J Gen Philos Sci (2014) 45:33–44 DOI 10.1007/s10838-014-9267-3 ARTICLE

How Much Philosophy in the Philosophy of Chemistry? Alexandru Manafu

Published online: 12 October 2014  Springer Science+Business Media Dordrecht 2014

Abstract This paper aims to show that there is a lot of philosophy in the philosophy of chemistry—not only in the problems and questions specific to chemistry, which this science brings up in philosophical discussions, but also in the topics of wider interest like reductionism and emergence, for which chemistry proves to be an ideal case study. The fact that chemical entities and properties are amenable to a quantitative understanding, to measurement and experiment to a greater extent than those in psychology or biology, makes chemistry an ideal case study for those interested in reductionism and emergence. Keywords physics

Philosophy of chemistry  Reductionism  Emergence  Completeness of

1 Introduction This paper argues that the topics of reductionism and emergence can receive exquisite treatments if one takes chemistry as a case study. The message that this paper wants to convey is that there is a lot of philosophy in the philosophy of chemistry—not only in the problems and questions specific to chemistry, which this science raises in philosophical discussions, but also in topics of wider interest like reductionism and emergence, for which chemistry proves to be an ideal case study.

2 Chemistry and Reductionism Reductionism has been and continues to be a much debated topic in philosophy of science. Despite the fact that a lot of the initial enthusiasm for reductionism has dwindled in the

A. Manafu (&) Institute for History and Philosophy of Science and Technology, University of Paris 1 Panthe´on-Sorbonne, 75006 Paris, France e-mail: [email protected]

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light of the many problems that the positivistic philosophy of science had faced, reductionism continues to gain a lot of attention; new reductionist accounts or improvements and defences of the old ones have continued and continue to appear in many sub-fields of philosophy of science, including philosophy of biology and in philosophy of cognitive science (Bickle 1998; Dizadji-Bahmani et al. 2010; Esfeld and Sachse 2011). I want to argue that one can shed more light on this topic if one takes chemistry as a case study. This idea is supported by several observations: 1.

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Chemical properties are amenable to a quantitative understanding, to measurement and experiment to a greater extent than those in psychology or biology (Scerri and McIntyre 1997; Humphreys 1997). Chemistry is free from the difficult problems afflicting our understanding of the mind, such as the phenomenal character of conscious experience—what Chalmers (1995) called ‘‘the hard problem of consciousness’’, and which he argued has no analog in the physical sciences (1996).1 Chemistry is the discipline that is in some sense closest to physics, and therefore it could provide us with the first and most clear example of domain reduction (or the failure thereof).2 Both chemistry and physics are mature sciences, and one can hope that this would allow us to scrutinize the relations between them with great clarity.

Despite the clarifying potential of choosing chemistry as a case study, when it comes to the reduction of chemistry to physics, the degree of disagreement amongst philosophers is astounding. Some consider the reduction of chemistry to physics as a prototypical example of intertheoretic reduction. For example, in his well-known account of reduction, Nagel claims that ‘‘only the stupendous mathematical difficulties involved in making the relevant deductions from quantum theoretical assumptions seem to stand in the way’’ of the complete reduction of chemistry to the quantum theory of atomic structure (1961, 365). In a very influential classical paper on reduction, Kemeny and Oppenheim claimed that ‘‘a great part of classical chemistry has been reduced to atomic physics’’ (1956). And in a recent encyclopedia of philosophy entry on reductionism, it is claimed that the reduction of chemistry to physics is ‘‘a standard example’’ of reduction (Ney 2008). On the other hand, many of those working in the philosophy of chemistry (chemists or philosophers) are more reserved, to say the least. For example, in reaction to the view of Kemeny and Oppenheim, Primas has claimed that the philosophical literature on reductionism is teeming with scientific nonsense (1991, 163); Also, many philosophers of science including Bunge (1982), Scerri (1991, 1994, 2004), Hendry (2006), and Needham (2005) have expressed various kinds of doubts about the reducibility of chemistry to the underlying microphysics. One well-known disputed case is that of water being H2O. Whether one construes this strictly as a question of reductionism or one of microstructuralism, the problem of whether water is H2O has attracted a lot of attention (by e.g., van Brakel 2000, ch. 3; Needham 2000, 2002, 2008a, b; Hendry 2008b; Chang 2012, inter alia).3 One balanced view is that ‘‘water is the 1

This doesn’t mean that chemistry doesn’t pose difficult problems of its own, for example the problem of chemical substance, matter or material, element, bond, etc. But it does mean that chemistry is free of one of the most vexing problems of contemporary philosophy, and this is a good thing.

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Chemistry is the first ‘‘special science’’ according to the layer-cake model of the sciences introduced by the classical article of Putnam and Oppenheim (1958).

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Microstructuralism is the view that chemical kinds can be individuated solely in terms of their chemical microstructure (Bird and Tobin 2008).

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substance that has H2O molecules as its ingredients, although most live on in the water’’ (Hendry 2008b, 113). Some aspects of chemistry that cannot be easily reconciled with reductionism relate to chemical practice. As Bensaude-Vincent and Simon (2008) have argued, when science is seen as an essentially unified homogeneous project aimed at solving well-defined ultimate ontological questions, then it is quite natural to oppose physics and chemistry, with the latter playing a secondary role. But they claim that this is not the only way of presenting the philosophical issues; they suggest that chemists have come to define their own exigencies with respect to matter, and have developed their own characteristic philosophical territory, independent from physicists. They argue that this was possible insofar as chemists rejected the hierarchical model of science that underlies reductionism. A related point was made by van Brakel (2000) who distinguished between a manifest and a scientific image, and argued for the primacy of the manifest image and against reductionism. Far from being detrimental to our understanding of reductionism in science, this degree of disagreement about the reduction of chemistry could in fact be fruitful. For in philosophy progress is often measured not by answering a question one way or the other, but by clarifying the question. Thus, taking chemistry as a case study has the potential to expose confusions and misunderstandings, and this, in turn, could help the two parties reach a consensus. One of these confusions is between two concepts of reduction, a weak one and a strong one. On the weak concept of reduction, chemical compounds and substances are viewed as being composed of nothing else except the entities that microphysics talks about. Also, on this weak concept of reduction, chemical phenomena do not involve anything else except microphysical phenomena, and they all happen in accordance with microphysical laws. Assuming this weak concept of reduction is tantamount to assuming physicalism: chemical entities and properties are solely the result of the interaction of microphysical entities and properties; in other words, there are no sui generis chemical entelechies, entities, or forces. On the strong concept of reduction, reduction is conceived as a relation between theories (or, more generally, theoretical frameworks, including models). The strong concept of reduction requires that all chemical terms be given explicit definitions in microphysical terms, and that claims and observations made by chemistry be deduced from the claims of microphysics. The strong concept of reduction goes beyond physicalism: it excludes not only the existence of sui generis chemical forces, entities, or entelechies, but also that of sui generis chemical properties, kinds, laws, and explanations. Both the weak and the strong concept of reduction have been challenged. Weak reduction would be threatened if we discovered a kind of emergence that denies the causal closure of physics. If chemicals have physical effects that cannot be in principle entirely explainable in terms of physical causes, then there must be some sui generis chemical entities, properties, or forces. That is, it would follow that chemicals cannot be understood exclusively in a physicalistic framework; consequently, we would have to expand our ontology to include sui generis chemical entities, properties, or forces. To most contemporary philosophers of science, the prospects for such an expansion are dim. However, a few philosophers of science who looked at chemistry more carefully have given reasons to think that chemistry calls into question physicalism. I will return to this issue later, after I discuss emergence. For now I will just say that the truth of physicalism and the principle of the causal closure of physics are important topics in contemporary philosophy of science, which could receive exquisite treatments if one takes chemistry as a case study. The results obtained would have ramifications in metaphysics and in other philosophical areas, such as the philosophy of mind.

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Strong reduction has also encountered difficulties. I give below a typology of these difficulties and suggest that taking chemistry as a case study might help philosophers bring into focus these difficulties, better understand them, and find strategies to solve them. 2.1 Computational Difficulties Reduction is often conceived of as deduction; but computational difficulties stand in the way of such deductions. Analytical solutions to the Schro¨dinger equation can be obtained only for very few chemical species like hydrogenic atoms and the diatomic hydrogen cation (Scerri 1991, 1994; Scerri and McIntyre 1997; Seck 2012). The true philosophical significance of this fact for reduction has not yet been fully elucidated. By choosing chemistry as a case study, philosophers interested in the technical aspects of the reductive process would have a lot of work to do. One question which is worth examining in detail is whether the computational difficulties in obtaining chemically relevant information about atoms and molecules (like electron configurations, bond angles and dipole moments) by applying the Schro¨dinger equation to a complex quantum–mechanical system pose just a practical difficulty for reduction, or whether they create deeper concerns. For example, if the satisfactory ab initio deduction of the properties of a chemical species requires more computational power than there could possibly exist in our universe, which might be the case for big molecules (Luisi 2002), then maybe the ‘in principle versus in practice’ dichotomy must be supplemented with a third category—in this case, the deduction would not only be impossible in practice, but it will be in principle impossible in practice (Bedau 2008). How the existence of this third category of impossibility bears upon the reduction of chemistry is currently an unexplored topic. But this topic is inherently philosophical: it brings into focus metaphysical issues (e.g., physicalism, modality) as well as epistemological issues (e.g., having to do with the nature of scientific knowledge, especially when it is obtained by computation and deduction). 2.2 Appeal to Approximations and Idealizations Approximations and idealizations, as well as models, have been getting a lot of attention in philosophy of science. A lot of the philosophical research on these topics has happened in the philosophy of physics and philosophy of biology. But when it comes to approximations and idealizations, chemistry is equally worthy of philosophical attention. By looking at chemistry, philosophers of science would expand their base of case studies with a large number of very clear examples. One such example is the Hartree–Fock method of approximating the wave-function of a quantum many-body system in stationary state. This method allows physical chemists to solve the time-independent Schro¨dinger equation for a multi-electron atom or molecule. Since most chemical species are many-electron systems for which the Schro¨dinger equation cannot be solved analytically, their solutions must be found numerically. To achieve this task, the Hartree–Fock method relies on a number of approximations and idealizations. One of these is the Born–Oppenheimer approximation (BOA), which represents the wave-function of a molecule as a product of its electronic and nuclear components: Wwholemolecule ¼ Welectrons  Wnuclei . In doing this, BOA assumes that the nuclei are fixed. But this is strictly speaking false, since molecules have parts that rotate and/or vibrate. Another approximation is that the momentum operator is assumed to be non-relativistic; thus, relativistic effects are not taken into account. Yet another approximation in the Hartree–Fock method is that the solution is assumed to be a linear

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combination of a finite number of basis functions, and the finite basis set is assumed to be approximately complete. Another approximation used in physical chemistry is the orbital approximation, and it is a strategy that allows us to learn about the wavefunction of a multi-electron atom on the basis of the individual electrons. The orbital approximation assumes that the wavefunction of a multielectron atom can be thought of as resulting from the separate contributions of each electron wavefunctions, and it can be written as a product of individual atomic orbitals: wðr1 ; r2 ; . . .rn Þ ¼ /1 ðr1 Þ/2 ðr2 Þ. . ./n ðrn Þ. It is a useful tool in the attempts at solving the Schro¨dinger equation for atoms that have more than one electron. When applied to multi-electronic atoms, the (atomic) orbital approximation assumes that each electron behaves independently of the others, and thus the electronic Hamiltonian can be b1 þ H b2 þ    þ H b n . The be ¼ H separated into as many components as there are electrons: H correlation between the spatial position of electrons due to their Coulomb repulsion is not taken into account. What is the philosophical import of these approximations? On most reductionist accounts devised by philosophers of science, reduction is typically conceived as explanation. However, on most models of explanation, including the deductive nomological (DN) model and cognate models, the explanations need to be true. But approximations such as those used in physical chemistry are strictly speaking false. Now, explaining with falsehoods is not a new problem in philosophy of science. What chemistry brings to the table is a set of very clear examples of approximations which could potentially support or provide counterexamples to an existing solution, or help us find a new solution. For those interested in the technical aspects of reduction, or in approximations, idealizations, and modeling, quantum chemistry provides interesting case studies. 2.3 Appeal to Rules and Principles for Which We Don’t Really Possess a Quantum– Mechanical Justification, or for Which the Justification is Disputed The determination of the electron configuration of atoms, ions or molecules makes use of rules and principles for which we don’t really possess a quantum–mechanical justification. The Aufbau principle consists in a number of explicitly stated rules that allow us to understand the atom as a physical system that is build by successively adding electrons around the nucleus: 1.

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The principle of the minimum energy. The electrons occupy atomic orbitals in such a way that the total energy of the atom is a minimum; they fill orbitals starting at the lowest possible energy states before filling higher states. The Pauli exclusion principle. Every electron in an atom is described by its own distinct set of four quantum numbers, not shared with any other electron. This entails that a given orbital is to be occupied by no more than two electrons, case in which their spins, denoted by the ms quantum number, are paired. The Madelung rule. Orbitals with a lower n ? l value are filled before those with higher n ? l values. Hund’s rule of maximum multiplicity. Electron pairing will not take place in orbitals of the same sub-shell until orbitals are singly filled by electrons with parallel spin.

The Pauli exclusion principle, the Madelung rule, and Hund’s rule of maximum multiplicity are useful tools in determining electron configurations. Some attempts to provide

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good theoretical foundations for these rules have been proposed (Lo¨wdin 1969; Weinhold and Bent 2007; Ostrovsky 2003, 2005), but they have been criticized (e.g., Scerri 2009). What are the consequences of these theoretical shortcomings for the question of reduction? The answer is debatable. Hall (1986) argued that chemistry cannot be said to have been reduced to quantum mechanics because the reduction would require the use of the Pauli exclusion principle, for which the quantum–mechanical derivation is disputed. However, Scerri (1995) has argued that Hall is mistaken in attaching any importance to the lack of derivability of the exclusion principle from quantum mechanics. This leaves us with the following question: if the lack of a non-disputed quantum–mechanical derivation of these principles does not preclude the reduction of chemistry to physics, then how should we best construe inter-theoretic reductions? Answering this question is a philosophical matter, but it is occasioned by reflections on chemistry. 2.4 Chemical Terms that Don’t Seem to Have a Microphysical Reference Another apparent obstacle in the way of the reduction of chemistry to physics is the existence of chemical terms that don’t seem to have a microphysical (quantum– mechanical) reference: chemical bond (Primas 1983), chemical structure (Woolley 1978), orbital (Scerri 1991; Scerri and McIntyre 1997; Post 1974). For example, Primas (2004) has claimed that ‘‘most theoretical concepts of chemistry have not yet been successfully reduced to quantum mechanics, and it is an open question whether such a reduction can always be achieved.’’ Primas gives as an example the concept of valence, which is of great importance to chemistry, but for which we have no theory, he claims, because all valence models have counterexamples. How does the existence of chemical terms which cannot be defined in the terms of quantum mechanics affect the reduction of chemistry to physics? One answer might be that it doesn’t. On the model of reduction advocated by Kemeny and Oppenheim (1956), Putnam and Oppenheim (1958), reduction obtains when any part of the observational data explained by a certain theory (the reduced theory, which is typically coarse-grained) is explainable by means of another, typically more fine-grained theory (the reducing theory). So a direct connection of the terms present in the vocabularies of the two theories is not necessary on this model of reduction; the connection between two theories may be effected indirectly, via their respective connections with the observational data. However, arguably the most discussed model of reduction (offered by Nagel 1961) does require a direct connection between the vocabularies of the two theories. This requirement is formulated quite explicitly by Nagel as the ‘‘connectability condition’’: to effect a reduction between two theories, the terms of the reduced theory must be translated in the terms of the reducing theory by appeal to ‘‘bridge laws’’. On Nagel’s model, the reduction of chemistry to microphysics would depend on connecting the chemical terms with the microphysical terms. However, if there are chemical terms which cannot be defined in microphysical terms, the reduction of chemistry would be, on the Nagelian schema, impossible. This situation prompts the following question: if there are terms that don’t have a microphysical reference, how should we construe reduction? Is the model advocated by Kemeny, Oppenheim and Putnam superior to the model advocated by Nagel? Or maybe an eliminativist model of reduction (in which the end result is the elimination of chemistry in favour of quantum mechanics) would be more appropriate? Is there a single model of reduction which is suitable for the reduction of theories regardless of the domain, or rather different domains or sciences demand their own model of reduction? These questions are

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inherently philosophical, and choosing a science like chemistry as a case study brings them into focus. 2.5 The Multiple Realization of Chemical Properties Many chemical properties are defined functionally, by pointing to a certain functional role or behaviour. For example, properties like being an acid, a base, a metal, a reductant, an oxidant are defined not by pointing to a single microphysical feature that is present in all acids, bases, etc., but by specifying their behaviour in chemical reactions. Since there are many systems of electrons and nuclei that can carry out the same chemical behaviour, there is prima facie evidence that many chemical properties are multiply realized. Now, the topic of multiple realization has received a lot of attention in philosophy of science. It has inspired antireductionist arguments in the philosophy of mind (Fodor 1974; Putnam 1975a, b), philosophy of biology (Kitcher 1984, 1999; Kincaid 1990), and it was also discussed in the philosophy of physics (Batterman 2000, 2002). But the alleged negative consequences of multiple realizability for reduction have been challenged in several ways. For instance, it has been argued that multiple realization does not impede reduction (Richardson 1979; Sober 1999; for a response see Marras 2005). Also, it has been argued that the thesis of multiple realizability has been accepted too uncritically by philosophers (Shapiro 2000); some have claimed that empirical findings coming from neuroscience contradicted the multiple realizability thesis in psychology (Bickle 1998). It has also been claimed that the thesis of multiple realizability is underdeveloped and vague. That some philosophers took the mammalian eye and the octopus eye as virtually identical realizations of an eye (Putnam 1975b), while others treated them as distinct realizations (Block and Fodor 1981) is an ironic fact, which is symptomatic of this. Philosophy of chemistry has a lot to contribute to these discussions. Since there are many prima facie cases of multiple realizability in chemistry, taking this discipline as a case study could be extremely useful not only for clarifying and giving an adequate formulation of the multiple realizability thesis, but also for examining how multiple realizability affects reduction. Chemical systems are much simpler than biological or psychological systems, and this may enable philosophers of science to examine the scientific and philosophical complexities of the multiple realizability thesis with increased clarity and depth. This does not mean that chemistry is less interesting than biology or psychology; it just means that the examples it affords allow for a more thorough examination of certain philosophical questions.

3 Chemistry and Emergence Besides being frequently given as an example of reduction, chemistry is just as often (or perhaps even more often) given as an example of emergence. It has been claimed that ‘‘chemistry seems to offer the most plausible example of emergent behaviour’’ (Broad 1925, 65), and that chemistry is ‘‘the embodiment of emergence’’ (Luisi 2002, 184). This situation is a bit puzzling, because reduction and emergence are often (though not always) regarded as mutually exclusive. This is possible because ‘‘reduction’’ and especially ‘‘emergence’’ are unclear concepts. This led to philosophers talking past each other and to what is often perceived as a stalemate in the reductionism versus emergence debate. Taking chemistry as a case study by those interested in emergence could contribute to this

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much discussed topic by offering extremely clear examples and detailed analyses of their consequences, and could contribute to breaking the stalemate. As some philosophers have noted, emergence is not a very well defined philosophical position (Kim 2006). Nonetheless, there is a common set of features that many emergentist positions share. Typically they hold some form of the view that the world consists of a hierarchy of levels or ontological strata (e.g., the microphysical, the macrophysical, the chemical, the biological, the psychological); each level is regarded as dependent on the previous level, but irreducible (in a strong sense) to it. The relation between the lower level and the higher level is usually thought to be supervenience: higher level properties are said to supervene on the lower level properties. These higher level properties (emergents) arise from those at the lower level, but they cannot be predicted on the basis thereof. Emergents are often deemed to have novel causal powers, i.e., they have the capability to produce effects in a way that cannot be anticipated. Often, emergents are said to be capable of downward causation—to have the ability to influence the basal conditions from which they arise. Also, sometimes it is held that emergents involve global rather than merely local properties, and thus they arise only when the basal conditions are characterized by a certain amount of complexity. Thinking about chemistry as emergent has a long and rich history in philosophy. While it is true that one of the main motivations for the development of what it has been called ‘‘the theory of emergence’’ was to account for mental properties (which seem resistant to a purely mechanistic explanation), for most traditional emergentists (i.e., British emergentists like Mill or Broad) the paradigmatic example of emergence was that of chemical compounds. For instance, Mill (1882) talked about a chemical mode of causation, which defied the principle of composition of causes (the principle stating that the effect produced by two causes acting together is the sum of the effects of each cause acting independently). For Mill, chemical compounds have properties that are not ‘‘the sum’’ of the properties of their components taken separately or simply juxtaposed, as in a mixture. Mill gives as an example the blue color of copper sulfate, CuSO4, which is not a mixture of the colors of sulfuric acid (transparent) and copper(II) oxide (black), from which it is produced; he also gives as an example the sweet taste of lead diacetate, Pb(C2H3O2)2, which is not the sum of the tastes of its component elements, acetic acid and lead or its oxide. Broad also thought about chemistry as emergent. For Broad, chemistry ‘‘seems to offer the most plausible example of emergent behaviour’’ (1925, 65). Broad linked emergence to unpredictability. According to Broad, the emergent properties of chemical compounds cannot be predicted even in principle from exhaustive knowledge of the properties of the parts—the only way to learn about them is to study samples of those compounds. Broad’s claim about the in-principle unpredictability of the properties of chemical compounds has empirical content—there is a matter of fact whether contemporary physical chemistry can determine the chemical properties of compounds or other chemically relevant information ab initio, purely from first principles. This question is very relevant to the topics of reductionism and emergence, and it cries out for philosophical attention. However, not a lot of ink has been spilled by contemporary philosophers of science trying to answer it. This could be because most philosophers of science may think that the issue is settled and the answer is undoubtedly affirmative; however, as I showed earlier in this paper, the answer might not be as straightforward as it seems. By taking chemistry as their case study, the philosophers interested in emergence as unpredictability might do a better job at determining whether emergence-as-unpredictability exists in reality or only as a theoretical possibility.

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Broad thought that the inability to predict the properties of a chemical compound on the basis of the properties of the components was due to a new type of forces which occur when particles are arranged in certain configurations. These forces, which have been called ‘‘configurational forces’’ (McLaughlin 2008), were thought to be responsible for downwards causation. McLaughlin (2008) thought that contemporary science offers no scintilla of evidence for Broad’s view on emergence, because it offers no evidence for configurational forces. However, contemporary authors like Hendry have revived Broad’s view (Hendry 2006, 2008a). Hendry adopts McLaughlin’s terminology, but he formulates it in terms of Hamiltonians, rather than forces. Using the quantum chemistry of the molecule, Hendry aims to show that there is downward causation in chemistry by showing that there are ‘‘configurational Hamiltonians’’ governing the behaviour of molecules. Hendry asserts that if the behaviour of some composite systems is governed by configurational Hamiltonians, then it is not determined by the more general laws governing their constituents. He gives as an example the CO2 molecule. One could regard the parts of this molecule as quantum mechanical harmonic oscillators and rigid rotators. But to do this, one needs to assume a certain structure for the whole molecule. We can use quantum mechanics to explain the motions of parts of the molecule, but we do it within the context of a given structure for the molecule as a whole (in this case, a linear structure). Then he argues that rather than deriving this structure using resultant Hamiltonians, we put it ‘‘by hand’’—we assume ‘‘configurational Hamiltonians’’. Hendry claims that to the extent that the behaviour of any subsystem is affected by the supersystems in which it participates, the emergent behaviour of complex systems must be viewed as determining, but not being fully determined by, the behaviour of their constituent parts. For Hendry, this is an example of downward causation. Hendry’s view has been questioned by Scerri (2012). Scerri accuses Hendry that he has failed to heed his own warning that ontological considerations should not be anchored to the present state of our theoretical understanding. Also, he invokes Primas (1983) who blames the problem of the apparent ungroundedness of molecular structure on the fact that the molecules are considered as isolated systems. Scerri argues that considering molecules as interacting with other molecules in the environment and being subject to quantum decoherence helps alleviate the problem of molecular structure. He also suggests that the present state of our knowledge does not justify belief in emergence and downward causation, and that an agnostic attitude is the most appropriate. To overcome agnosticism about emergence and downward causation and to establish whether they are real features of our world or just metaphysician’s fictions, more research (both scientific and philosophical) is needed. If one focuses this research on the plausible candidates offered by chemistry, one can hope that progress can be made on this front. In addition to Luisi, Hendry and Scerri, a number of authors have discussed emergence in chemistry, including McIntyre (2007) and Llored (2012).

4 Conclusion So far, many of the discussions about reductionism, emergence, and downward causation took place in the philosophy of mind. But minds are extraordinarily complex systems; the brain is the most complicated physical object in the known universe. Also, neuroscience is in its infancy and many fundamental discoveries are ahead of us. Chemistry, on the other hand, is a mature science; its fundamental principles are well understood, and scientists are not expecting major fundamental discoveries. With regard to its foundations, chemistry is

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in many ways a completed science. This can be seen from the type of research that was awarded the Nobel Prize in the past two decades. Physics is not yet a completed science with respect to its foundations (and this can also be seen by looking at what type of research was awarded the Nobel Prize in physics lately); however, that part of physics which is relevant to chemistry is by and large well known. There are still some notoriously unresolved problems in quantum physics, and it cannot be guaranteed that they won’t have consequences for chemistry, but in general the quantum–mechanical underpinnings of chemistry have been elucidated. By focusing on systems like the ones studied in chemistry, philosophers preoccupied with reductionism and emergence have better chances of making progress on these topics. At the end of his evaluation the arguments for emergence and downwards causation in chemistry, Scerri (2012) called for more philosophical work on these topics: What is required is more work on the questions of reduction, emergence, and causation in the context of the borders between physics and chemistry. At present, the literature contains a few isolated studies […]. This is a little surprising given the frequent calls for more attention to the ‘special sciences’ made by philosophers of science. It is also surprising given that the manner in which chemistry interfaces with physics represents perhaps the ‘first step’ in the reductive hierarchy dealing with the special sciences and their relationship to the fundamental science of physics. (Scerri 2012, 25) I second Scerri’s thoughts. To determine whether emergence (as the British emergentists or contemporary philosophers like Hendry conceived it) is a real phenomenon rather than a metaphysician’s fiction, we need more research, both philosophical and scientific. It would be a good idea for the philosophers preoccupied with these topics to take chemistry as their case study. When it comes to the topics of reductionism and emergence, chemistry has a privileged position relative to the other special sciences: it is the discipline that immediately follows microphysics in the traditional hierarchy of the sciences, and the examples it affords are simpler and amenable to a more rigorous treatment than those of the other special sciences. If the future scientific and philosophical research will support the existence of emergence and downward causation in chemistry, then this will have significant consequences for our image of the world. If downward causation is real feature of some chemical systems, then it would seem that we would have to reject the causal completeness or causal closure of physics—the thesis that ‘‘all physical events are determined (or have their chances determined) entirely by prior physical events according to physical laws’’ (Papineau 1990). Arguably, this would mean no less than the end of physicalism (Hendry 2003). This, in turn, will have repercussions throughout philosophy, in areas such as metaphysics, philosophy of mind, and even ethics.4 Given what is at stake, and the privileged position of chemistry relative to the other special sciences, it is almost imperative that the philosophers interested in reductionism and emergence pay even more attention to chemistry and its philosophy. Acknowledgments I would like to thank two anonymous referees of JGPS for useful comments and suggestions.

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The existence of emergence and downward causation would have a great impact on the problem of free will, which is relevant to philosophy of mind and ethics.

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