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Full Idea
To say that everyone believes what is the case, is to concede the truth of the oppositions' beliefs; in other words, the person has to concede that he himself is wrong.
Gist of Idea
If you claim that all beliefs are true, that includes beliefs opposed to your own
Source
Plato (Theaetetus [c.368 BCE], 171a)
Book Ref
Plato: 'Theaetetus', ed/tr. Waterfield,Robin [Penguin 1987], p.64