What was Fisher's fundamental theorem of natural selection and what was it for?

Fisher's 'fundamental theorem of natural selection' is notoriously abstract, and, no less notoriously, many take it to be false. In this paper, I explicate the theorem, examine the role that it played in Fisher's general project for biology, and analyze why it was so very fundamental for Fisher. I defend Ewens (1989) and Lessard (1997) in the view that the theorem is in fact a true theorem if, as Fisher claimed, 'the terms employed' are 'used strictly as defined' (1930, p. 38). Finally, I explain the role that projects such as Fisher's play in the progress of scientific inquiry.