The indefeasibility of the inference that if not-A, then not-C

Content and, to a lesser degree, epistemic or inferential conditionals regularly invite conditional perfection as a non-monotonic inference or conversational implicature. Conditional perfection (henceforth CP) is the natural language tendency to perfect conditionals (if A then C) into their corresponding biconditionals (if and only if A, then C) through the mediation of an if not-A, then not-C conditional. In the literature there is some controversy regarding the pragmatic principle by which CP is derived, but there seems to be silent agreement about the defeasibility of CP. In this paper we argue that the if not-A, then not-C CP inference is indefeasible with particular conditional utterance types where cancellation contrasts with the additional propositional attitude expressed by those conditional utterance types. The indefeasibility of the if not-A, then not-C CP inference can further be accounted for in terms of the pragmatic principle by which it is derived.