In the first section of this paper I offer a necessary condition for members of a particular class of arguments to be acceptable asproofs. In the second section, I point out that a plausible extension of this principle reveals that a number of additional arguments cannot function successfully as proofs. Finally, I note that a number of theological arguments, particularly cosmological and ontological arguments, are suspect in the light of this extended principle. Standardly in the ontological argument, criticism falls on (...) the conditional, modality-enriching premise which involves a move from possibility to actuality. I offer two defences of this premise, and argue that—as the use of Moore's extended maxim shows—the problem lies with the premise that asserts mere possibility. (shrink)