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Reductionism is the hypothesis that science is unified by chains of intertheoretic reductions across disciplines, with theories from basic physics providing the ultimate ground. The classic work on intertheoretic reduction in the philosophy of science is chapter 11 of Ernest Nagels The Structure of Science. For Nagel, reduction is logical deduction (derivation) of the statements of the reduced theory TR from those of the reducing theory TB. In interesting cases from sciences history, the TRs contain terms that do not occur within the descriptive vocabulary of TB (e.g., equilibrium thermodynamics contains pressure and temperature, which do not occur in statistical mechanics and the kinetic/corpuscular theory of matter). Such cases are especially prominent in the psychological and social sciences because their theories developed mostly independently of one another. To derive such TRs from TBs in something other than a trivial fashion, Nagel insisted that the premises of the derivation require bridge principles connecting terms across the theories (e.g., temperature in a gas is mean kinetic energy of molecular constituents). Furthermore, most historical scientific reductions are correctivethey indicate where the TR is false. Thus Nagel insisted that the premises of the deduction must contain not only the TB and the necessary bridge principles but also various limiting assumptions and boundary conditions on TBs application, some of which are contrary to fact. These elements circumscribe the falsity in the premises of the valid derivation of a false conclusion (TR ) away from the (presumed true) TB. Challenges to Nagels account were quickly raised by philosophers of science, but virtually every alternative account of intertheoretic reductionfor example those by Kenneth F. Schaffner in 1967 and Clifford Hooker in 1981emerged as a direct response to Nagels details.

With physics providing the ultimate reducing theories, reductionism is allied with physicalism about mental and social phenomena. As an explicit program, it has focused primarily on cases from biology and psychology rather than the social sciences. The rationale is straightforward: Claims about accomplished reductions in the former remain controversial, and if these controversies cannot be resolved, then the plausibility of the program for the latter seems remote. However, social phenomena do make an explicit appearance in one of the classic papers on reductionism in the philosophy of science. Paul Oppenheim and Hilary Putnam (1958, p. 7) illustrate the working hypothesis of the unity of science as follows:

6.......... Social groups

5.......... (Multicellular) living things

4.......... Cells

3.......... Molecules

2.......... Atoms

1.......... Elementary particles

Each level is related as parts (below) to wholes (immediately above), with micro-reductions hypothesized to obtain between theories explaining phenomena at a lower and an immediately higher level. And while Oppenheim and Putnam admit that accomplished microreductions from levels 6 to 5 have not advanced very far, they cite individual choice theories in economics and the principal theoretical approaches in sociology (Marxist, Veblenian, Weberian, Mannheimian) as examples of attempted micro-reductions to individualist psychology.

One popular general criticism of reductionism focuses on the multiple realizability of given higher-level kinds on lower-level mechanisms. (Bickles 2006 article Multiple Realizability provides a survey of these arguments and many reductionist responses, with an extensive bibliography.) An example from the social sciences often appealed to is Greshams law (colloquially stated, that bad money drives out good, and more precisely stated in terms of what happens in monetary exchanges under specific exchange conditions). Monetary exchange is realized by a wide variety of physical phenomenaexchanging paper bills or minted coins or strings of wampum, signing checks, or depressing the Enter button on ones desktop computer, to name just a few. It seems a safe empirical bet that no single physical kind, described and studied by some lower-level physical science, obtains in all these cases (not to mention in the myriad possible realizations of monetary exchange). One could disjoin all the lower-level realizers of such a high-level kind, but it seems an even safer bet that the resulting disjunctive kind does not occur in the explanatory laws of any physical science from which the laws of the higher-level science can be derived.

From the mid-1970s until the mid-1990s this argument held sway against reductionism. A few replies emerged during this time. Some reductionists advocated domain-specific reductions, limited to classes of realizers that shared lower-level mechanisms. Others pointed out examples of accomplished reductions from sciences history that included multiply realized kinds in the reduced theories, showing that multiple realizability is not sufficient by itself to block intertheoretic reduction. Since the late 1990s some critics have appealed to different grains in the characterizations of higher- and lower-level kinds. Advocates of multiple realizability tend to tolerate a wide variety of functional differences in their specification of higher-level kinds shared across distinct realizers, but they insist that a single (minute) physical difference indicates distinct lower-level kinds. When grains are matched across higher- and lower-level theories, multiple realizability vanishes. Others have noticed a dilemma for advocates of multiple realizability. The more that the physical mechanisms across distinct realizers are similar, the less likely is multiple realization of the same higher-level kind. The more that the physical mechanisms are dissimilar, the easier it is to find functional differences across realizers at the higher level and hence the less likely is multiple realization of the same higher-level kind. At the time of this writing, anti-reductionists appealing to multiple realizability owe responses to these challenges.

A more diffuse anti-reductionist argument focuses on the complexity of social phenomena. Most likely it is this intuition that lies behind critiques of reductionist approaches by critics of neoclassical economics: Reducing social phenomena to the dynamics of individual decision makers will fail to explain some crucial economic or social features. (Reductionism to individuals is often characterized as doctrinaire or simplistic in the social sciences.) But turning this intuition into rational argument is not easy. One recent attempt applies the sophisticated mathematics of complexity theory to social sciencesdynamical systems, chaotic nonlinearity, trajectories through high-dimensional state spaces, and the like. Explanations in the form of differential equations are proposed to account for the behavior of systems, but not in terms of the mechanical outputs of its components. (Timothy van Gelders application of complexity theory to cognitive psychology is a detailed attempt of this sort.) The principal worry is whether the reductionism targeted here is a straw man. If the behavior of systems requires these explanatory resources, then so will the lower-level sciences that appeal not only to the structure but also the dynamics and organization of the systems components. The philosopher William Bechtel (2001), for example, replies that complexity is compatible with reduction, in the form of decomposition and localization explanatory strategies. He argues not just on conceptual grounds but also by appealing to empirical work by cognitive neuroscientists studying memory.

SEE ALSO Choice in Economics; Choice in Psychology; Determinism; Economics; Economics, Neoclassical; Individualism; Marginalism; Materialism; Microeconomics; Objectivism; Psychology; Rational Choice Theory; Sociology; Sociology, Micro-


Bechtel, William. 2001. The Compatibility of Complex Systems and Reduction: A Case Analysis of Memory Research. Minds and Machines 11: 483-502.

Bickle, John. 2006. Multiple Realizability. In Stanford Encyclopedia of Philosophy, ed. Edward N. Zalta.

Hooker, Clifford. 1981. Towards a General Theory of Reduction. Part 1, Historical and Scientific Setting. Part 2, Identity in Reduction. Part 3, Cross-Categorial Reductions. Dialogue 20: 38-59, 201-236, 496-529.

Nagel, Ernest. 1961. The Structure of Science: Problems in the Logic of Scientific Explanation. New York: Harcourt, Brace and World.

Oppenheim, Paul, and Hilary Putnam. 1958. Unity of Science as a Working Hypothesis. In Concepts, Theories, and the Mind-Body Problem, Vol. 2 of Minnesota Studies in the Philosophy of Science, ed. Herbert Feigl, Michael Scriven, and Grover Maxwell, 3-36. Minneapolis: University of Minnesota Press.

Schaffner, Kenneth F. 1967. Approaches to Reduction. Philosophy of Science 34 (2): 137-147.

Van Gelder, Timothy. 1998. The Dynamical Hypothesis in Cognitive Science. Behavioral and Brain Sciences 21: 615-628.

John Bickle


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When theoretical statements use terms that refer to objects and properties whose existence seems awkward, puzzling, redundant, or ontologically problematic, there is motivation to analyze or reduce such statements to others that employ better understood terms. Reductionism must be distinguished both from eliminativism and supervenience. Consider two domains of properties M and P (e.g., the mental and the physical). Eliminativism claims that since only P exists, M can be eliminated (e.g., there is no such thing as demonic possession, but only a biochemical problem in the brain). Supervenience asserts that both M and P are real and distinct, though M is determined by P (e.g., headache pain is real, and while not identical to neurophysiological processes, is nonetheless realized by such processes). Reduction, however, asserts that there is but one thing that is both M and P, with P having explanatory priority (e.g., Mary's particular headache pain is just a particular complex neurological event).

Semantic and theoretic reduction

Examples of reductions in philosophy include logicism (reducing statements about numbers into statements of logic and set theory), phenomenalism (reducing statements about external macro-objects into statements of actual and possible experience), logical behaviorism (reducing statements about mental states into stimulus-response conditionals), logical positivism (reducing statements employing theoretical entities to ones referring only to observed objects), and naturalism (reducing normative ethical statements to ones whose terms refer to natural properties only). All these philosophical reductions are semantic, for all use definitional equivalences linking terms of the reduced to those of the reducing statements, (i.e., statements in the reduced theory just mean equivalent statements in the reducing theory). Broadly speaking, semantic reductions have been out of favor in philosophy since the 1950s. This is due in part to four developments: the heightened sensitivity to the "paradox of analysis" (i.e., if a semantic reduction is successful it is not informative and if it is informative it cannot be successful); the realization of the enormous practical difficulties of actually carrying out the proposed reductions; an increasing recognition of the holistic nature of sentence meaning; and the growing doubt about the very possibility of foundational discoveries.

Of more interest to the science-religion conversation is the status of scientific reductions. Consider physics, chemistry, biochemistry, biology, physiology, neuroscience, psychology and sociology. How are these various disciplines related? How does one connect hadrons, atoms, chemical compounds, amino acids, cells, synapses, thoughts, and cultural tendencies? If physicalism is true in asserting that all that ultimately exists are those entities referred to in the most basic physical theory, then in what sense can thoughts and cultural tendencies exist? Should talk of such things be eliminated, or should we understand theories making reference to them to be reducible to more basic theories, and ultimately to theories referring to fundamental physical entities? Theoretic reduction in the philosophy of science attempts to show how entire theories, and the entities and properties specified by them, are reducible to more basic theories.

Unlike semantic reduction, theoretic reduction understands the biconditionals connecting theoretical terms in the reducing and reduced theories to be empirically discoverable bridge laws specifying coextensive property instantiations. While statements in the reduced theory mean something different from statements in the reducing theory, it is nonetheless true that the reduced theory statements are true if and only if their reducing statements are true. Examples of theoretical reduction within science include the reduction of chemistry to physics, the reduction of thermodynamics to statistical physics, the reduction of Mendelian genetics to molecular genetics, and the partial reduction of psychiatry to neurophysiology.

Reductions can also be found in theology and religion, though they are not often presented as such. For example, Immanuel Kant (17241804) semantically reduced talk of God to discourse about morality, while Friedrich Ernst Schleiermacher (17681834) reduced it to modifications of the feeling of absolute dependence. Karl Marx (18181883), Sigmund Freud (18561958), and Emile Durkeim (18581917) attempted theoretically to reduce religion to economics, psychology, and sociology respectively.

Varieties of reduction

There are different types of reduction, and also different typologies of these reduction types. One might distinguish methodological, epistemological, and ontological reduction. Accordingly, the first is a research strategy in which the behavior of complex wholes is analyzed into their component parts; the second an explanatory strategy claiming that theories and laws at the higher levels are analyzable or otherwise explainable in terms of the theories and laws of the lower levels; and the third an ontological strategy holding that reality is ultimately comprised of nothing but simple components (e.g., quarks, strings) organized in particular ways.

This "nothing but" relation can be understood as reduction's defining characteristic: M reduces to P if and only if M is nothing but P. Accordingly, one can distinguish ontological, property, semantic, theoretical, and causal reduction. Ontological reduction claims that upper-level entities and events are nothing but complex configurations of lower-level entities and events; property reduction asserts that the instantiation of every upper-level property is nothing but the instantiation of a particular lower-level property; semantic reduction declares that the meaning of statements in the reduced theory is nothing but the meaning of statements in the reducing theory; theoretic reduction claims that laws of the reduced theory are nothing but the laws of the reducing theory; and causal reduction asserts that the causal powers of upper-level entities are nothing but the causal powers exhibited by their lower-level physical realizers.

Property reduction and causal reduction are of particular interest in the science-theology discussion. One can hold that while only physical particulars exist, property dualism nonetheless obtains because higher-level properties are not reducible to, and thus not coextensive with, any specific lower-level properties. Some in the science-theology discussion believe such a nonreductive physicalism of emergent mental properties can protect religious discourse and experience from reduction or elimination.

Causal reduction is extremely important for the question of the ontological status of putative emergent entities and properties. If entities at the upper-levels wholly inherit their causal powers from the lower-levels, and if ontological status only pertains to causally efficacious entities, then it seems that emergent phenomena are not fully real. The question of the causal status of emergent properties is at the heart of the controversy about downward causality. Some in the science-theology discussion suggest that the emergent itself can effect the causal distribution at the lower-levels, not just the lower-level realizers of that emergent, (e.g., consciousness itself is causally efficacious.) But if particular lower-level actualizations are sufficient for the instantiation of an emergent property, then it seems that these actualizations are also sufficient for the effects this emergent property is said to cause.


Many in the science-theology discussion wish to provide an account of emergent phenomenon that does not presuppose reductive explanation. Unfortunately, even in the absence of the straightforward reduction of the emergent, the admission of its physical realization seems to accomplish much of what reduction initially sought, for the causal loop still gets closed at the lowest physical levels. It seems that the "something more" of the emergent may be "nothing more" when it comes to the issue of causal reduction. This is not a result that would cheer many in the science-theology conversation.

See also Behaviorism; Causality, Primary and Secondary; Causation; Downward Causation; Materialism; Naturalism; Physicalism, Reductive and Nonreductive; Supervienience


beckermann, ansgar; flohr, hans; and kim, jaegwon, eds. emergence or reduction? essays on the prospects of nonreductive physicalism. berlin and new york: walter de gruyter, 1992.

bielfeldt, dennis. "how does the mental matter?" center for theology and natural science bulletin 19, no. 4 (2000): 11-21.

charles, david, and lennon, kathleen, eds. reduction, explanation, and realism. oxford: oxford university press, 1993.

kim jaegwon. supervenience and mind: selected philosophical essays. cambridge, uk: cambridge university press, 1993.

kim, jaegwon. mind in a physical world: an essay on the mind-body problem and mental causation. cambridge, mass.: mit press, 1999.

murphy, nancey. "supervenience and the nonreducibility of ethics to biology." in evolutionary and molecular biology: scientific perspectives on divine action. eds. robert john russell, william stoeger, and francisco ayala. vatican city and berkeley, calif.: vatican observatory publications and the center for theology and the natural sciences, 1998.

murphy nancey. "physicalism without reduction: toward a scientifically, philosophically, and theologically sound portrait of human nature." zygon 34 (1999): 551-572.

nagel, ernest. the structure of science: problems in the logic of scientific explanation. new york: harcourt, 1961.

peacocke, arthur. "reductionism: a review of the epistemological issues and their relevance to biology and the problem of consciousness." zygon 11, no. 4 (1976): 307334.

dennis bielfeldt


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reductionism In its most general usage, the term reductionism denotes any intellectual strategy for reducing apparently diverse phenomena to some primary or basic explanatory principle. For example, reductionist forms of materialism as applied to the natural sciences have attempted to explain the distinctive properties and powers of living beings in terms of the concepts and laws of chemistry. Attempts to explain patterns of difference in measured human intelligence, or social differences between men and women, in terms of genetic or physiological differences are commonly criticized by sociologists as examples of misguided biological reductionism. Some forms of Marxism, in which economic relations are supposed wholly to determine social and political life, are also commonly criticized as economic reductionism. Distinctions may usefully be made between logical reduction, in which the aim is to reduce the laws of the more superficial science to the more basic one; semantic reduction, in which the language of one science is defined in terms of the other; and explanatory reduction, in which the aim is to show that the phenomena of one science can be explained as effects of mechanisms identified in another.


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re·duc·tion·ism / riˈdəkshəˌnizəm/ • n. often derog. the practice of analyzing and describing a complex phenomenon, esp. a mental, social, or biological phenomenon, in terms of phenomena that are held to represent a simpler or more fundamental level, esp. when this is said to provide a sufficient explanation.DERIVATIVES: re·duc·tion·ist n. &·duc·tion·is·tic / riˌdəkshəˈnistik/ adj.