Reductionism is a metaphysical thesis, a claim about explanations, and a research program. The metaphysical thesis reductionists advance (and antireductionists accept) is that all facts, including all biological facts, are fixed by the physical and chemical facts; there are no non-physical events, states, or processes, and so biological events, states and processes are “nothing but” physical ones. The research program can be framed as a methodological prescription which follows from the claim about explanations. Antireductionism does not dispute reductionism's metaphysical claim, but rejects the explanatory claim and so the methodological moral. To a first approximation what reductionists and anti-reductionists disagree about is whether explanations in functional biology can be or need to be explained or completed or perhaps replaced by explanations in terms of molecular biology. And this disagreement over the adequacy of explanations in functional biology drives a significant methodological disagreement with consequence for the research program of biology.