  He then argued that this is a tautology if \(x = y\), and a contradiction if \(x \neq y\). This is the subject on which he eventually sparred with Wittgenstein, when the latter wrote a letter in 1927 (reproduced in [NP] 340–341). Wittgenstein argued that either way this formula is nonsensical and cannot be substituted for x = y. Two drafts of a rejoinder by Ramsey are preserved ([NP] 342–346), but the matter was dropped subsequently, although Wittgenstein came back to it repeatedly after Ramsey’s death, for example in (Wittgenstein [BT], § 113). (For further discussion see Marion 1995; Sullivan 1995; and Methven 2015, chapter 7.)