Because men know not the real essence of substances.
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“of which essences I confess I have no distinct ideas at all; and, I am apt to suppose, others, when they examine their own knowledge, will find in themselves, in this one point, the same sort of ignorance. 7. Because men know not the real essence of substances. Now, then, when men apply to this particular parcel of matter on my finger a general name already in use, and denominate it GOLD, do they not ordinarily, or are they not understood to give it that name, as belonging to a particular species of bodies, having a real internal essence;” source