To suppose the soul to think, and the man not to perceive it, is, as has been said, to make two persons in one man. And if one considers well these men’s way of speaking, one should be led into a suspicion that they do so.
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“And I say, it is as possible that the soul may not always think; and much more probable that it should sometimes not think, than that it should often think, and that a long while together, and not be conscious to itself, the next moment after, that it had thought.
19. That a Man should be busy in Thinking, and yet not retain it the next moment, very improbable.
To suppose the soul to think, and the man not to perceive it, is, as has been said, to make two persons in one man. And if one considers well these men’s way of speaking, one should be led into a suspicion that they do so. For those who tell us that the SOUL always thinks, do never, that I remember, say that a MAN always thinks. Can the soul think, and not the man? Or a man think, and not be conscious of it? This, perhaps, would be suspected of jargon in others.” source