Moore’s Fourth Condition

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.