Moore’s Fourth Condition

abstract: G. E. Moore’s “Proof of an External World” has long vexed readers. That Moore could regard the proof as cogent, despite its apparent circularity, has often been read as a failure to register the worry, underwriting the charge of philosophical naivete. This paper offers a new interpretation. Drawing on newly discovered archival material, I argue that “Proof” is best read as the culmination of Moore’s efforts to come to grips with a “fourth condition” on proof—beyond premise–conclusion distinctness, known premises, and validity—namely, a proof’s capacity to extend knowledge. On this interpretation, Moore’s proof embodies a more complex account than commentators have recognized, clarifying both its seemingly paradoxical character and the unease it provokes in readers.