Accelerating developments in molecular biology since 1953 have strongly encouraged the advocacy of reductionism by a number of important biologists, including Crick, Monod, and E. O. Wilson, and strong opposition by equally prominent biologists, especially Lewontin, along with most philosophers of biology.

Reductionism is a metaphysical thesis, a claim about explanations, and a research program. The metaphysical thesis that reductionists advance (and antireductionists accept) is physicalism, the thesis that all facts, including the biological facts, are fixed by the physical and chemical facts; there are no nonphysical events, states, or processes, and so biological events, states, and processes are “nothing but” physical ones. This metaphysical thesis is one reductionists share with antireductionists. The reductionist argues that the metaphysical thesis has consequences for biological explanations: they need to be completed, corrected, made more precise, or otherwise deepened by more fundamental explanations in molecular biology. The antireductionist denies this inference, arguing that nonmolecular biological explanations are adequate and need no macromolecular correction, completion, or grounding. The research program that reductionists claim follows from the conclusion about explanations can be framed as the methodological moral that biologists should seek such macromolecular explanations.