Arthur Prior's Proofs of the Necessities of Identity and Difference

This paper draws attention to a proof of the necessity of identity given by Arthur Prior. In its simplicity it is comparable to a proof of Quine’s, popularised by Kripke, but it is slightly di ff erent. Prior’s Polish notation is transcribed into more familiar idiom. Prior’s proof is followed by a proof of the necessity of di ff erence, possibly the first such proof in the literature, which is also repeated here and transcribed. The paper concludes with a brief discussion of Prior’s views on identity and di ff erence over time.