Evolution as fact and theory

Evolution as fact and theory is a phrase describing how scientists and philosophers of science treat evolution both as an established fact and as a well substantiated explanatory theory, popularized by the paleontologist Stephen Jay Gould in a 1981 article. Gould defined a scientific fact as data confirmed to such a degree that withholding provisional assent to it would be perverse, and a scientific theory as a well substantiated explanation of such facts. The evidence for evolution comes from three main sources: processes that can be directly observed in living organisms, structural imperfections in living things that reveal their evolutionary history, and transitions recorded in the fossil record. Much of the apparent disagreement over whether evolution is only a theory arises because ordinary usage treats theory as speculation, while among scientists it denotes a well substantiated explanation, so the dispute is often more about language than about the underlying science.