Intertheoretic reduction is the claim that one scientific theory can be derived from or explained by another, typically more fundamental, theory. The classical model (Ernest Nagel, 1961) requires that the laws of the reduced theory be logically derivable from the reducing theory, possibly with the help of bridge laws connecting the vocabularies of the two.
The standard examples are the reduction of thermodynamics to statistical mechanics (temperature is mean molecular kinetic energy) and the proposed reduction of psychology to neuroscience. In each case, the question is whether the higher-level theory’s explanations, laws, and categories can be fully captured by the lower-level theory or whether something is lost in translation.
Opponents of reduction argue that higher-level theories capture patterns (multiple realizability, emergent organization) that are invisible at the lower level. Jerry Fodor’s argument that psychological kinds cross-classify physical kinds, and Philip Kitcher’s defense of explanatory autonomy, both challenge the assumption that lower means more fundamental. The debate connects to broader questions in philosophy of science about explanation, emergence, and the unity or disunity of science.