Anass El Aouni

Khalid Daoudi

Khalid Minaoui

Hussein Yahia

Robust Detection of the North-West African Upwelling From SST Images

Keywords: Sea surface temperature (SST) images, upwelling, upwelling identification and extraction, upwelling index

Analysis and study of coastal upwelling using sea surface temperature (SST) satellite images is a common procedure because of its coast effectiveness (economic, time, frequency, and manpower). Developing on the Ekman theory, we propose a robust method to identify the upwelling regions along the northwest African margin. The proposed method comes to overcome the issues encountered in a recent method devoted for the same purpose and for the same upwelling system. Afterward, we show how our method can serve as a framework to study and monitor the spatio-temporal variability of the upwelling phenomenon in the studied region.

I. INTRODUCTION

T HE Atlantic coastline of north Africa is characterized by the presence of the upwelling phenomenon almost all around the year. This phenomenon can be identified by seasonally variable cold and nutrient-rich water [START_REF] Aouni | Physical and biological satellite observations of the northwest African upwelling: Spatial extent and dynamics[END_REF]. Thus, the automatic identification and detection of this phenomenon is of fundamental importance in the study of productivity enhancement [START_REF] Aouni | Surface mixing and biological activity in the north-west African upwelling[END_REF], [START_REF] Aouni | The contribution and influence of coherent mesoscale eddies off the North-West African Upwelling on the open ocean[END_REF] and associated fish management [START_REF] Chassot | Satellite remote sensing for an ecosystem approach to fisheries management[END_REF].

In the literature, several techniques and algorithms have addressed the problem of automated identification of upwelling regions in sea surface temperature (SST) images. For instance, Cayula and Cornillon [START_REF] Cayula | Edge detection algorithm for SST images[END_REF] proposed an unsupervised classification method where neural network is trained based on the k-means clustering [START_REF] Hartigan | Algorithm AS 136: A K-means clustering algorithm[END_REF], [START_REF] Likas | The global K-means clustering algorithm[END_REF]. Nieto et al. [START_REF] Nieto | Mesoscale frontal structures in the canary upwelling system: New front and filament detection algorithms applied to spatial and temporal patterns[END_REF] proposed a histogram-based separation method, where the bimodality of SST histogram is interpreted to represent two different water. Sousa et al. [START_REF] Sousa | Identification of upwelling areas on sea surface temperature images using fuzzy clustering[END_REF] proposed an algorithm to extract upwelling regions in the coastal ocean of Portugal. This algorithm is based on fuzzy clustering and yields good segmentation accuracy in that particular area. More recently, Tamim et al. and Anass El Aouni et al. [START_REF] Tamim | An efficient tool for automatic delimitation of moroccan coastal upwelling using SST images[END_REF]- [START_REF] Aouni | An improved method for accurate computation of coastal upwelling index using sea surface temperature images[END_REF] improved the latter algorithm and developed a method that showed good results in the southern part of the Moroccan Atlantic coast. However, this method fails to provide good segmentation on the whole coast of Morocco (including the central and Manuscript received January 13, 2020; accepted March 25, 2020. This work was supported by the French-Moroccan Partenariats Hubert Curien (PHC)-Toubkal under Project TBK/16-24 and Project PPR2-6.

(Corresponding author: Anass El Aouni.)

Anass El Aouni, Khalid Daoudi, and Hussein Yahia are with INRIA Bordeaux Sud-Ouest, 33405 Talence, France (e-mail: anas.elaouni@gmail.com; khalid.daoudi@inria.fr; hussein.yahia@inria.fr).

Khalid Minaoui is with the Faculty of Sciences, Mohammed V University in Rabat, LRIT URAC 29, Rabat 10004, Morocco (e-mail: khalid.minaoui@um5.ac.ma).

Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LGRS.2020.2983826 northern regions). This is mainly due to the high variability of the latitudinal distribution of temperatures.

In this letter, we develop a robust approach devoted to the identification of the upwelling regions from SST images along the central part of the Canary upwelling Ecosystem. Our method is based on a nonlinear normalization of SST images, it is simple to implement and it overcomes the problems encountered while using the method developed in [START_REF] Tamim | An efficient tool for automatic delimitation of moroccan coastal upwelling using SST images[END_REF] and [START_REF] Tamim | A simple and efficient approach for coarse segmentation of Moroccan coastal upwelling[END_REF]. Also in this letter, we show how our method can serve as a framework to monitor the spatio-temporal variability of the upwelling intensity and extent.

The letter is structured as follows: Section II presents the database and the area of study used throughout this letter. Section III describes the methodology for the automatic delimitation of upwelling area from SST images. In Section IV, we show how our method can serve as an input to characterize and monitor the evolution of the upwelling variability in the region. Finally, a conclusion is drawn in Section V.

II. REGION OF INTEREST AND DATA SETS

In this letter, we use a total of 720 thermal level 3 infrared MODIS-T images of the Moroccan Atlantic coast spanning the region 20 • 510-36 • 21N and 4 • 26-19 • 97W, for the time period from January 2001 to December 2015. These data are extracted from http://oceancolor.gsfc.nasa.gov/ a with spatial resolution of 4 × 4 km. Fig. 1(a) shows an example of SST images over the study area, obtained on the first week of 2007. We consider eight-day SST images because they constitute a good compromise between the variability of the pixels values and the necessity to provide a continuous spatio-temporal coverage. SST images are included in the database if they have at least 25% of valid information in the area between the coast and 200 km offshore.

III. SEGMENTATION OF UPWELLING AREAS

In this section, we present a robust method to extract the upwelling regions along the Moroccan Atlantic coast, this method complements and improves the method introduced in [10] and [START_REF] Tamim | A simple and efficient approach for coarse segmentation of Moroccan coastal upwelling[END_REF]. Indeed, the latter's region of interest is the south part of Morocco. Although it yields accurate segmentation in that region, it fails however, to provide good segmentation on the whole coast of Morocco (including center and north). The main reason behind this failure is the high variability of the latitudinal distribution of temperatures. Hence, for example, upwelling temperature in the southern part can be closer to the offshore temperature in the northern region. An example of such phenomena is shown in Fig. 1. In this figure, the green curve represents the latitudinal variation of the maximal temperature in the offshore along the SST image in Fig. 1(a). The blue curve represents the latitudinal variation of the mean temperature over the first 80 km. Here, the red line represents the Fuzzy C-means algorithm (FCM), which separates the two clusters (upwelling and nonupwelling), and this figure shows that a part of the green curve falls in the cluster presenting the upwelling water. To bypass this problem, we propose a latitudinal normalization by offshore temperatures, that is, we consider "region-dependent" difference of temperatures rather the temperatures themselves. The procedure is described in Section III-A.

A. Latitudinal Normalization of SST Images

As described in [START_REF] Ekman | On the influence of the Earth's rotation on oceancurrents[END_REF], Ekman transport can generate a 90 • movement of surface water to the right of the wind direction, which results in coastal upwelling in the case of the north-west African margin. Thus, in order to identify the upwelling, it is natural to compare the temperature of the inshore water with the offshore water within a perpendicular line to the coast. Within each perpendicular line, apply the method developed in [START_REF] Tamim | An efficient tool for automatic delimitation of moroccan coastal upwelling using SST images[END_REF] and [START_REF] Tamim | A simple and efficient approach for coarse segmentation of Moroccan coastal upwelling[END_REF] and find what pixels belong to the upwelling. This turns out to be similar to normalizing every line with respect to its maximum temperature found in the offshore. In this letter, we thus partition each image into r lines l which are perpendicular to the coast, as shown in Fig. 2, where l are pixel of width and n pixels of length starting from the coast line. All the lines present the same angle to the coast line in order to cover all the pixels of the image as follows: We first define the general orientation of the Moroccan coast as a virtual line L lat,lon = (P 1 , . . . , P r ).

(

) 1
Starting from the northern point P 1 till the southern one P r . This line will serve to define the perpendicular lines as

l 1 , L = 0 . . . l r , L = 0. (2)
On each line, we first define the maximal offshore temperature and normalize the line with respect to its maximum found in the offshore up to the limit of the image boundary. To do so, we consider a line l ∈ {l 1 , . . . , l r } composed of temperature pixels l = {T n i ; i = 1, . . . , n}. Let T l m be the maximal temperature on line l

T l m = max i T l i . ( 3
)
We now define the offshore difference of temperature as

δT l i = T l m -T l i . ( 4
)
Fig. 3(a) displays the image of Fig. 1(a) after this normalization. As we see, this image keeps the same thermal structures as in the SST image. Moreover, the upwelling area, which now corresponds to high pixel intensities, is visually more clear than in the original SST image. However, there exists some unnatural lines caused by the fluctuations presented in the vector of maximal temperatures T r m [red line in Fig. 3(c)] used to generate this image. To overcome this problem, a moving average algorithm with a window size of five is used to smooth out the vector of maximal temperatures [blue line in Fig. 3(c)]. Fig. 3(b) shows the final normalized image computed using the smoothed T r m vector. Fig. 4 displays the "dual" information of Fig. 1 where the green curve represents the minimal pixel in each line of the normalized image and the blue curve represents the mean pixels over the first 80 km of each line. It is clear that offshore information (green curve) is now normalized over the whole coast and is clearly discriminated from the inshore information (blue curve).

B. Upwelling Segmentation

Once the normalization is applied on an SST image, the upwelling area corresponds to the region(s) with the highest intensities. Segmentation of this area goes back to separate the latter from pixels with the lowest intensities. To do so, we use the same approach developed in [START_REF] Tamim | A simple and efficient approach for coarse segmentation of Moroccan coastal upwelling[END_REF] and [START_REF] Tamim | An efficient tool for automatic delimitation of moroccan coastal upwelling using SST images[END_REF] but applied over the normalized image this time.

Fig. 1(a) displays the segmentation obtained with the method developed in [START_REF] Tamim | A simple and efficient approach for coarse segmentation of Moroccan coastal upwelling[END_REF] applied on the SST images in Fig. 1(a). As stated above, this method fails to provide a good segmentation along the whole coast. Fig. 4(a) shows the segmentation obtained after applying the same method on the normalized SST images. It is so clear that the new approach yields a very accurate segmentation on the whole coast, therefore, solved the problem encountered in the northern parts of the upwelling system.

IV. FRAMEWORK FOR UPWELLING MONITORING

This section shows how the proposed method can be used as an input to study and monitor the evolution of the upwelling phenomenon along the studied area. Several authors have used SST images to characterize and quantify the spatio-temporal variability of the upwelling intensity, generally calculated as a simple thermal difference between the inshore water and its (a) Binary result of FCM algorithm applied on the normalized SST image in (b). White color refers to upwelling water while black color refers to offshore water. (b) Binary result after the application of the regiongrowing algorithm [START_REF] Chang | Adaptive image region-growing[END_REF] on the image (a). (c) Latitudinal comparison between the upwelling and the offshore pixels values. reference in the offshore direction [START_REF] Van Camp | Upwelling and boundary circulation off northwest Africa as depicted by infrared and visible satellite observations[END_REF]- [START_REF] Marcello | Seasonal and temporal study of the northwest African upwelling system[END_REF] I l = T l max -T l min .

(

) 5
In the literature, T l min is defined as the minimum temperature recorded in the coastal band from the coast up to the continental slope [START_REF] Van Camp | Upwelling and boundary circulation off northwest Africa as depicted by infrared and visible satellite observations[END_REF]- [START_REF] Santos | Decadal changes in the Canary upwelling system as revealed by satellite observations: Their impact on productivity[END_REF]. For the Moroccan coast, the latter is generally chosen to be 500 km.

Here, we simply define T l min as the minimal temperature encountered within the extracted upwelling region as shown in Fig. 5. That is, let S be the extracted upwelling region, then given a line l ∈ {l 1 , . . . , l r } of temperature pixels l = {T l i ; i = 1, . . . , n}, T l min is defined as

T l min = min i∈S T l i . ( 6
)
This approach is simple, very intuitive, and dynamic because it takes into account the spatial extent of the upwelling, therefore, it ensures that all the (necessary) low temperatures are considered in the calculation. We show in Fig. 6 the interannual variability of the upwelling intensity via a space-time Hovmoller diagram computed from the definition (5 and 6) at 4 km/8-day spatio-temporal resolution. The upwelling intensity displays high variability both in space and time, with the entire period being marked by substantial oscillations between strong and weak intensity, and a main upwelling pattern that can be divided into three characteristic regions. 1) The region between 33 • N and 36 • N where the upwelling intensity is weak due to the unfavorable orientation of the coast line in this region.

2) The region between 27 • N and 33 • N, where the upwelling intensity is strong in summer and autumn.

3) The region between 21 • N and 27 • N where the upwelling is intense throughout the year with a maximum from spring to autumn.

V. CONCLUSION

This letter contributes to the study and monitoring of the upwelling phenomenon over the north-west African margin. Previous works have addressed the issue of the automatic identification of upwelling regions from the SST images along different parts of the Canary current upwelling system. However, the problem of upwelling extraction from SST images along the central part of the Canary Current upwelling system (CCUS) shows several challenges in the literature. In this letter, an algorithm that successfully the upwelling regions from the SST images over the whole central part of CCUS is proposed. The proposed approach is performed in two steps: the first one where a nonlinear normalization is applied over each SST image, and the second where a clustering algorithm is applied over the normalized SST images to separate the region of upwelling from the offshore water. This method showed to be very accurate and more robust than a recent state-of-the-art method. Moreover, the proposed method can be served as a framework to study and monitor the spatio-temporal variability of the upwelling dynamics over the studied area.

Fig. 1 .

 1 Fig. 1. (a) Eight-day SST images obtained during the first week of January 2007. (b) Result of the approach developed in [11] applied on the SST image from (a). White color refers to upwelling water, whereas black color refers to offshore water. (c) Latitudinal comparison between the upwelling and the offshore temperature; green curve represents the maximal temperature on each latitudinal step of the SST image in (a), blue curve represents the mean temperature over the first 80 km of each latitudinal step.

Fig. 2 .

 2 Fig. 2.Illustration of the nonlinear normalization of SST images. L represents the general orientation of the Moroccan coast, l 1 , . . . , l r represents the perpendicular lines to the coastline.

Fig. 3 .

 3 Fig. 3. (a) Normalized SST image computed from the SST images in (a). (b) Smoothed normalized SST image. (c) SST max vector before (red) and after (blue) being smoothed (increased by 1).

Fig. 4 .

 4 Fig. 4.(a) Binary result of FCM algorithm applied on the normalized SST image in (b). White color refers to upwelling water while black color refers to offshore water. (b) Binary result after the application of the regiongrowing algorithm[START_REF] Chang | Adaptive image region-growing[END_REF] on the image (a). (c) Latitudinal comparison between the upwelling and the offshore pixels values.

Fig. 5 .

 5 Fig. 5. (a) Final segmentation achieved by the proposed method. Also shown in the image, an example of a line within which the coastal upwelling I l index is computed. (b)-(d) shows the results of the proposed method applied on different eight-day images covering different periods of time (April, June, and September).

Fig. 6 .

 6 Fig. 6. Space-time Hovmoller plot of the interannual variability of the upwelling intensity I l computed from the definition (5) and (6) from 2001 to 2015.


ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

At the beginning of the 20 th century, philosophy of science defined itself mainly in opposition to metaphysics [START_REF] Mach | Die Mechanik in ihrer Entwicklung historisch-kritisch dargestellt[END_REF][START_REF] Duhem | La Théorie physique[END_REF][START_REF] Carnap | Die Überwindung der Metaphysik durch logische Analyse der Sprache[END_REF], which was taken to be a field lacking rigorous methods, thus being hostage to arbitrariness and dogmatism [START_REF] Hahn | Wissenschaftliche Weltauffassung -Der Wiener Kreis[END_REF]. Logical empiricism, first promoted in Vienna and Berlin in the 1920s and the beginning of the 1930s, recommended abandoning metaphysical theories with their shaky epistemic status and suggested a new start based on the hypothesis of the unity of science [START_REF] Hahn | Wissenschaftliche Weltauffassung -Der Wiener Kreis[END_REF]. According to logical empiricism, scientific knowledge is better justified than metaphysics because it rests on a solid foundation: the pillars of observation and logic. Metaphysical statements, which cannot be justified by observation or logic, were taken to be strictly speaking meaningless. Given this conception of metaphysics and science that has been dominant in much of the 20 th century, it may seem surprising, and even paradoxical, to open up a new field of research under the name of "metaphysics of science" or "scientific metaphysics", with the aim of pursuing metaphysical questions in the light of contemporary science or of interpreting contemporary science in metaphysical terms. How can it be legitimate to raise questions about the structure of reality revealed by the sciences in metaphysical terms?

According to the doctrine of logical empiricism, metaphysical questions that arise about the objects of scientific research can only be addressed in a rational and coherent way once they have been translated into questions bearing on scientific theories, construed as sets of propositions. To the question of what is meant by the statement that a physical or biological event causes another event, logical empiricism substituted the analysis of the logical form of causal explanations provided by the sciences [START_REF] Hempel | Studies in the Logic of Explanation[END_REF]. To the question: "what is a dispositional property?" (such as elasticity or electrical conductivity in physics, reactivity in chemistry, or fitness in biology), it substitutes the analysis of the logical form of disposition statements (Carnap 1936/7). In this conception, there is no room for metaphysical questions as such; metaphysics is seen at best as a conceptual framework that makes scientific research possible but has itself the status of a convention [START_REF] Carnap | Empiricism, semantics, and ontology[END_REF].

However, during the second half of the 20 th century it turned out that the distinctions that helped to justify the neat separation between science and metaphysics, that is, between the empirical content and the conventional form of scientific propositions, were themselves ill grounded. There is no difference in principle but only in degree between analytic and synthetic statements (Quine 1953(Quine /1980b); furthermore, it has become controversial whether there is a principled distinction between observational and theoretical statements, which might ground a metaphysical distinction between "observable" and "theoretical" entities [START_REF] Maxwell | The Ontological Status of Theoretical Entities[END_REF][START_REF] Kuhn | The Structure of Scientific Revolutions[END_REF][START_REF] Hacking | Representing and Intervening[END_REF]).

This evolution explains why it has become legitimate again to address metaphysical issues on a background of contemporary science. It now appears artificial to separate the analysis of the general conceptual tools that are used throughout the sciences, such as cause, law, individual object, process, mechanism, reduction, from the metaphysical analysis of the structure of reality as represented by science.

The earliest form this "return to metaphysics" has taken is the conception of ontology developed by Quine (1953Quine ( /1980a)): the ontology of a given (scientific or popular) theory consists of the objects to which the theory is "ontologically committed", that is, the objects that exist if the theory is correct. Technically, Quine's proposal is to take quantification as a criterion of ontological commitment. His hypothesis is that a theory is committed to the existence of a set of objects if and only if they belong to the domain in which the bound variables appearing in the axioms and theorems of the theory take their values. More simply, the idea is to look at the universal ("every object x is such that...") and existential ("there is an object x such that...") generalizations that are part of the theory. What exists according to the theory is the set of objects in which the variables x take their values. However, Quine's conception cannot be adequately used when many formal versions of the same theory exist (e.g., in classical mechanics) or when many interpretations of the theory are sustainable (e.g., in quantum mechanics). Moreover, metaphysics is wider than ontology; if the aim of ontology is to enquire into the fundamental kinds of entities that exist, metaphysics also explores concepts expressing the structure, dynamics and interaction of these entities, such as the concepts of law, causation, necessity and possibility, and reduction and emergence. In metaphysics of science, both questions -what exists? And how do the entities that exist evolve and interact? -are addressed on the background of contemporary science.

Today, the field of metaphysics of science is blossoming. Unsurprisingly for such a fundamental branch of philosophy as metaphysics, the scope, aims and methods, and even the very possibility of the discipline remain controversial. The common denominator of all work lying within this new domain is to pursue metaphysics in the light of contemporary scientific knowledge.

The articles collected in this special issue were presented at a workshop on New Trends in the Metaphysics of Science, organized by IHPST (Institute for the History and Philosophy of Science and Technology), University of Paris 1, Panthéon-Sorbonne, CNRS, and held at the Sorbonne library, December 16-18, 2015, as part of the research project "Metaphysics of Science" (ANR-12-BSH3-0009) funded by ANR.

These papers represent a significant part of the spectrum of topics covered in present-day metaphysics of science. Foundational issues concern the very possibility of metaphysics of science, the nature and epistemic status of claims made in this field, and the criteria of their justification and evaluation. Some authors [START_REF] Ladyman | Every Thing Must Go: Metaphysics Naturalized[END_REF] have presented metaphysicians with a simple alternative: according to them, one possibility of pursuing metaphysics takes it to be a form of enquiry proceeding by conceptual analysis on purely a priori grounds, drawing its justifications exclusively from common sense intuitions and the analysis of ordinary language statements, and taking its concepts, theses and tools from the history of philosophy. The alternative promoted by Ladyman and Ross consists in replacing traditional metaphysics by what they call "scientific metaphysics": it takes metaphysics to be a purely a posteriori enterprise, its concepts and theses being exclusively derived from and evaluated in light of contemporary science.

The first two papers in this special issue, by Alexandre Guay & Thomas Pradeu and Bryant, challenge the simple alternative just sketched and elaborated by several authors (among others, [START_REF] Redhead | From Physics to Metaphysics[END_REF][START_REF] Maudlin | The Metaphysics Within Physics[END_REF][START_REF] Esfeld | Physique et Métaphysique: Une Introduction à La Philosophie de La Nature[END_REF][START_REF] Esfeld | Metaphysics and Science[END_REF][START_REF] French | The Structure of the World: Metaphysics and Representation[END_REF]. In their paper "Right Out of the Box: How to Situate Metaphysics of Science in Relation to Other Metaphysical Approaches", Guay & Pradeu first distinguish metaphysics applied to science from scientific metaphysics. The first examines how such traditional metaphysical concepts as causality, law, individual or reduction, can be enriched by contemporary science and help interpret it. The aim of the second is to elaborate a worldview that takes account of current science. However, they show that the latter leaves open a whole array of options: scientific metaphysics can be developed in realist or non-realist frameworks; it need not be based exclusively on physics but can be based on other sciences. Scientific metaphysics need not be, in [START_REF] Strawson | Individuals[END_REF] terms, "revisionary", i.e. aim at correcting the worldview of common sense, but can also be developed in a "descriptive" way. This can be done by naturalizing human conceptual structure through cognitive science. Guay & Pradeu situate metaphysics of science with respect to a whole array of conceptions of metaphysics: 1) metaphysical theses can be justified a priori or a posteriori, 2) they can be descriptive or revisionary, and 3) the justification and evaluation of metaphysical theses can rely on science, but also on common sense and/or traditional metaphysics. It would take an argument to show that the former excludes the latter. Amanda Bryant's paper "Keep the Chickens Cooped: The Epistemic Inadequacy of Free Range Metaphysics" distinguishes two ways of conceiving metaphysics: scientific or naturalized metaphysics and what she calls "free range metaphysics". Science has only a nominal role in the latter's construction and evaluation of metaphysical theses and theories. While she argues that such non-scientific constraints on theory choice as consistency, simplicity, and intuitive plausibility are not sufficiently robust and fail to provide free range metaphysics with epistemic warrant, it can nevertheless be fruitful (in the sense of producing "collateral benefits"), by developing techniques for making clear statements, for suggesting hypotheses that may become integrated into science later (Popper 1983). Bryant shows that although free range metaphysics is epistemically inferior to scientific metaphysics, on count of both robustness and epistemic warrant, free range metaphysics plays a useful role by developing conceptual tools that can be used in scientific metaphysics [START_REF] French | Thinking Outside the Toolbox: Towards a More Productive Engagement Between Metaphysics and Philosophy of Physics[END_REF].

All other papers in the special issue make constructive contributions to metaphysics of science, either concerning science in general or specific fields of science. Michael Esfeld's paper "A proposal for a minimalist ontology" generalizes the primitive ontology approach to quantum mechanics. He constructs a general ontological framework that allows for a realist interpretation of physical theories but that does not fall prey to the pessimistic induction, in terms of an extremely parsimonious ontology that postulates only permanent matter points individuated by changing distance relations. Where other realist interpretations of science take properties and laws of nature relating them to be part of the ontology, Esfeld argues that both properties and laws belong to the representation that science constructs in order to account in the best and simplest way for the evolution of the configuration of matter.

In his paper "Physical Modality, Laws and Counterfactuals", James Woodward elaborates a way of accounting for the metaphysical concept of modality within physics without postulating any "non-Humean entities" such as powers or dispositions. Physics makes or justifies claims about what is and is not possible for certain systems. Woodward suggests that it is possible to account for such modal claims in terms of the concepts of invariance and independence, in a framework that relies on a strict distinction, in the description and scientific explanation of facts, between laws and initial conditions. Manuel Martinez paper "Synergic kinds" suggests an improvement on the concept of natural kinds as "homeostatic property clusters". In a similar spirit as Woodward's paper, Martinez elaborates the concept of a natural kind without positing any special "metaphysical entities", such as essences, but rather explores the conditions under which the use of natural kinds can be useful in science, as a conceptual tool for organizing the practice of scientific inference. Using conceptual tools from information science, he shows that the concept of a homeostatic property cluster is too narrow to cover all cases in which science uses natural kinds. In synergic kinds, the basis for inductive inferences is more complex than in homeostatic kinds: In synergic kinds, the presence or absence of several properties forms a non-redundant basis for inferences to other properties; whereas in cluster kinds, the property of one (sometimes but not necessarily an essential) property is sufficient as an inference base, information about other properties in the cluster being redundant.

The last two papers explore metaphysical consequences of case studies in specific sciences. In his paper "Neural Correlates without Reduction: The Case of the Critical Period", Muhammad Ali Khalidi shows that what is presently known about the neural correlates of the cognitive phenomenon of a critical period is incompatible with various forms of reductionism and eliminativism. He shows how it is possible to evaluate metaphysical theses such as reductionism and eliminativism by paying close attention to the ways the different relevant sciences, i.e. psychology and ethology on one side and neuroscience on the other, construct their categories. As a thesis in metaphysics of science, reductionism claims that every theoretical category of a given "higher-level" science such as psychology or ethology corresponds to a unique category of some underlying lowerlevel science, such as neuroscience, in such a way that it is in principle possible to derive eventually all hypotheses about the higher-level category from hypotheses about the lower-level category. Khalidi shows, for the case of the critical period, that reduction is blocked not only because different neural mechanisms have been discovered to implement critical periods of different perceptual and cognitive capacities in different species but that one and the same neural mechanism can, in different neural and nonneural contexts, implement different cognitive phenomena. Such a "many-to-many" or "cross-cutting" relationship between neural and cognitive kinds is incompatible with reduction. In his paper "Incubating a Future Metaphysics: Quantum Gravity", Joshua Norton argues for a strong version of scientific metaphysics, in which even the most abstract and conceptual parts of metaphysics are constrained by contemporary science, and more specifically by fundamental physics. According to contemporary physical theories of quantum gravity, such as loop quantum gravity theory and string theory, spacetime is not a fundamental aspect of reality. These theories are the last in a series of scientific theories that have influenced metaphysics. Other examples are Newtonian physics challenging Aristotle's metaphysical conception of heavenly bodies and their movements, quantum mechanics challenging Leibniz' principle of the identity of indiscernibles, and general relativity challenging Kant's doctrine of space as a form of intuition structured according to Euclidean geometry. Norton argues that the fact that spacetime, as traditionally understood, has no role in contemporary theories of quantum gravity justifies a revolution in metaphysics: It undermines all classical interpretations of the distinction between abstract and concrete. If either string theory or loop quantum gravity theory is true, none of the traditional conceptions of the distinction between mathematical and physical objects is correct.

Introduction:

  New Trends in the Metaphysics of Science Max Kistler Université Paris 1 Panthéon-Sorbonne, CNRS, ENS UMR 8590 IHPST -Institut d'Histoire et de Philosophie des Sciences et des Techniques Synthese 197 (2020), pp. 1841-1846. Published online 24 may 2018. DOI 10.1007/s11229-018-1799-3.