermodynamics) at the end of this period [START_REF] Kuhn | Energy Conservation as an Example of Simultaneous Discovery[END_REF][START_REF] Kuhn | Energy Conservation as an Example of Simultaneous Discovery[END_REF]. Correlative to the principle of conservation, the idea that all "forces" (whether physical, electrical, magnetic, chemical, physiological, etc.), being of the same nature, are interconvertible, became established. 3 The principle of conservation of energy was thus at the center of a new physicalist current from the 1840s onwards (Assoun, 1981, pp. 59-65). Physicalist physiology was represented by a generation of scientists that included Virchow, Du Bois-Reymond, Helmholtz, Brücke and Ludwig; the latter four founded the Physikalische Gesellschaft zu Berlin in 1845. In 1842, Du Bois-Reymond pronounced the now famous "physicalist oath", which was to clarify the epistemological foundations of this Society: "Brücke and I pledged a solemn oath to put into effect this truth: "No other forces than the common physical-chemical ones are active within the organism" (cited by Jones, 1953Jones, -57/1964, pp. 62-63), pp. 62-63).

After Mayer, Helmholtz explicitly sought to bridge the gap between physiology and physics, precisely by putting forth the idea that the "forces" described by these two disciplines are substantially identical (Assoun, 1981, p. 158;Tran The et al., 2018, pp. 3-5). While discussing the two conceptions of Mayer and Helmholtz, Ostwald asserted the existence of psychical phenomena that can be conceived like every other phenomenonenergetic phenomena (Assoun, 1981, p. 172;Tran The et al., 2018, p. 8). Therefore, physicalism first brought physics closer to chemistry [START_REF] Duhem | Le potentiel thermodynamique et ses applications à la mécanique chimique et à l'étude des phénomènes électriques [The Thermodynamic Potential and its Applications to Chemical Mechanics and the Study of Electrical Phenomena[END_REF] applied the laws of thermodynamics to chemical phenomena), then to biology, physiology and medicine (as evidenced by [START_REF] Fechner | Elements of Psychophysics[END_REF][START_REF] Fechner | Elements of Psychophysics[END_REF],

Helmholtz [1847/2003] or [START_REF] Lotze | Allgemeine Pathologie und Therapie als mechanische Naturwissenschaften[END_REF]), and eventually to psychology and psychoanalysis at the end of the century (with [START_REF] Wundt | Principles of Physiological Psychology[END_REF]Wundt [ /1919[START_REF] Wundt | Outline of psychology[END_REF] in Germany, [START_REF] Ribot | The Diseases of the Will[END_REF][START_REF] Ribot | The Diseases of the Will[END_REF] in France and Freud in Austria). Freud's incorporation of the principle of energy conservation within his metapsychology must be understood in regard to this epistemological context of physicalism.

During his medical studies, having spent several years in Brücke's laboratory, Freud was constantly exposed to energetic conceptions such as Helmholtz's or Fechner's (Assoun, 1981, pp. 145-187;Rabinbach, 1992, p. 170;[START_REF] Tran The | The Epistemological Foundations of Freud's Energetics Model[END_REF], p. 5, 2020, pp. 2-3).

The energetic ontology

At that time, many scientists sought to close the scientific field by marking their distance from philosophy. In the German-speaking countries, Du Bois-Reymond, Helmholtz, Liebig and Mayer thus explicitly rejected Naturphilosophie by faulting its mystical, speculative and nonquantitative dimensions (Prigogine & Stengers, 1979, pp. 101-103, p. 124). It is no less clear that these scholars conveyed a cosmological vision whose origins can be traced back to nineteenth century Romanticism and Naturphilosophie. The notion of energy conservation appears indeed as a metaphysical principle of harmonic unification of Nature as a Whole (Blay, 2017, pp. 266-267;[START_REF] Ellenberger | The Discovery of the Unconscious. The History and Evolution of Dynamic Psychiatry[END_REF]Ellenberger, /1979, p. 540;, p. 540;[START_REF] Kuhn | Energy Conservation as an Example of Simultaneous Discovery[END_REF]Kuhn, /1977, pp. 94-100;, pp. 94-100;Prigogine & Stengers, 1984, p. 109;Rabinbach, 1992, pp. 54-56). Helmholtz (1856) for instance described his principle of energy conservation (here still named "force" 4 ) as follows:

[…] Nature as a whole possesses a store of force which cannot in any way be either increased or However, as shown by Rabinbach (1992, pp. 45-68) or [START_REF] Blay | L'ordre économico-cosmique énergétiste [The Energetic Economic-Cosmic Order[END_REF], this principle of energy conservation is not only ontology-laden, it is also ideology-laden. We are now going to see how economic concerns permeated the energetic conception of nineteenth century scholars, in the context of the industrial revolution in Western countries.

The industrial revolution and the division of labor

The eighteenth century witnessed the correlative development of science as technique (with, for example, the creation of the first engineering schools in France) and the empowerment of the economic and productivity spheres (with an accumulation of capital due to the considerable increase in trade). By the end of that century, the economic theories of the classical liberal school were born. In England, the first industrial revolution played a major role in the birth of this school, marked by the publication of Adam Smith's The Wealth of Nations in 1776. [START_REF] Smith | An Inquiry into the Nature and Causes of the Wealth of Nations[END_REF][START_REF] Smith | An Inquiry into the Nature and Causes of the Wealth of Nations[END_REF] defended the thesis that "The greatest improvement in the productive powers of labour […] seem to have been the effects of the division of labour" (p. 13). After an initial accumulation of capital in the late seventeenth century, the second half of the eighteenth century saw the development of industrial machinery and the generalization of the division of labor.

Although Smith was not the first economist to theorize the notion of labor, he nevertheless gave it a new role: it was no longer the value of the marketable object itself that determined its price, but the labor necessary to produce it (Blay, 2017, pp. 233-234;[START_REF] Foucault | The Order of Things. An archaeology of the human sciences[END_REF]Foucault, /2002, pp. 241-242), pp. 241-242). The worker was no longer paid directly because he produced an object, but because he "spent" his "energy" in the process of providing work (Blay, 2017, p. 235) With the rationalization of production methods, the development of industrial machines went hand in hand with a reification of the worker himself. As a result, in the nineteenth century, a vision of Nature conceived as a machine and the new figure of the human machine flourished 5 (Blay, 2017, pp. 235-260;Rabinbach, 1992, pp. 45-68).

Nature and humans as machines

The formulation of the principle of energy conservation was the direct result of the development of industrial machines (in particular the steam engine) from the very early nineteenth century [START_REF] Kuhn | Energy Conservation as an Example of Simultaneous Discovery[END_REF](Kuhn, /1977, pp. 83-94;, pp. 83-94;[START_REF] Mirowski | More Heat than Light[END_REF]Mirowski, /1995, p. 24;, p. 24;Rabinbach, 1992, pp. 45-46). The engineering arts played an important role in the diffusion of the ideological vision of Nature and humans as machines in the scientific field 6 (Blay, 2017, pp. 215-225;[START_REF] Kuhn | Energy Conservation as an Example of Simultaneous Discovery[END_REF]Kuhn, /1977, pp. 90-94), pp. 90-94).

In 1824, Carnot compared nature to an enormous machine with a huge reservoir of driving forces (Blay, 2017, pp. 236-237). A few years later, Coriolis (1829, pp. 2-3, p. 17) proposed for the first time to replace the term "force" by "work" in dynamics, so that the former would be used only to refer to static phenomena. He then committed to "an approach [...] that enabled him to transpose the notion of work from the economic domain, or even from the common language, into the field of physics" 7 (Blay, 2017, p. 242). Coriolis's writings (e.g., 1829, p. 27) were explicitly geared towards the industrial and economic applications of his concept of work.

They likened not only animals and human beings, but also the whole Nature, to machines and sources of driving forces (Blay, 2017, pp. 245-260;[START_REF] Kuhn | Energy Conservation as an Example of Simultaneous Discovery[END_REF]Kuhn, /1977, p. 91), p. 91). This theme of Nature as a gigantic machine and a reservoir of resources also comes up in the work of scholars such as Helmholtz, Du Bois-Reymond, Haeckel and Ostwald (Rabinbach, 1992, p. 49).

Helmholtz (1856), for example, stated: "We cannot create mechanical force, but we may help ourselves from the general storehouse of Nature" 8 (p. 501). Thus, from the point of view of these scientists, to consider a human as a machine was to recognize that he can produce work as long as he is given some energy, which comes from the reservoir of Nature.

The entropy principle

In 1852, Thomson formulated, after Carnot, the principle of entropy, 9 the second "law" of thermodynamics. The conservation principle "was based on the model of an idealized machine, whose utopian output would not be subject to any loss" (Tran The et al., 2020, p.5). On the contrary, Thomson started from a context that was no longer ideal, taking concrete mechanical energy losses into account. By emphasizing the irreversibility of these energy losses, this principle dismissed the possibility of perpetual motion (Merleau-Ponty, 1983, p. 9).

Accordingly, perpetual motion was only defined negatively, as the radical impossibility for an engine to indefinitely provide "work" without an external source capable of supplying it with energy (Blay, 2017, pp. 263-265;Merleau-Ponty, 1983, pp. 8-9).

One might wonder how the two laws of thermodynamics could be compatible, that is, how can energy be "lost" if everything in nature is conserved (Guedj, 2010, p. 116;[START_REF] Mirowski | More Heat than Light[END_REF]Mirowski, /1995, p. 60;, p. 60;Rabinbach, 1992, pp. 47-49, p. 63)? [START_REF] Thomson | XLVII. On a universal tendency in nature to the dissipation of mechanical energy[END_REF] answers this:

[…] there is an absolute waste of mechanical energy available to man when heat is allowed to pass from one body to another at a lower temperature, by any means not fulfilling his criterion of a "perfect thermo-dynamic engine," established, on a new foundation, in the dynamical theory of heat.

As it is most certain that Creative Power alone can either call into existence or annihilate mechanical energy, the "waste" referred to cannot be annihilation, but must be some transformation of energy.

(p. 304)

Here we are led to understand that energy is "wasted" to humans even though it is preserved in Naturewhich makes it apparently possible for the two principles of thermodynamics to remain compatible (Guedj, 2010, pp. 116-123;Harman, 1982, p. 52;Merleau-Ponty, 1983, p. 14;Raitt, 2002, p. 67). The research on a perpetual motion machine (which kept many a scientist busy in the eighteenth and early nineteenth centuries) actually reflected a concern for the importance of the resources available to humans, as well as the fantasy of a world with inexhaustible resources (Blay, 2017, pp. 262-264;Rabinbach, 1992, p. 58;Raitt, 2002, p. 64, p. 67). As we are about to see, the tension that exists between the two laws of thermodynamics (conservation and entropy) can be understood in light of the industrial-capitalist ideology of the nineteenth century.

The nineteenth century matter of scarcity

After the industrial revolution, and with the naturalization of the capitalist system of production, the idea of scarcity appeared to be more problematic than it was at the beginning of the nineteenth century [START_REF] Foucault | The Order of Things. An archaeology of the human sciences[END_REF](Foucault, /2002):

In Classical thought, scarcity comes about because men represent to themselves objects that they do not have; but there is wealth because the land produces, in some abundance, objects that are not immediately consumed […]. Ricardo inverts the terms of this analysis: the apparent generosity of the land is due, in fact, to its growing avarice; what is primary is not need and the representation of need in men's minds, it is merely a fundamental insufficiency. (p. 279)

The naturalization of the problem of scarcity was justified by classical liberals according to a scenario that combined the classical theme of natural right with the newer political-economic problem of population 10 to make work a necessity for humans: contrary to the "state of nature", and because of the growing demography, they must work to ensure their subsistence. Malthus inherited from Linnaeus the theme of the regulation of plant and animal populations through the "balance of nature" and applied it to human society, insisting on the natural character of the phenomenon of population growth [START_REF] Canguilhem | Idéologie et rationalité dans l'histoire des sciences de la vie [Ideology and Rationality in the History of the Life Sciences[END_REF](Canguilhem, /2009, pp. 117-118), pp. 117-118). Ricardo pointed out that the more the human population grows, the more humans must work: this growth forces them to farm less and less fertile land, as work becomes more and more difficult and less and less profitable.

Those economic considerations have played an important role in the problematization of the concept of entropy by scientists. In the vision of the machinist ideology, the worker is a machine that produces goods, but it is an imperfect machine, which constantly requires resources (wages) to "function" and is exhausted by its task (Rabinbach, 1992, p. 68). Nature's reservoir of raw materials poses, on another scale, a comparable problem: 11 its resources, which are necessary for production, are seen as subject to an increasing scarcity (Prigogine & Stengers, 1984, pp. 114-116). Having renounced the possibility of perpetual motion, the new objective of nineteenth-century scientists was to find technical solutions to the problem of the scarcity of the resources required for humans and machines to work 12 . Thus, the thermodynamic cycle modelled by Carnot can be conceived as "a set of tricks to avoid irreversible conduction.

Thermodynamics was thus built up about irreversibility but also against it, seeking not to know it but to save it" 13 (Prigogine & Stengers, 1979, p. 270). The tension between the two laws of thermodynamics and the nearly paradoxical description of a world in which energy is never destroyed and yet dissipates reflect the concerns of "pessimistic" economists, who feared an incompatibility between a virtually infinite desire for consumption and the finiteness of the Earth's exploitable resources 14 . Returning now to Freud, we will now see that this quasiparadox is also to be found in his economic theory.

The second industrial revolution and the death drive

From thermodynamics to metapsychology

We have seen previously that Freudian metapsychology exhibits a tension between the pleasure principle (i.e., the tendency towards discharge) and the reality principle (or constancy, i.e., the tendency towards stability). This tension in metapsychology is reminiscent of the tension in thermodynamics between the principle of conservation (as an ideal of inexhaustible cosmic energy) and that of entropy (as the concrete impossibility of perpetual motion) (Rabinbach, 1992, pp. 63-64;Tran The et al., 2020, p.5). In Thomson's case, the consideration of energy losses implied the "passage" from the first to the second law of thermodynamics (Tran The et al., 2020, p.5). In the same way, the acknowledgement of a discharge involved, for Freud, the "passage" from constancy to pleasure [START_REF] Raitt | Psychic waste: Freud, Fechner and the principle of constancy[END_REF]:

[…] as far as the psyche was concerned, the first law of thermodynamics as it was expressed in the principle of constancy implied the apocalyptic pessimism of the second. If, in order to keep the level of psychic excitation constant, the mind was discharging energy, like the universe it would surely eventually run out of steam. (p. 67) This analogy should not be surprising when considering that Freud's economics is rooted in energetic ontology and its figure of the human machine (Lacan, 1954(Lacan, -55/1991, pp. 60-62), pp. 60-62).

Machines, according to the engineers of his day, functioned thanks to the energy they tend to disperse (entropy); if we replace machines by humans, their death is conceivable as the outcome of an entropic process (total loss of energy) (Tran The et al., 2020, p.6).

However, there is a slight yet significant difference between thermodynamic theories and Freudian metapsychology. In the former, the principle of conservation is akin to an "ideal" to strive for, by opposing the tendency of mechanical systems to disperse their energy. Thus, having given up the possibility of a perpetuum mobile, nineteenth century engineers designed technical devices that optimize efficiency by limiting energy losses. In contrast, in Freud's

Beyond the Pleasure Principle, death (i.e., the result of "organism entropy") is conceived as a purpose, whereas the tendency to maintain tension at a nonzero level (i.e., alive) is perceived as an accident. In other words, Freud's model implied that expending energy is an aim, an idea that contrasts with the fear of scarcity that haunted the nineteenth century (see Table 2

below).

While previous developments on the genesis of the energetic ideology and the human machine metaphor have helped us to understand how Freudian economic principles (realityconstancy and pleasure-discharge) can be understood as transpositions of the two thermodynamic principles (conservation and entropy) in psychoanalysis, we must now try to figure out why, despite this borrowing, Freudian and thermodynamics theories differ. I will argue that the divergence between the "ideals" of thermodynamic engineering and of Freudian metapsychology is to be found in the evolution of Western societies between the mid-nineteenth century (when the law of entropy was formulated) and 1920 (when Beyond the Pleasure Principle was published).

Principles of thermodynamics

Principles of metapsychology "Purpose" ←

Conservation (1 st law) Reality (constancy) → "Actuality" "Actuality" ← Entropy (2 nd law) Pleasure (discharge) → "Purpose"

Table 2: Overview of the differences between the principles of thermodynamics and metapsychology

Scarcity after the second industrial revolution

In the late nineteenth century, European countries experienced a series of economic and political crises. The spread of discourses (such as Nietzsche's) on the "end of history" at the time was a manifestation of the fear of waste as well as of the social mutations that characterized the period [START_REF] Foucault | The Order of Things. An archaeology of the human sciences[END_REF](Foucault, /2002, pp. 282-286;, pp. 282-286;Prigogine & Stengers, 1984, p. 116). Likewise, the Great War, both an expression and an outcome of the disintegration of the old empires, would be perceived by many as the ultimate symptom of the decline of the West [START_REF] Ellenberger | The Discovery of the Unconscious. The History and Evolution of Dynamic Psychiatry[END_REF](Ellenberger, /1979, p. 548), p. 548). At the dawn of the twentieth century, Vienna was the setting for a cultural and political crisis of liberalism, a crisis that constituted the premise of a second modernity (Schorske, 1980, pp. XXVI-XXVII, pp. 184-185). After the war, the fall of the Austro-Hungarian Empire created a situation of social anomie, which was anxiety-inducing for the dominant classes and influenced [START_REF] Freud | Group Psychology and the Analysis of the Ego[END_REF]Freud ( /1955) ) in the writing of Group Psychology and the Analysis of the Ego [START_REF] Ellenberger | The Discovery of the Unconscious. The History and Evolution of Dynamic Psychiatry[END_REF](Ellenberger, /1979, p. 528), p. 528). The influence of conflict and of the post-war period on Freud's elaboration of the concept of death drive, particularly on the pessimism of this concept, has already been widely noted (e.g., [START_REF] Koteska | Freud on the First World War (Part 1)[END_REF]. About this concept and its context of formulation, [START_REF] Clair | Freud, l'idole et la parole [Freud, the Idol and the Word[END_REF] wrote: "What stronger image could be proposed of the Viennese sensibility and the rift it went through, a modernity both desired and rejected -"Secession" but not Revolution?" 15 (p. 23).

Indeed, the post-war period also marked the advent of the second modernity on the old continent. At that time, mass consumer society, a by-product of the second industrial revolution that had begun in the late nineteenth century, experienced a boom. This industrial boom and the massification of production sparked a wave of "economic optimism" in Western countries (Zaretsky, 2004, p. 134). In the United States and then in Europe, Fordism increased the control of workers (via a rationalized production chain), but also enabled them (thanks to higher wages) to achieve consumer "status" (Rabinbach, 1992, p. 282;Zaretsky, 2004, pp. 138-139). At the turn of the twentieth century, electric and combustion engines, which were much more energyefficient, replaced steam engines. With the expansion of domestic electrical networks, the democratization of the automobile, the development of the chemical industry and the rationalization of the work chain, scarcity now seemed to be much less of a looming prospect.

In a sense, the new methods for producing and harnessing energy appeared to have actually made the dream of nineteenth century engineers, that of a society with inexhaustible resources, come true 16 (Zaretsky, 2004, p. 140). As noted by [START_REF] Zaretsky | Secrets of the Soul. A Social and Cultural History of Psychoanalysis[END_REF], "Fordism thus gave rise to a paradox: beginning as an attempt to regiment both work and family, it generated the utopian idea that human life need no longer be subordinated to the imperatives of production" (p. 140).

Ultimately, however, while mass production addressed (through technical innovation) the problem of access to and exploitation of raw materials, it could not conclusively solve the problem of their progressive exhaustion. The utopia of second modernity precisely consisted in highlighting the desire to consume and an apparent abundance to conceal the insoluble problem of the finiteness of the Earth's resources. 17 Although latent, the death drive hypothesis can be understood as the result of this tension between the desire to consume and the fear of scarcity.

Death drive and the contradiction of the second modernity [START_REF] Jameson | The Political Unconscious. Narrative as a socially symbolic act[END_REF][START_REF] Jameson | The Political Unconscious. Narrative as a socially symbolic act[END_REF] proposes to analyze the text "as the imaginary resolution of a real contradiction" (p. 62). More specifically, he argued that the subtext of the narrative expressed a social collision resulting from the encounter of two antagonistic modes of production coexisting in the writer's society 18 [START_REF] Jameson | The Political Unconscious. Narrative as a socially symbolic act[END_REF](Jameson, /2002, 66-68, pp. 81-84), 66-68, pp. 81-84). In light of this, we can assume that the thesis of Beyond the Pleasure Principle reflects not simply its "epoch", but a socio-economic contradiction provoked by the rise of the second modernity. In this case, the antagonism in question is not between the bourgeoisie and the proletariat, which was particularly important to the history of nineteenth century Europe. It is, however, between the nineteenth-century large-scale industry mode of production, spurred by consumption ideology yet restrained by the fear of resource exhaustion, and the twentieth-century Fordist mode of production, characterized by the "democratization" of consumerism and the repression of the scarcity problem. How is this opposition expressed into Freud's metapsychology?

Freud was able to consider energy expenditure ("human entropy") as an "aim" because the early twentieth-century massification of production truly turned consumption (which requires energy expenditure) into an end in itself. Indeed, as soon as resources ceased to appear as a "problem" (of scarcity) and demand preceded supply within industrialized societies, consumption was presented (particularly to the hitherto deprived working classes) as a genuinely new way of life. In Freud's economic model, this relationship to consumption is reflected by the idea that energy is not something to be accumulated or saved (a measure that could have seemed wise in the nineteenth century), but to be spent. On the other hand, as we have seen, this hypothesis leads Freud to the oddly pessimistic claim that the organism actively seeks death (i.e., the total discharge of body energy). Crucially, this pessimistic assertion is reminiscent not only of the nineteenth-century fear of scarcity, but also of the latent persistence of the resource depletion problem in the beginning of the twentieth century. The recent "democratization" of consumption did not solve the old matter of scarcity, that is, the fact that producing more and more consumer goods requires more and more energy from a finite world.

This paradox (in the new ideology of consumerism) is to be found in Freudian metapsychology:

the body aims to discharge its energy (we must spend energy to produce goods) though this leads it to a certain death (even if this results in the complete depletion of the Earth's resources).

Thus, the death drive hypothesis condenses two themes, symbolizing the encounter of two modes of production from two distinct eras: the large-scale industry model of the nineteenth century, in which the production of resources could not match consumerist dreams, and twentieth century Fordism, which found in new techniques and technologies the means to realize those dreams (see Table 3 for a schematic presentation of these equivalencies). The uncanny conclusion of Beyond the Pleasure Principle (although a total discharge of energy is equivalent to death, death is in fact sought by the organism) is the involuntary expression of the meeting and superimposition of these two modes of production, a phenomenon that characterized the advent of the second modernity in the postwar period.

Conclusion

This paper has demonstrated the economic dimension of Freudian metapsychology is rooted in mid-nineteenth-century thermodynamic theories. The thermodynamic principle of energy conservation finds its counterpart in Freud's principle of constancy (or reality), while the idea of entropy finds an equivalent in the pleasure principle (discharge). Freud's way of dealing with Mode of production: Nineteenth century large-scale industry Twentieth century Fordism (mass production)

Expression in the death drive hypothesis:

A total expenditure of energy is equivalent to death Energy expenditure is an end in itself death (i.e., the "organism entropy") nonetheless expresses an approach to energy that differs from that of nineteenth-century scientists, who were concerned with the problem of scarcity.

This difference in perspective results from the advent of the second modernity in the early twentieth century, and in particular of the new relationship to energy triggered by the second industrial revolution: energy consumption was now thought as an end in itself.

In focusing on the genealogy of energetics, my objective was not to highlight all the historical determinants that contributed to Freud's formulation of the death drive in 1920. 19 While I used the methodology of historical epistemology, my aim was also not to propose an commentary on Freudian concepts or texts, but rather to frame them within the long term. As noted by Davidson (1987, pp. 256-257), this kind of method may be of particular interest in the case of the history of psychoanalysis, where two historiographical approaches tend to conflict: one focusing on the person of Freud as a creator (Freud as a "genius"), the other denying any originality to his theories by showing how much they owe to the science of its time (Freud as an "usurper"). Here, the method of historical epistemology suggests going beyond the individual-centered level of analysis. It is indeed the history of the scientific field and of its concepts that determines the possibility, for a scientist embedded with this field and its history, of becoming the author of a given theory.

terminology, see The Economic Problem of Masochism [START_REF] Freud | The Economic Problem of Masochism[END_REF](Freud, /1961, pp. 159-160), pp. 159-160). In this paper, Freud situates the pleasure principle next to the "demands of libido" and the "life instincts", while opposing them to a "Nirvana principle" expressing the "trend of the death instinct".

2 [START_REF] Kuhn | Energy Conservation as an Example of Simultaneous Discovery[END_REF]Kuhn ( /1977, pp. 66-68) , pp. 66-68) cites the following authors, noting that the list is not exhaustive: Sadi Carnot, Marc Séguin, Karl Holtzmann, G. A. Hirn, C. F. Mohr, William Grove, Faraday, Liebig, Mayer, Joule, Colding and Helmholtz.

3 It is possible to trace the concept of energy conservation back to Leibniz's 1686 concept of "living force", or even to the idea of conservation of movement formulated by Descartes in 1644 (Mayrargue, 2010, pp. 21-24;[START_REF] Mirowski | More Heat than Light[END_REF]Mirowski, /1995, pp. 16-19), pp. 16-19). For his part, Newton formulated his "first law" of inertia, which was central to the classical physics of forces, and stated that "Every body perseveres in its state of being at rest or of moving uniformly straight forward except insofar as it is compelled to change its state by forces impressed" [START_REF] Newton | The Principia: Mathematical Principles of Natural Philosophy[END_REF](Newton, /1999, p.62, p.62). However, it was not until the mid-nineteenth century that the terms "energy" and "thermodynamics" became established in English science (particularly by Thomson, e.g., 1852Thomson, e.g., , 1853)), and then exported into the continent (particularly via Helmholtz, who met and developed relationships with many English scholars who used these terms, such as Thomson; Cahan, 2012, pp. 57-58;Wegener, 2009, pp. 267-270). The new term of energy also reflected conceptual changes: the "key differentiation here was the slow transmutation of the concept of Kraft, or force, from a general and quite vague synonym for "activity" into a Kantian substance; a process that was only completed […] with the meiosis of the concept into a more substance like "energy" and a more characteristically active "force"" [START_REF] Mirowski | More Heat than Light[END_REF](Mirowski, /1995, p. 45), p. 45). However, scientists disagreed as to whether the new concepts of thermodynamics could merge with or replace those of classical physics, especially matter; by the late nineteenth century, Newtonian dynamic mechanicsthe science of moving massesseemed hardly compatible with thermodynamic physicsthe science of energy exchanges (Prigogine & Stengers, 1984, pp. 121-122, pp. 247-253).

& Stengers, 1979, pp. 206-207). In the early twentieth century, the possibility of reversible processes was recognized by the theory of relativity: "a model of field interactions was proposed where the field itself could be considered to be self-generating, since it acts upon its own source.

[…] Something could come from nothing" [START_REF] Mirowski | More Heat than Light[END_REF](Mirowski, /1995, p. 81, p. 81). In the field of economics, Walras and Jevons independently developed, from the 1870s onwards, the theories at the origin of neoclassical thought. Their works are explicitly inspired by physics, and in particular by energetics; their economic notion of equilibrium is thus the counterpart of the thermodynamic principle of energy conservation [START_REF] Mirowski | More Heat than Light[END_REF](Mirowski, /1995, pp. 238-241, pp. 254-261), pp. 238-241, pp. 254-261). However, entropy has no equivalent in Jevons' or Walras' theories [START_REF] Mirowski | More Heat than Light[END_REF](Mirowski, /1995, p. 254), p. 254). 17 Although bracketed in the first half of the twentieth century, this problem of resources scarcity would resurface during its second half. In the 1970s, the oil shocks dramatically illustrated the dependence of Western countries on a limited resource. In economics, the matter of scarcity was extensively debated from the 1970s to the 1990s.

Against the models of the neoclassical economists Solow and Stiglitz, who assumed that the productivity of resources could be increased indefinitely (thanks to technical progress in particular), the heterodox economist Georgescu-Roegen and his student Daly used the concept of entropy to restate the problem of increasing scarcity (for an overview of these debates, see [START_REF] Couix | Natural resources in the theory of production: the Georgescu-Roegen/Daly versus Solow/Stiglitz controversy[END_REF]. 18 Jameson, who is a literary critic, focused his analysis on romance literature. I mean to show here that the extension of this method to scientific literature is heuristically profitable. 19 As mentioned above, the developments of Beyond the Pleasure Principle borrow as much from the biology of its epoch as from a certain philosophical-cum-speculative reflection. As such, to fully understand this essay, one should take a detour through the history of philosophy as well as that of biologywhich is unfortunately beyond the scope of this paper. Regarding the "philosophical pessimism" of this essay, I shall simply point out here the well-documented influence of the pessimistic philosopher Schopenhauer on Freud (e.g., [START_REF] Cybulska | Freud's Burden of Debt to Nietzsche and Schopenhauer[END_REF]. In Beyond the Pleasure Principle, [START_REF] Freud | Beyond the Pleasure Principle[END_REF]Freud ( /1955) ) mentions "Schopenhauer's philosophy": "For him death is the "true result and to that extent the purpose of life", while the sexual instinct is the embodiment of the will to live" (p. 50). Regarding its biological inspirations, let me just note that with the birth of modern biology at the turn of the nineteenth century, reflections on the relationship between the living and the nonliving, between life and death, took a new turn. In 1780, John Brown's (1780/1803) Elements of Medicine already included a statement that could be compared to Freud's future proposition: "it is certain that life is not a natural, but a forced state; that the tendency of animals every moment is to dissolution; that they are kept from it, not by any power in themselves, but by foreign powers […]; and then, from the necessity of their fate,

  The ego's instincts of self-preservation Function (purpose): To reduce the quantity to zero To keep the quantity at a constant level to travel from thermodynamics into a field like psychoanalysis? Secondly, what brought Freud

  diminished, and […] therefore the quantity of force in nature is just as eternal and unalterable as the quantity of matter. Expressed in this form, I have named the general law "The Principle of the Conservation ofForce." (p. 501)

Table 1 :

 1 Overview of the economic model presented in Beyond the Pleasure Principle

Table 3 :

 3 Expression in metapsychology of the two modes of production coexisting in the second modernity

nd preprint version -reviewed and edited before publication

On the shift from "force" to "energy", see note 3 above. On the prominent role of the relationship between Helmholtz and the British scientists in this shift from the 1850s onwards, see[START_REF] Wegener | Science and Internationalism in Germany: Helmholtz, Du Bois-Reymond and Their Critics[END_REF] or[START_REF] Cahan | Helmholtz and the British scientific elite: From force conservation to energy conservation[END_REF].

Descartes, in particular, had already disseminated the idea that animals could also be conceived in terms of a play of forces. In his view, the human machine differs from other animal machines in that it alone is endowed with reason[START_REF] Descartes | A Discourse on the Method of Correctly Conducting One's Reason and Seeking Truth in the Sciences[END_REF](Descartes, /2006, pp. 45-48), pp. 45-48). The new figure of the human machine that emerged in the nineteenth century made this distinction void: "The human body and the industrial machine were both motors that converted energy into mechanical work"(Rabinbach, 1992, p. 2).

In the scientific literature, those mutations resulted in a new way of looking at the concept of energy. In the area of physics, Boltzmann exposed his kinetic theory, an attempt to statistically capture the evolution of gaseous molecules, in 1872. While Boltzmann's aim was to reconcile classical dynamics with entropy, his model actually implied the existence of reversible phenomena, like "spontaneous" changes of temperature (Prigogine

they yield to death" (p. 116). On the place of Brown's theory in the history of medicine, see for example[START_REF] Canguilhem | Idéologie et rationalité dans l'histoire des sciences de la vie [Ideology and Rationality in the History of the Life Sciences[END_REF]Canguilhem ( /2009, pp. 57-67), pp. 57-67).

Acknowledgements

I want to thank Stéphanie Dupouy and Rémy Guichardaz for proofreading and feedback on the manuscript of this paper. I would also like to congratulate Jean-Yves Bart and the editing and translation department of the MISHA for the corrections and modifications made to the text.

Notes

1 The evolution of Freud's vision and vocabulary on the drive doctrine surely complicates the attempts to ascertain the terms he used. Freud took the opportunity of Beyond the Pleasure Principle to redefine some of his concepts, and later reconsidered the terms used in that essay as of 1924. On the developments of Beyond the Pleasure Principle, see for example Laplanche and Pontalis (1988, pp. 220-222). On the evolution of the

Author affiliation & information

Arthur Massot Archives Henri Poincaré -Philosophie et Recherches sur les Sciences et les Technologies UMR 7117 -CNRS -Université de Lorraine -Université de Strasbourg 7, rue de l'Université -Bureau 104 67 000 Strasbourg (France) Mail: a.massot@unistra.fr ORCID ID: https://orcid.org/0000-0002-1747-9690


Brenda Elizabeth Olivas-Padilla

Dimitris Papanagiotou
email: dimitris.papanagiotou@mines-paristech.fr

Gavriela Senteri
email: gavriela.senteri@mines-paristech.fr

Sotiris Manitsaris
email: sotiris.manitsaris@mines-paristech.fr

Alina Glushkova
email: alina.glushkova@mines-paristech.fr

Computational ergonomics for task delegation in Human-Robot Collaboration: spatiotemporal adaptation of the robot to the human through contactless gesture recognition

The high prevalence of work-related musculoskeletal disorders (WMSDs) could be addressed by optimizing Human-Robot Collaboration (HRC) frameworks for manufacturing applications. In this context, this paper proposes two hypotheses for ergonomically effective task delegation and HRC. The first hypothesis states that it is possible to quantify ergonomically professional tasks using motion data from a reduced set of sensors. Then, the most dangerous tasks can be delegated to a collaborative robot. The second hypothesis is that by including gesture recognition and spatial adaptation, the ergonomics of an HRC scenario can be improved by avoiding needless motions that could expose operators to ergonomic risks and by lowering the physical effort required of operators. An HRC scenario for a television manufacturing process is optimized to test both hypotheses. For the ergonomic evaluation, motion primitives with known ergonomic risks were modeled for their detection in professional tasks and to estimate a risk score based on the European Assembly Worksheet (EAWS). A Deep Learning gesture recognition module trained with egocentric television assembly data was used to complement the collaboration between the human operator and the robot. Additionally, a skeleton-tracking algorithm provided the robot with information about the operator's pose, allowing it to spatially adapt its motion to the operator's anthropometrics. Three experiments were conducted to determine the effect of gesture recognition and spatial adaptation on the operator's range of motion. The rate of spatial adaptation was used as a key performance indicator (KPI), and a new KPI for measuring the reduction in the operator's motion is presented in this paper.

I. INTRODUCTION

Industry 4.0 has resulted in a rise in research in the field of Human-Robot Collaboration (HRC). As a result, robotic agents are being integrated into the work routine, not to take the position of human operators but to assist them in accomplishing complicated and physically demanding tasks. By integrating collaborative robots properly, operators may be able to avoid developing work-related musculoskeletal disorders (WMSDs). WMSDs are a significant concern in the industry, constituting for the majority of work-related health problems in Europe [START_REF] De Kok | Work-related musculoskeletal disorders : prevalence, costs and demographics in the EU[END_REF]. These are caused by the repeated performance of difficult and repetitive operations that frequently demand operators to push themselves beyond their normal physical limitations.

Task delegation and coordination between operators and robots must be optimized while ergonomic aspects are considered to maximize ergonomics and production efficiency in industrial co-production cells. With this in mind, two hypotheses for ergonomically effective task delegation and HRC are proposed in this paper. The first hypothesis (H1) is that by utilizing motion capture (MoCap) data, operators' postures and movements can be accurately measured, allowing for a more thorough ergonomic analysis of their actions. The ability to record accurate measurements for ergonomic analysis is essential as it provides quantitative measures of operators' performance. In this work, the ergonomic analysis method developed is based on segmenting the professional tasks into motion primitives with known ergonomic risk levels. The tasks are initially recorded using a minimized set of inertial sensors to facilitate the implementation of the proposed framework in industrial settings. After evaluation, the collaborative robot is then given the tasks from which the most dangerous motions are detected, while the operators are given the ergonomically safe or supervisory control tasks.

The second hypothesis (H2) is that human gesture recognition and spatial adaptation enable a more natural HRC by performing only motions that are convenient for the operators and avoiding unnecessary movements that may expose them to ergonomic risks. Human gesture recognition is one of the methods used to achieve contactless communication between a robot and a human operator. It can be defined as the process of transforming movements into a form that a machine can easily interpret. On the other hand, spatial adaptation is the process through which the robot adjusts its movements according to the anthropometric characteristics of each operator. By adding spatial adaptation in the HRC, the robot can assist operators in reducing their range of motion and, consequently, the physical effort necessary to fulfill their professional duties.

Fig. 1 illustrates the pipeline for ergonomically optimizing an industrial HRC scenario. The first hypothesis is tested using real professional tasks recorded in a co-production cell for television manufacturing. Next, the proposed HRC is evaluated using two key performance indicators (KPI) to test the second hypothesis. Section II discusses the current state of the art in ergonomic analysis using MoCap data and HRC frameworks. Following that, Section III explains the methodology for ergonomically evaluating professional tasks and the results obtained. Section IV details the HRC's framework and the KPI results. Finally, Section V presents the conclusion and future work.

II. STATE OF THE ART

As mentioned earlier, the activities performed by manual laborers in the industrial sector are becoming more challenging and complex in order to meet market demands within certain time limits, job specifications, and budget constraints. Operators must go beyond their natural physical limitations to undertake repetitive jobs for long periods of time in order to complete the tasks required of them. Being subjected to such constant physical strain leads to work-related musculoskeletal disorders (WMSDs). Ergonomists have developed a variety of methods for evaluating work-related tasks. The methods based their analysis on theoretical knowledge of human physical limitations and abilities indicated by known standards (e.g., ISO 11226:2000 andEN 1005-4). Some of the most popular methods are the Rapid Upper Limb Assessment (RULA) [START_REF] Mcatamney | RULA: A survey method for the investigation of work-related upper limb disorders[END_REF], Rapid Entire Body Assessment (REBA) [START_REF] Mcatamney | Rapid Entire Body Assessment[END_REF], European Assembly Worksheet (EAWS) [START_REF] Schaub | The European Assembly Worksheet[END_REF], and Ovako Working Posture Analysing System (OWAS) [START_REF] Karhu | Correcting working postures in industry: A practical method for analysis[END_REF]. To implement these methodologies, the ergonomist observes an operator executing the task under evaluation and annotates various body part postures on a worksheet. The ergonomic score of the task is then calculated using these annotations. This method of scoring determines which tasks should be changed for better ergonomics. However, because these approaches rely on the ergonomist's perception and experience, scoring can be subjective and have a lot of inter-variability. Alternative sensor-based ergonomic evaluation approaches are now being developed by researchers. Optical and inertialbased motion capture systems have frequently been used to extract upper body posture for ergonomic evaluation [START_REF] Manghisi | Real time RULA assessment using Kinect v2 sensor[END_REF], [START_REF] Plantard | Validation of an ergonomic assessment method using Kinect data in real workplace conditions[END_REF], [START_REF] Shafti | Real-time Robot-assisted Ergonomics[END_REF]. Using inertial measurement units, Vignais et al. created a real-time ergonomic assessment based on RULA (IMUs). Similarly, Yan [START_REF] Yan | Wearable IMUbased real-time motion warning system for construction workers' musculoskeletal disorders prevention[END_REF] used inertial sensors to track the torso inclination of construction workers for ergonomic monitoring.

Collaborative robots are becoming increasingly common in industrial settings as an automated solution for making workplaces more ergonomic, cost-effective, and flexible. As a result, early research focused on creating HRC frameworks that physically couple humans and robots. For example, some applications are for co-carrying [START_REF] Agravante | Collaborative human-humanoid carrying using vision and haptic sensing[END_REF], [START_REF] Yu | Human-Robot Co-Carrying Using Visual and Force Sensing[END_REF], [START_REF] Sirintuna | Human-Robot Collaborative Carrying of Objects with Unknown Deformation Characteristics[END_REF] and co-manipulation [START_REF] Peternel | A Human -Robot Co-Manipulation Approach Based on Human Sensorimotor Information[END_REF], [START_REF] Wu | Reference Trajectory Reshaping Optimization and Control of Robotic Exoskeletons for Human-Robot Co-Manipulation[END_REF], [START_REF] Al-Yacoub | Robotics and Computer-Integrated Manufacturing Improving human robot collaboration through Force / Torque based learning for object manipulation[END_REF]. However, the majority of existing real-time HRC frameworks are developed around the concept of human avoidance to assure safety, i.e., preventing unintentional accidents between humans and robots carrying sharp or heavy industrial goods [START_REF] Liu | Intention Recognition in Physical Human-Robot Interaction Based on Radial Basis Function Neural Network[END_REF], [START_REF] Sharkawy | Human-robot collisions detection for safe human-robot interaction using one multi-input-output neural network[END_REF], [START_REF] El-Shamouty | Towards Safe Human-Robot Collaboration Using Deep Reinforcement Learning[END_REF]. Further research is needed to determine how ergonomics should be considered when a robot collaborates with a human. For instance, when asked to perform the same action, different users of the same setup have different anthropometric characteristics and behaviors. These distinctions must be taken into account in HRC frameworks.

Moreover, meaningful key performance indicators (KPIs) that evaluate either the operators or the robot are also required to measure the performance and interaction of these adaptive HRC. Previous research had offered metrics that primarily assessed the collaborative robot's performance [START_REF] Goodrich | Seven principles of efficient human robot interaction[END_REF], [START_REF] Steinfeld | Common metrics for human-robot interaction[END_REF]. Therefore, new KPIs must be developed that take into consideration human factors to evaluate the job quality of operators in the HRC, and not only its productivity performance.

For designing ergonomic HRC scenarios, previous studies used biomechanical simulations to compute ergonomic metrics (posture, physical effort, and energy spent during the task) [START_REF] Maurice | Experimental assessment of the quality of ergonomic indicators for collaborative robotics computed using a digital human model[END_REF], [START_REF] Kim | Anticipatory robot assistance for the prevention of human static joint overloading in human-robot collaboration[END_REF], [START_REF] Marin | Optimizing contextual ergonomics models in human-robot interaction[END_REF]. However, the main downside of these approaches is that they are hard to incorporate into industrial applications that demand rapid reconfigurability. This is because the human ergonomic analysis in these studies is done offline and in a laboratory. In most cases, for an accurate performance evaluation, it is required to use optical motion capture technology, which is only available in specialized laboratories, or the measurements of numerous sensors distributed throughout the operators' bodies. The recordings inside laboratories can cause inaccurate measures since they lack authenticity and are not real workplace scenarios. After simulating professional tasks, an offline biomechanical analysis is done to compute the ergonomic metrics for the workstation redesign. Consequently, methodologies that employ technologies that are simple to implement in real-world scenarios and that can quickly and effectively estimate the ergonomic risk level of any representative set of manipulation actions performed in the industry are still needed.

To summarize, highly adaptable and rapidly reconfigurable frameworks with real-time data processing capabilities are necessary to handle the industry's ergonomics challenges while simultaneously ensuring productivity. This paper expands on the authors' prior work on adaptive HRC [START_REF] Papanagiotou | Egocentric Gesture Recognition Using 3D Convolutional Neural Networks for the Spatiotemporal Adaptation of Collaborative Robots[END_REF], and automatic ergonomic monitoring [START_REF] Olivas-Padilla | Motion analysis for identification of overused body segments : the packaging task in industry 4 . 0[END_REF], [START_REF] Olivas-Padilla | Hidden Markov Modelling And Recognition Of Euler-Based Motion Patterns For Automatically Detecting Risks Factors From The European Assembly Worksheet[END_REF], [START_REF] Olivas-Padilla | Stochastic-biomechanic modeling and recognition of human movement primitives, in industry, using wearables[END_REF], with the goal of creating a unified ergonomic and reconfigurable HRC framework.

III. AUTOMATIC ERGONOMIC EVALUATION OF PROFESSIONAL TASKS

This section describes the task delegation methodology followed to improve the ergonomics of a real-world workplace scenario.

A. Motion capturing of the television assembly scenario

For the initial study and recording of tasks performed on a television (TV) production line, it was used the BioMed bundle motion capture system from Nansense Inc. 2 . This system consisted of a full-body suit composed of 52 IMUs strategically placed throughout the body and hands. At a rate Two healthy adults, one male and one female, wore the IMU-based suit during the MoCap recording session. The recording took place on a real production line over the course of an eight-hour shift, with the operators placing electronic circuit cards into the frame of the televisions. The entire assembly procedure can be broken down into four main tasks, each illustrated in Fig. 2. The first task is grabbing a circuit card from a container (T 1 ); the second is taking a wire from a second container (T 2 ); the third involves connecting the card and wire and placing them on the TV frame (T 3 ); the fourth task corresponds to drilling the circuit cards on the TV frame (T 4 ). The whole assembly was recorded 108 times. Following the recordings, the MoCap data was offline pre-processed before analysis. Low pass filtering was used to reduce noise, and the common zero velocity update algorithm was used to eliminate drift caused by electromagnetic interference.

B. Modeling of motion primitives for ergonomic evaluation

The methodology described in a previous study [START_REF] Olivas-Padilla | Hidden Markov Modelling And Recognition Of Euler-Based Motion Patterns For Automatically Detecting Risks Factors From The European Assembly Worksheet[END_REF] was followed to detect dangerous motions performed in the television assembly tasks. In this study, HMMs were trained to recognize motion primitives with varying ergonomic risk levels according to EAWS. The ergonomic score for each task was then determined based on the motions detected.

For the generation of the data set with the motion primitives, MoCap recordings were made in a laboratory. The neutral environment sessions involved ten healthy individuals, three females and seven males. None of them suffered from any musculoskeletal injury. The protocol contained 28 different motion primitives, each recorded three times. The motions can be classified into three broad categories: those performed standing, those performed while seated in a chair, and those performed while kneeling. The small movements progressed from comfortable to increasingly uncomfortable postures (rising arms, bending forward, rotating the torso). Each motion was assigned an EAWS risk score, which ranges from 0.5 to 26.5. The higher the ergonomic risk score, the greater the risk.

Only motion primitives where subjects were standing were used to train the HMMs, as operators in the television production line are always required to be standing. Thus, HMMs were trained to recognize only 14 motion primitives with different ergonomic risk levels. Additionally, in a second prior study, where these motion primitives were analyzed by modeling their spatiotemporal dynamics through a Gestural Operational Model (GOM) [START_REF] Olivas-Padilla | Stochastic-biomechanic modeling and recognition of human movement primitives, in industry, using wearables[END_REF], it was discovered that with only five inertial sensors, high recognition accuracy can be achieved. These sensors were placed on the lumbar spine, left upper arm, right shoulder, right upper leg, and left forearm. Therefore, the 14 HMMs were trained using joint angle sequences obtained exclusively from these five sensors and using the same HMM topology and number of states recommended by the study.

C. Automatic postural evaluation of TV assembly motions

To conduct the ergonomic evaluation, the recordings of the professional tasks were first segmented into windows of similar duration as the motion primitives (approximately five seconds), then provided to the 14 pre-trained HMMs. The resulting likelihoods were used to determine the detected motion primitive. All the motion primitives recognized throughout each task were annotated and used to compute each task's mode and average ergonomic score. The ergonomic score was calculated using the equations in [START_REF] Olivas-Padilla | Hidden Markov Modelling And Recognition Of Euler-Based Motion Patterns For Automatically Detecting Risks Factors From The European Assembly Worksheet[END_REF], and the statistics were used to infer which tasks expose operators to a higher ergonomic risk. These were assigned to the collaborative robot, and only the safer tasks in the TV assembly were left to the human operators.

D. Results and discussion of the ergonomic evaluation

Prior to segmenting professional tasks into motion primitives, the 14 HMMs were configured with a left-right topology and seven internal states. Then, their recognition performance was evaluated using the all-shots approach. The overall F-score obtained was 95.65%, which is adequate for discriminating between motion primitives. Table I illustrates the mean, standard deviation, and mode of the ergonomic scores calculated for each task. According to these results, the majority of iterations of tasks T 1 and T 2 can be classified as medium-risk motions, while iterations of tasks T 3 and T 4 are classified as lowrisk motions. For T 1 , the motion primitives detected most frequently resembled motions in which the elbows are raised above shoulder level while the torso is laterally bent. These results are expected based on the movements performed in T 1 , as operators must rotate and laterally bend their torsos to retrieve a circuit card from a container. Due to the container's location, operators must also raise their arms above shoulder level, as illustrated in Fig. 2(a). The motion primitives detected for T 2 corresponded more to motions where there is both bending and rotation of the torso and stretching of the arms. These results match T 2 , where the operators bend to pick up a wire from a container and then connect it to the circuit card. The motion primitives detected for T 3 and T 4 were primarily torso rotations and working with arms bent around 90°degrees. These motion primitives are visible in Fig 2(c) and 2(d). In these tasks, operators were only required to slightly rotate their torso due to the constant movement of the TV frame caused by the conveyor belt, but they did not need to stretch their arms or strongly bend forward to reach the TV frame.

Since T 1 and T 2 involve assuming awkward postures such as rotating the torso while bending forward or raising the arms above shoulder level, they represent a major ergonomic risk than T 3 and T 4 . EAWS recommends that, if possible, tasks with moderate risk must be redesigned; otherwise, the risk must be controlled through other means. Therefore, it is proposed to delegate T 1 and T 2 to the robot and leave T 3 and T 4 to the operators.

IV. OPTIMIZATION OF WORK-SPACE SCENARIO

As suggested in the preceding section, it is necessary to modify the existing scenario in order to avoid operators developing musculoskeletal injuries. Thus, this section details the integration of a collaborative robot into the television manufacturing process. The framework includes a real-time gesture recognition module that enables natural HRC by performing only gestures that are convenient for the operator. Additionally, pose estimation was integrated using a skeleton-tracking algorithm to supply the robot with human pose information, allowing it to spatially adapt its movements to the operators' anthropometrics, thereby improving the operators' posture. Three experiments were conducted to determine whether gesture recognition and pose estimation can reduce the operator's range of motion. Two KPIs were utilized to evaluate the proposed HRC: the percentage of spatial adaptation achieved by the robot and a new KPI for collaborative systems presented in this paper. The new KPI quantifies the reduction in operator motion caused by the inclusion of gesture recognition.

A. Gesture recognition with 3DCNNs

3D Convolutional Neural Networks (3DCNNs), a type of Deep Learning architecture, are used for the gesture recognition part that concerns the communication between the operator and the robot. These networks were trained on a television assembly dataset generated using an RGB camera in an egocentric view. The challenge with using egocentric data for recognition is that the head and hands move in parallel but do not always follow each other. Even if the hands are a significant part of this module, they can be prominent within the frame but can also be partly or even totally out of the main view. Nevertheless, the added value of an egocentric perspective is that the network will provide gesture recognition results independent of the users' anthropometric characteristics and position in the environment.

To train the proposed network, the routine of assembling a television is divided into sub-tasks performed either by the robot or operators. The operators are assigned the ergonomically safe task that also requires decision-making. The dataset created from this HRC scenario consists of RGB images recorded with a GoPro Black, placed with a headband on the head of the human operator, providing the egocentric view. The recording was made at an 848x480 resolution and 20 frames per second. A group of 14 operators, four females and ten males, were recorded performing six gestures and five postures. Thus, there are 11 classes in total, each corresponding to a unique command for the collaborative robot. Fig. 3 presents the commands contained in the data set. The objects involved in this TV assembly process are a TV frame and two circuit cards: the power supply (PSUgold card) and the mainboard (chassis-green card). An earlier study [START_REF] Papanagiotou | Egocentric Gesture Recognition Using 3D Convolutional Neural Networks for the Spatiotemporal Adaptation of Collaborative Robots[END_REF] demonstrated the network's ability to recognize the 11 gestures with a 98.5% accuracy.

B. Human Robot Collaboration for professional environments

For the proposed HRC scenario, the UR3 robotic arm 3 from Universal Robots was used. The external parts that were used for grasping and releasing the circuit cards, were from ROBOTIQ (gripper: 2F-140 4 & force torque sensor: FT-300-S 5 ). For the control of the robotic arm, the Robot Operating System (ROS) was used. robotic arm receives real-time ID messages from the gesture recognition module through a UDP communication protocol. In the new routine, the operator performs the Start gesture (G 1 ) to notify the robot that the TV assembly routine starts. The robot approaches the card container, retrieves the initial green card, and hands it to 3 https://www.universal-robots.com/products/ur3-robot/ 4 https://robotiq.com/products/ft-300-force-torque-sensor 5 https://robotiq.com/products/2f85-140-adaptive-robot-gripper the operator in a predefined handover position. By pressing the Force sensor, the operator releases the card and verifies its functionality. If the card is functional, the operator performs G 2 and places it on the television frame (G 3 ), while the robotic agent moves towards the card box to retrieve the gold card. If the green card is not functional, the operator executes G 10 to notify the robot, which then brings a replacement green card. When the robotic arm delivers a functional card, the operator performs G 2 and then places (G 3 ) and screw (G 4 ) the card on the television frame. This procedure is repeated until both the green and gold cards are placed on the television frame. Then, the human operator executes G 11 to signal the routine's completion.

The gesture recognition module was designed with a dual-level of control to ensure the operator's safety in the HRC scenario. More precisely, the robot receives the ID of the recognized gesture only after it has been correctly recognized for 20 consecutive time frames. This may raise concerns about any latency observed during the television assembly process. However, it was observed that the time interval between capturing a frame and its correct recognition was between 0 and 0.08 ms, indicating that no significant latency was observed. The second level of control was performed within ROS, where the robot accepted only gestural IDs that corresponded to the TV assembly workflow as defined by the working routine. As a result, if an unintended incident occurs, the operator retains full control because the robotic arm strictly adheres to the routine sequence.

The introduction of the robotic arm and a gesture recognition module requires only minor torso rotations from the human operator. To facilitate natural collaboration and assist the operator in performing only ergonomically safe motions, a posture estimation module was added that enables the robot to spatially adapt to the operator. Papanagiotou et al. [START_REF] Papanagiotou | Egocentric Gesture Recognition Using 3D Convolutional Neural Networks for the Spatiotemporal Adaptation of Collaborative Robots[END_REF] presented this posture estimation module and spatial adaptation procedure in a previous study. The spatial adaptation refers to the fact that the robotic arm does not place the cards in a fixed position but rather adapts to the operator's anthropometric characteristics. This procedure ergonomically improves the operator's posture. An Intel-RealSense RGB-D camera was utilized to capture the operator for the robot's spatial adaptation. The X and Y axes of the camera were first aligned with the X and Z axes of the robot. Then, the operator's skeleton and the position and velocity of the operator's wrists were extracted. The robotic arm tracks the operator's hand and approaches it with a circuit card when it remains motionless and in an accessible position for the robot.

C. Key performance indicators

As a key performance indicator (KPI), the percentage of robot spatial adaptation was used [START_REF] Papanagiotou | Egocentric Gesture Recognition Using 3D Convolutional Neural Networks for the Spatiotemporal Adaptation of Collaborative Robots[END_REF]. In addition, this paper presents a second KPI for determining how much operator effort is reduced as a result of the gesture recognition module.

The KPI of robot spatial adaptation SA represents the ratio of the distance covered by the robot without spatial adaptation to the distance covered when the robot adjusts to the operator-specified position. The following formula is used to determine this KPI: SA(%) = AHP -W P -P HP -W P P HP -W P

Where SA is spatial adaptation, AHP the adapted handover position, W P is the waiting point and P HP the particular handover position. Centimeters are used to measure distances. The higher the rate of adaptation, the more effort the operator had to put in during the robot collaboration.

The second KPI quantifies the difference in operators' motion before and after the introduction of gesture recognition. This KPI was calculated as follows:

RiOM (%) = M woGR -M wGR M woGR (2)
In equation 2, RiOM corresponds to the reduction in operator's motion, M woGR the motion without gesture recognition and M wGR is the motion with gesture recognition. This KPI measures the amount of effort reduced by the operator as a result of gesture recognition.

D. Experimental results

In order to evaluate the HRC scenario in terms of collaboration and operator performance, 14 operators were recorded performing the proposed work routine in three separate experiments. In the initial experiment, gesture recognition and spatial adaptation were disabled. Therefore, operators were required to interrupt their routine and inform the robotic arm of their current action by pressing its force-torque sensor. The gesture recognition module was enabled for the second experiment, but not the spatial adaptation, so the operators received the circuit cards from a pre-defined handover position. Finally, in the third experiment, gesture recognition and spatial adaptation were enabled and used to continuously provide information about the operators' actions to the robotic arm. The purpose of these three experiments was to assess the robotic arm's adaptation and the operator's motion during each experiment.

The KPIs for each operator were determined based on the experiments and are shown in Table II. Note that the smaller the percentage for SA, the better, as the robot placed the circuit cards closer to the operator's hand, reducing the necessity for the operator to reach for the card. On the other hand, for RiOM , the bigger the percentage is, the better because it means a greater reduction in operator motion when gesture recognition is used.

E. Discussion

Both KPIs demonstrated a consistent decrease in the amount of physical effort required of the operator in the proposed HRC with gesture recognition and spatial adaptation. The average rate of spatial adaptation and reduction in the operators' motion among the 14 subjects was 29.37% and 28.37%, respectively. The optimized HRC configuration significantly decreased the operators' motion compared to the configuration without gesture recognition. This is also illustrated in Fig. 4, where, in Fig. 4(a), which shows the HRC configuration without gesture recognition, the operator was required to rotate his torso in order to touch the robot's sensor, which is located outside the TV frame. In Fig. 4(b), where there is gesture recognition, the robot recognizes when the operator has completed his task and can proceed to the next action in the work routine. These results provide solid evidence that the collaborative robot can adapt to human factors and that the proposed HRC scenario contributes to improved ergonomics and productivity.

V. CONCLUSION AND FUTURE WORK

This paper presents a methodology for task delegation and a human-robot collaboration framework that maximize ergonomics and production efficiency in a television coproduction cell. At first, professional tasks performed on a television production line were captured using a minimal set of inertial sensors. Then, motion primitives with known ergonomic risks were detected across the entire professional tasks in order to estimate the tasks' ergonomic scores and identify the most dangerous ones. An optimized HRC was proposed, in which hazardous tasks were delegated to the collaborative robot. The HRC scenario was enhanced by applying gesture recognition and spatial adaptation that allowed the human operator to collaborate with the robot using gestures while avoiding unnecessary movements that could cause physical strain.

Future research will focus on developing a frame-by-frame risk prediction method. The system would be capable of predicting future motions and their ergonomic risk level. This type of system may be advantageous for HRC for various reasons. For example, the robot could provide preventive feedback to operators or protect them from ergonomic risks by actively assisting them in adopting safer postures before and during a task.

ACKNOWLEDGMENT

The research leading to these results has received funding by the EU Horizon 2020 Research and Innovation Programme under grant agreement No. 820767, project CoLLab- oratE. The authors like to express their gratitude to Arc ¸elik A.S. for their support in this work.

Fig. 1 :

 1 Fig. 1: Pipeline for ergonomically optimizing industrial co-production cells with HRC

Fig. 2 :

 2 Fig. 2: Professional tasks for TV assembly. (a) T 1 : Grab the circuit card from a container; (b) T 2 : Take a wire from a container; (c) T 3 : Connect the circuit card and wire and place them on the TV frame; (d) T 4 : Drilling circuit cards to the TV frame.

Fig. 3 :

 3 Fig. 3: Professional gestures of the egocentric gesture recognition module.

Fig. 4 :

 4 Fig. 4: Professional gestures for TV assembly with and without gesture recognition. (a) MwoGR: Press robot to start; (b) MwGR: The robot recognizes 'Waiting gesture' (G 7 ) to start.

TABLE I :

 I Summary statistics of the EAWS scores calculated for each task.

	Tasks	EAWS scores Mean STD Mode
	T 1	16.02	2.65	17.50
	T 2	15.02	3.43	16.00
	T 3	10.76	3.68	8.50
	T 4	11.50	3.19	12.50

TABLE II :

 II Measured KPIs for each operator .30 21.10 27.50 30.40 31.90 27.10 31.80 13.40 33.90 43.50 32.10 18.70 27.40 RiOM (%) 31.40 33.10 24.40 27.10 32.10 27.30 24.50 26.80 37.40 20.60 45.90 20.80 21.30 24.50

	KPI	1	2	3	4	5	6	Operator 7 8	9	10	11	12	13	14
	SA (%)	39.10 33

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