What perception is, every one will know better by reflecting on what he does himself, what he sees, hears, feels, &c. or thinks, than by, any discourse of mine. Whoever reflects on what passes in his own mind, can-not miss it: and if he does not reflect, all the words in the world cannot make him have any notion of it.
John Locke, Essay on Humane Understanding Vol 2 p. 121 CH 9 S1