Full Idea
If you can't apprehend being you can't apprehend truth, and so a thing could not be known. Therefore knowledge is not located in immediate experience but in thinking about it, since the latter makes it possible to grasp being and truth.
Gist of Idea
Thought must grasp being itself before truth becomes possible
Source
Plato (Theaetetus [c.368 BCE], 186c)
Book Reference
Plato: 'Theaetetus', ed/tr. Waterfield,Robin [Penguin 1987], p.90