 A final form of empiricism is what we may call justificatory empiricism. In its strongest form, an empiricism of this kind is indifferent as to the origins of our ideas and can tolerate inferences to unobservable entities. It merely insists that wherever our ideas come from and whatever they claim, they cannot be regarded as justified until they have been successfully tested against experience. Within twentieth-century philosophy, this position is associated with a sharp distinction between the “context of discovery” and the “context of justification” (Reichenbach 1938: 6–7), with justificatory empiricists focusing on the latter. For Karl Popper (1902–1994), for instance, the question of how a theory is discovered is irrelevant to its epistemic status; what matters is whether that theory has explanatory power and has withstood severe testing (Popper 1935 [2002: 7–8]). On the face of it, this does seem a recent position. Perhaps no philosopher before Popper would have regarded the origins of an idea as entirely irrelevant to its standing. But there are hints of it in the work of some ancient and medieval thinkers, who held that factual claims arrived at by deductive reasoning lack credibility if they have not also been tested against experience.