I am, I exist. This is certain. How often? As often as I think. For it might indeed be that if I entirely ceased to think, I should thereupon altogether cease to exist. I am not at present admitting anything which is not necessarily true; and, accurately speaking, I am therefore [taking myself to be] only a thinking thing, that is to say, a mind, an understanding or reason-terms the significance of which has hitherto been unknown to me. I am, then a real thing, and really existent. What thing? I have said it, a thinking thing