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Scientific Reduction... Though conservative realism is the norm, some reductionists take a more anti-realist view. In such cases the reducing phenomena are taken to replace the prior phenomena which are in turn eliminated. The idea of mental illness as a type of psycho-neural disorder replaces the idea of demon possession


Scientific Reduction

First published Tue Apr 8, 2014; substantive revision Wed Feb 27, 2019

The English verb ‘reduce’, derives from the Latin ‘reducere’, whose literal meaning ‘to bring back’, informs its metaphorical use in philosophy. If one asserts that the mental reduces to the physical, that heat reduces to kinetic molecular energy, or that one theory reduces to another theory, one implies that in some relevant sense the reduced theory can be brought back to the reducing theory, the mental can be brought back to the physical, or heat can be brought back to molecular kinetic energy. The term ‘reduction’ as used in philosophy expresses the idea that if an entity x reduces to an entity y then y is in a sense prior to x, is more basic than x, is such that x fully depends upon it or is constituted by it. Saying that x reduces to y typically implies that x is nothing more than y or nothing over and above y.

Though the term ‘reduction’ in this use may not correspond to everyday use nor to scientific discourse, its technical meaning is not fixed by mere stipulation. ‘Reduction’ is a term of natural language, and, building upon its common metaphoric meaning philosophers use it to designate relations of particular philosophical importance in a number of closely related fields, especially in the philosophy of science, the philosophy of mind, and metaphysics.

The notion of scientific reduction as used in contemporary analytic philosophy differs from conceptions of reduction according to which we learn about the instantiation of reduction-relations on a purely a priori basis from basic religious, metaphysical or epistemological principles. ‘Scientific reduction’ applies to reductionist claims supposedly justified by scientific evidence and the success of science.

Different accounts of scientific reduction have shaped debates about diverse topics including scientific unification, the relation between (folk-)psychology and neuroscience, the metaphysics of the mind, the status of biology vis à vis chemistry, and the relation between allegedly teleological explanations and causal explanations. Understanding the relevant notions is thus a prerequisite for understanding key issues in contemporary analytic philosophy. Moreover, the notion of reduction itself has become a target of recent philosophical discussion, especially in the philosophy of science and in metaphysics.

1. Historical background

Conflicts between reductionist views and their dualist and pluralist metaphysical rivals have loomed large in the history of philosophy. The classical debate between materialism and mind-body dualism is simply the most familiar of many such disputes. The Cartesian dualist is committed to distinct types of substances—minds and bodies each with its own essence. Different versions of dualism—interactive, epiphenomenal, parallelist – disagree about whether the two causally interact, but they all assert the ontological difference between the mind and body. Neither reduces to the other. They are distinct, and both are real.

Most contemporary reductionist as well as eliminativist positions include some commitment to materialism or physicalism—the view that the physical or material provides the fundamental reductive base. However reductionism per se is ontologically neutral. The concept of reduction entails no specific ontological positions. It is logically independent of physicalism or materialism. Indeed reductionism could be true of the entities in a totally nonphysical world as long as it had some base and everything in that world reduced to that base. Reduction is a general relation between entities or theories that might hold in many sorts of specific cases.

Some historical reductivists have been resolutely opposed to materialism and physicalism. Bishop Berkeley’s phenomenal idealism reduced ordinary tables and chairs to collections of ideas and denied the existence of matter. On Berkeley’s reductive view everything real reduces to minds and ideas. Idealism, as proposed by Fichte can also be given a reductive interpretation. On this view, the non-mental does not exist over and above the mental. The non-mental can be fully explained in terms of the mental, and it can be ultimately assimilated to the mental. According to this interpretation, everything just is ultimately mental. Idealists are reductionists whose reductive base is mental.

Idealism does not have many contemporary supporters, but there are current reductive theories that are logically independent of physicalism though consistent with it. Nominalism, for example, can be regarded as a reductive theory about kinds. The nominalist argues that the supposed reality of abstract objects—whether properties and kinds or numbers and sets—can be reduced to facts about concrete objects and our ways of talking about them. French structuralists can be seen as reductionists about subjects, or subjectivism; they suggest that the functioning of individual intentions and goals reduces to the inner workings of larger discourses.

Carnap’s mid-20th century unificationism presents an interesting example of ontologically neutral, phenomenological reductionism. Carnap’s reductionism states that:

… science is a unity, [such] that all empirical statements can be expressed in a single language, all states of affairs are of one kind and are known by the same method. (Carnap 1934: 32)

This interpretation covers several aspects—semantic ones (concerning the expressive power of a language), metaphysical ones (states of affairs being of one kind), and methodological ones. Assume that any empirical statement, or proposition, can be expressed in a single language. This language is, on Carnap’s view, a language that employs observational concepts only. This seems to amount to an ontologically neutral form of reductionism. In principle, any reductionist—the idealist as well as the materialist—can subscribe to this thesis. Disagreement will start only when it comes to specifying the ontological status of the kinds designated by this language’s observational expressions. Are they external mind independent properties of objects or are they phenomenal properties directly present in perceptual experience?

In sum, though most contemporary reductionists are also physicalists, there are many actual reductionist positions that are independent of materialism and physicalism. Such positions have played an important role in the history of philosophy, and some are actively supported today even as forms of scientific reduction.

The notion of reduction itself has become a target of philosophical investigation. If science is to provide (at least partial) answers to questions of reduction, such as whether the mental reduces to the neural, then we need to understand what reduction requires. What are the necessary conditions and how might scientific evidence show whether they are fulfilled?

J.J.C. Smart provided an influential, tentative description of the reduction relation according to which an entity x reduces to an entity y only if x does not exist ‘over and above’ y (Smart 1959: 143). Smart frames the question as one of existence, an ontological question: What really exists? How many basic kinds of things exist? Approaching the issue from a different slant, the philosophical notion of ‘reduction’ might be cashed out in terms of an explanatory relation. If x reduces to y, then it can in a relevantly strong sense be explained in terms of y. Take an example from Ernest Nagel. Describing the possible reduction of headaches, he writes that when

the detailed physical, chemical, and physiological conditions for the occurrence of headaches are ascertained … an explanation will have been found for the occurrence of headaches (Nagel 1961: 366)

Alternatively, reductions can be conceived of as consisting in a specific form of assimilation—a description that can also be found in Nagel (1961):

[In a reduction, a] set of distinctive traits of some subject matter is assimilated to what is patently a set of quite dissimilar traits. (Nagel 1961: 339f.)

Reductivists are generally realists about the reduced phenomena and their views are in that respect conservative. They are committed to the reality of the reducing base and thus to the reality of whatever reduces to that base. If thoughts reduce to brain states and brain states are real, then so too are thoughts. Though conservative realism is the norm, some reductionists take a more anti-realist view. In such cases the reducing phenomena are taken to replace the prior phenomena which are in turn eliminated. The idea of mental illness as a type of psycho-neural disorder replaces the idea of demon possession. Demons and their voices have no role or reality in the new theory. []


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