Full Idea
Sight or touch might make someone take eleven for twelve, but he could never form this mistaken belief about the contents of his mind.
Gist of Idea
You might mistake eleven for twelve in your senses, but not in your mind
Source
Plato (Theaetetus [c.364 BCE], 195e)
Book Reference
Plato: 'Theaetetus', ed/tr. Waterfield,Robin [Penguin 1987], p.106