I am reading Doubt Truth to be a Liar by Graham Priest. In it he uses the symbol ? as a logical connective, and I am unsure of it's meaning.

Given his use of ? (a ? a) to denote the Law of Identity, my first guess would be that it's a stand in for =, but the usage also seems to imply that it has at least two parities (1 & 2). I'm stuck.

Here is the full example: Doubt Truth to be a Liar, p.13

1 Answer 1


This was a misprint of the pdf. I checked other pdfs of the text and it the law of identity reads "it is necessary that a implies a" in the first case.

If you've got the same rendering of the text that I had, I suggest finding another.

  • Ah, man, and here I thought maybe Priest was sprinkling a little bit of erotetic logic into his system. Commented Nov 27, 2021 at 10:03
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    This is correct. I have a printed copy of the book and the text is □(α → α). Priest interprets the law of identity in terms of propositions rather than individuals, and his point in context is that the law of non-contradiction can be derived from the law of identity within Aristotle's system, so it is not a first principle. (Page 13)
    – Bumble
    Commented Nov 27, 2021 at 13:13

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