The special sciences are generally taken to include all the sciences above physics, including biochemistry, genetics and the various biological sciences, the brain sciences, cognitive science, psychology, and economics, amongst many others. Because of their growing success over the last century, the special sciences, and their results, play an increasingly central role in philosophy. This is true of issues in the philosophy of mind and psychology, such as the mind-body problem or the nature of emotion, but also in central debates in ethics concerning a person's moral psychology and its implications, in metaphysics, for instance in discussions of personal identity and the possibility of freewill, and in epistemology, through the manifold issues affected by the nature of human cognitive capabilities. Consequently, debates over the nature, and status, of special sciences are understandably vigorous, though unfortunately they are also especially challenging because of the wide range of issues they incorporate, the often technical formulations of positions, and the implicit nature of many of their commitments. Given these difficulties, one must, first, illuminate the key questions about special sciences and then, second, provide a road map to the major positions and ongoing areas of dispute.
There have arguably been two primary, and hard-fought, foundational issues about the nature of special sciences, though for historical reasons only one of these questions has received widespread explicit discussion. First, there is the issue of the dispensability of the various special sciences themselves. That is, whether humans will be able to completely replace the special sciences, their theories, explanations, laws, and, ultimately, predicates (i.e., words or terms), with the predicates, laws, explanations, and theories of physics. Because there is clearly no practical opportunity of actually dispensing with the special sciences in the foreseeable future, the contested question is whether in principle special sciences, at some future point, could be dispensed with in favor of a more fully developed physics. Can humans, in principle, dispense with the special sciences, their predicates, such as neuron or diabetic, and the explanations couched in terms of them? (In discussing this issue, the in principle dispensability of special science predicates will be referred to for simplicity). On one side, the inter-theoretic reductionist argues that in order to fully explain and understand the natural world one ultimately only needs physics and its predicates, whereas, on the other side, the inter-theoretic anti-reductionist argues that humans cannot do without the special sciences and their proprietary vocabulary.
In contrast, rather than focusing upon words or explanations, the second foundational topic asks which entities, for example properties and individuals such as neurons and being diabetic, should be accepted as the truth makers of the best scientific explanations and theories. In this debate, in one corner is the ontological reductionist who argues that, when properly understood, the sole truth makers for scientific theories and explanations are the entities of physics—thus, really, only individuals like quarks and their properties of spin, charm, and charge should be taken to exist. In the other corner is the ontological non-reductivist who argues that, in addition to the entities of physics, it must also be accepted that the world contains the properties and individuals apparently posited by the special sciences—for example, individuals such as neurons and properties such as being diabetic.
Given the thoroughly ontological nature of this second question, it must be carefully noted that some philosophers accept the existence of only one genuine issue—the first. For example, many scientific anti-realists, such as the positivists and their intellectual descendents, take broad ontological questions to be, in some sense, illegitimate and argue that one must rest simply with the first kind of question about theories, explanations, and predicates. However, since the mid-1970s, scientific realism has reemerged and argues that the best scientific theories allow humans to know about entities in the world. As a consequence, many scientific realist philosophers now accept the legitimacy of both questions about what scientific predicates are in principle indispensable and the consequent issue of which worldly entities should be taken to be the truth makers of the true, and in principle indispensable, scientific theories and explanations using such predicates.
Bearing these two issues in mind, modern discussions of special sciences can be examined and arguably start, in the 1950s, with the positivists' account of special sciences, which grows from Ernest Nagel's (1961) model of inter-theoretic reduction. In its most plausible version, Nagel provided machinery that putatively allowed the laws of special sciences to be explained by using identity statements relating special science predicates and predicates of lower level sciences, in combination with the law statements of lower level sciences, to derive the law statements of special sciences. For this entry's purposes, what is important is that it was claimed that, in principle, one could consequently derive, and explain, all the laws of the special sciences from the laws of physics. Thus it was concluded that special sciences and their predicates are in principle dispensable. As a result, the Nagelian picture of special sciences takes them to be analogous to the line chefs who are needed in restaurants to speedily prep difficult and complex subject matter, but where ultimately the master chef, in physics and its predicates, would, in principle, suffice to get the job done (i.e., to explain and understand all phenomena).
As befits positivism's suspicion of ontology, the Nagelian picture is focused upon the relations of predicates, law statements, theories, and other semantic entities. However, obvious ontological conclusions flow from the Nagelian account, though, for the ideological reasons noted earlier, these implications were rarely made explicit. When one establishes identity statements, then one shows that there is only one entity referred to by two predicates, rather than two entities as was previously supposed. Through such identity statements one thus plausibly reduces one's ontology. Furthermore, if as a result of such identity statements one only needs physics and its predicates in order to account, in principle, for everything about the natural world, then, at least intuitively, parsimony considerations suggest that the entities of physics are the only entities that actually exist. In this manner, the Nagelian view of special sciences provides the background to recent debates with a trenchant defense of inter-theoretic reductionism, and the in principle dispensability of special sciences and their predicates, implicitly combined with a thorough ontological reductionism that merely accepts the existence of the entities of physics, such as quarks and their properties.
During the 1960s, 1970s, and 1980s, philosophers of science more closely examined the actual nature of particular special sciences, primarily psychology and biology, to show that, contrary to the Nagelian claims, such disciplines and their predicates are in principle indispensable. Though a range of evidence was used to defend this conclusion, Jerry Fodor's (1974) so-called Multiple Realization Argument was the most prominent of these defenses. The latter argument's crucial premise is the observation that the predicates of the special sciences refer to properties that are composed, or multiply realized, by heterogeneous combinations of the properties studied by physics. For example, the economic predicate "has monetary value" refers to the properties composed by the physically heterogeneous combinations of properties found in paper, metal, plastic, and even shells. Such multiple realization means that there is a failure in getting the identity claims necessary to drive the Nagelian program—having monetary value, and other special science properties, simply are not identical to any particular combination of physical properties.
As well as undermining the Nagelian's key argument for the dispensability of special sciences and their predicates, multiple realization was also used to provide positive arguments for the in principle indispensability of such predicates by Fodor, William Wimsatt (1976), Philip Kitcher (1984), and others. Though differing in their details, these positive arguments putatively show that—given the physical heterogeneity of the combinations of physical properties that realize special science properties—the predicates of physics will fail to articulate the commonalities between the multiply realized properties, like having monetary value, studied by the special sciences. For the predicates of physics simply frame the physical differences amongst the heterogeneous realizers of special science properties. Consequently, it is argued in various ways that the proprietary predicates of the special sciences are also necessary, in principle, to fully account for the multiply realized properties these disciplines study.
Though many philosophers of science have worked to articulate this position, for simplicity the latter account of special sciences will be referred to as the Fodorian view; and, as well as its claims about predicates, this position again lends itself to further ontological conclusions. In fact, the realization of special sciences properties by the properties of physics is explicitly combined by Fodor, and others, with the idea of the implementation of special science mechanisms by mechanisms of physics (and implicitly with the constitution of individuals of the special sciences by the individuals of physics).
As a result, the Fodorian view is apparently a version of ontological non-reductivism, for it assumes that the world is a compositional hierarchy containing many levels of distinct properties, individuals, and mechanisms bearing complex compositional relations to other levels of entities until one bottoms out (so far as we now know) with the entities studied by physics. The Fodorian picture thus takes a diametrically opposed view of the special sciences than the Nagelian account, arguing that, rather than leaving too many cooks preparing the broth, the special sciences and their proprietary predicates are, in principle, necessary in order to fully understand and explain the variegated levels of multiply realized properties, multiply constituted individuals, and multiply implemented mechanisms that the special sciences take as their objects of inquiry.
As the dominant position, the Fodorian view has received sustained critical attention and two tendencies are worth noting here. First, there is a significant, and continuing body of work that follows various strands of the Nagelian view, either by seeking to provide technical machinery that establishes the in principle dispensability of special science predicates, or by looking at a wider range of scientific cases to drive such machinery, or both (Hooker 1981, Bickle 1998). However, Jaegwon Kim has recently pioneered a second approach that diverges radically from the Nagelian framework's semantic focus. As a response to the Fodorian view, Kim (1998) instead champions what might be dubbed the "metaphysics of science"—the careful examination of ontological issues as they arise in sciences. The resulting strategy proceeds, first, by more carefully examining the nature of an ontological claim about the special sciences central to the Fodorian picture, and then, second, by seeking to show that when the metaphysics of this notion is properly understood it fails to support the conclusions claimed by the Fodorians.
Perhaps the most important of these critical arguments focuses on the realization relation itself. Crudely put, certain property instances, the realizers, realize another property instance only if the causal powers contributed by the realizers non-causally suffice for the powers individuative of the realized property, but not vice versa. Kim (1998) has consequently argued that, given this core feature of the realization relation, considerations of ontological parsimony make it prima facie plausible that it should only be accepted that there are realizer property instances. This grounds a new form of ontological reductionism, what Kim terms the functionalization model, which uses the Fodorian view's own commitment to realization relations in the special sciences to reduce the ontology of the sciences simply to the ultimate realizers—the properties of physics. Other important examples of such arguments driven by work in the metaphysics of science are found in Kim (1992) and Lawrence Shapiro (2000), which each use more precise metaphysical examinations of multiple realization to attack the scientific legitimacy of multiply realized properties, as well as explanations using predicates referring to them and any science that seeks to study them—again turning the Fodorian account of special sciences against itself.
Naturally, there have been responses to such critical arguments focused on the metaphysics of science (see, for instance, Fodor  and Gillett ) and, as yet, it is far from clear where this renewed ontological focus will finally lead. However, ongoing debates over special sciences have arguably changed in a fundamental way, not least by the range of new questions faced. Can the ontological non-reductivism, the levels position, which many assume is the backbone of the Fodorian view, be sustained or does it collapse upon itself as Kim's functionalizing reductionism seeks to show? And is there space for a third, previously unappreciated, type of view about special sciences that combines ontological reductionism, driven by the metaphysical argument underpinning Kim's functionalizing reduction, and a commitment to the in principle indispensability of special sciences, founded upon the Fodorian's reasoning that complex aggregates can only be fully understood using special science predicates? As well as these concerns about global views of special sciences, one also now confronts a prior set of more specific issues about the foundations of the special sciences. For example, what is the nature of composition generally in the special sciences, as well as particular compositional relations such as the realization relations between properties in the sciences?
The answers to the more particular questions in the metaphysics of science are important because, as has been seen, they underpin many of the ongoing disputes between proponents of competing global accounts of the special science themselves. Moreover, all of these questions about the foundations of the special sciences will only become more pressing. For as humans increasingly look to the sciences to understand their own nature, then what is said about special sciences, like genetics, neurophysiology, and psychology, will also have more and more obvious implications for what they must consequently conclude about themselves.
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Carl Gillett (2005)