As Jaegwon Kim points out in his excellent new book, “reductionism” has become
something of a pejorative term in philosophy and related disciplines. But originally
(eg, as expressed in Ernest Nagel’s 1961 The Structure of Science) reduction was
supposed to be a form of explanation, and one may wonder whether it is reasonable to
reject in principle the advances in knowledge which such explanations may offer.
Nagel’s own view, illustrated famously by the reduction of thermodynamics to
statistical mechanics, was that reduction is a relation between theories: theory A is
reduced to theory B by formulating “bridge laws” which link the terminology of the
theories, and using them to derive A from B. (An additional reductive claim is that Aphenomena
are identical with certain B-phenomena—as when the temperature of a
gas is identified with its mean molecular kinetic energy—but this kind of identity
claim is, strictly speaking, independent of the claim about theories.) Applied to the
case of mental states and brain states, a reduction would provide explanatory relations
between psychology and neuroscience, normally supplemented with the claim that
mental properties are identical with physical properties in the brain.