Reductionism refers to several related but different philosophical positions regarding the connections between phenomena, or theories, "reducing" one to another, usually considered "simpler" or more "basic". The Oxford Companion to Philosophy suggests that it is "one of the most used and abused terms in the philosophical lexicon" and suggests a three part division:

1. Ontological reductionism: a belief that the whole of reality consists of a minimal number of parts
2. Methodological reductionism: the scientific attempt to provide explanation in terms of ever smaller entities
3. Theory reductionism: the suggestion that a newer theory does not replace or absorb the old, but reduces it to more basic terms. Theory reduction itself is divisible into three: translation, derivation and explanation.