Full Idea
It is difficult to know whether you know something or not. For it is difficult to know whether or not our knowledge of something proceeds from its principles - and this is what it is to know something.
Gist of Idea
Knowledge proceeds from principles, so it is hard to know if we know
Source
Aristotle (Posterior Analytics [c.327 BCE], 76a25)
Book Reference
Aristotle: 'Posterior Analytics (2nd ed)', ed/tr. Barnes,Jonathan [OUP 1993], p.14