In the eyes of its practitioners, economics is both a deductive science and an empirical science. The starting point of its deductions might be thought of as first principles. But what is the status of such principles? The tension between foundationalism, the idea that there are necessary and secure first principles for economic inquiry, and fallibilism, the idea that no belief can be certified as true beyond the possibility of doubt, is explored. Empirical disciplines require some sort of falsifiability. Yet, empirical inquiries also require a starting place—if not a necessarily true one, at least an indubitable one, that is, one that is not actually doubted. Indubitability appears to have necessary consequences, undercutting fallibilism, while fallibilism threatens confidence in the de facto first principles that begin inquiry. This tension is examined in three well-known attempts to define economics and its method: John Stuart Mill’s economics as the science of wealth, Lionel Robbins’s economics as constrained optimization; and George Stigler and Becker’s attempt to reformulate neoclassical economics to square empiricism with Robbins’ deductivism.