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.