Some Remarks on Bonjour on Warrant, Proper Function, and Defeasibility

A number of counterexamples have recently been leveled against Alvin Plantinga's Proper Functionalism, counterexamples aimed at showing that Plantinga's theory fails to provide sufficient conditions for warrant — that elusive epistemic property which together with true belief yields knowledge. Among these counterexamples, Laurence Bonjour's is perhaps the most formidable and, if successful, shows that Proper Functionalism is simply too weak to serve as an acceptable theory of warrant. In this paper, I argue that, contrary to initial appearances, BonJour's counterexample is not successful. More exactly, I argue that, once it is recognized that a defeasibility constraint is deeply embedded within Plantinga's proper function condition for warrant — a constraint which says, in effect, that a belief B is warranted for an agent S only if S does not possess any defeaters against B — BonJour's counterexample to Proper Functionalism can be handled quite straightforwardly