When Descartes said, _I think_, he meant by this word every internal act, every phenomenon immediately present to the soul; he spoke not of thought taken in a purely intellectual sense, but included in it all that of which we have immediate consciousness. "By the word _thought_," he says, "I understand all that is done within us, in such a manner that we perceive it immediately by ourselves: this is why not only to understand, to will, to imagine, but also to feel, are here the same thing as to think. For if I say that I see, or that I walk, and thence I infer that I am; if I mean to speak of the action performed with my eyes or with my feet, this conclusion is not so infallible that I have no reason to doubt it; because it may be that I think I see, or walk, although I do not open my eyes or stir from my seat; for this sometimes happens when I am asleep, and the same might also happen even if I had no body; but if I mean to speak only of the action of my thought or of the feeling, that is to say, the knowledge that I possess, which makes it seem to me that I see, or that I walk, this same conclusion is so absolutely true that I cannot doubt it; because it relates to the soul, which alone has the faculty of feeling, or of thinking, in any other manner whatever."[12] [12] _Les Principes de la Philosophie._ 1^{ière.} partie, N. 9. 176. This passage shows very clearly Descartes' ideas; he destroyed every thing by doubt, excepting one thing which defied all his efforts, the consciousness of himself; and this consciousness he took for the basis, on which, with full certainty, he might build anew the edifice of science. Locke and Condillac did nothing else; they followed, indeed, a different path, but their point of departure was at all times the same. Locke says: "First, I shall inquire into the _original_ of those _ideas_, notions, or whatever else you please to call them, which a man observes, and is conscious to himself he has in his mind; and the ways whereby the understanding comes to be furnished by them."[13] "Since the mind, in all its thoughts and reasonings, hath no other immediate object but its own _ideas_, which it alone does or can contemplate, it is evident that our knowledge is only conversant about them."[14] "Whether, to speak metaphorically," says Condillac, "we ascend even to the heavens, or descend into the abysses, we do not go out of ourselves, and it is always our own thought that we perceive."[15] [13] _Essay on the Human Understanding._ Prologue. [14] _Essay on the Human Understanding._ Book iv., Chap. I., § 1. [15] _Essai sur l'Origine des Connaissances._ Première partie, C. 1., § 1.