Reductionism is a view that asserts that entities of a given kind are collections or combinations of entities of a simpler or more basic kind or that expressions denoting such entities are definable in terms of expressions denoting the more basic entities. Thus, the ideas that physical bodies are collections of atoms or that thoughts are combinations of sense impressions are forms of reductionism.

Two very general forms of reductionism have been held by philosophers in the 20th century: (1) Logical positivists have maintained that expressions referring to existing things or to states of affairs are definable in terms of directly observable objects, or sense-data, and, hence, that any statement of fact is equivalent to some set of empirically verifiable statements. In particular, it has been held that the theoretical entities of science are definable in terms of observable physical things, so that scientific laws are equivalent to combinations of observation reports. (2) Proponents of the unity of science have held the position that the theoretical entities of particular sciences, such as biology or psychology, are definable in terms of those of some more basic science, such as physics; or that the laws of these sciences can be explained by those of the more basic science.

The logical positivist version of reductionism also implies the unity of science insofar as the definability of the theoretical entities of the various sciences in terms of the observable would constitute the common basis of all scientific laws. Although this version of reductionism is no longer widely accepted, primarily because of the difficulty of giving a satisfactory characterization of the distinction between theoretical and observational statements in science, the question of the reducibility of one science to another remains controversial.

Excerpt from the Encyclopedia Britannica without permission.