Full Idea
A man cannot conceive himself capable of a greater certainty, than to know that any idea in his mind is such as he perceives it to be; and that two ideas wherein he perceives a difference, are different, and not precisely the same.
Gist of Idea
The greatest certainty is knowing our own ideas, and that two ideas are different
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.02.01)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.531
A Reaction
That leaves open the question of what you know when you know your ideas. Do you fully know the contents? The contingent truths expressed by some of them? Or just their meanings?