The feeling that he had found such a firm foundation, induced the father of modern philosophy, Descartes, to base the whole of human knowledge on the principle, "I think, therefore I am." All other things, all other processes, are independent of me. Whether they be truth, or illusion, or dream, I know not. There is only one thing of which I am absolutely certain, for I myself am the author of its indubitable existence; and that is my thought. Whatever other origin it may have in addition, whether it come from God or from elsewhere, of one thing I am sure, that it exists in the sense that I myself produce it. Descartes had, to begin with, no justification for reading any other meaning into his principle. All he had a right to assert was that, in apprehending myself as thinking, I apprehend myself, within the world-system, in that activity which is most uniquely characteristic of me. What the added words "therefore I am" are intended to mean has been much debated. They can have a meaning on one condition only. The simplest assertion I can make of a thing is, that it is, that it exists. What kind of existence, in detail, it has, can in no case be determined on the spot, as soon as the thing enters within the horizon of my experience. Each object must be studied in its relations to others, before we can determine the sense in which we can speak of its existence. An experienced process may be a complex of percepts, or it may be a dream, an hallucination, etc. In short, I cannot say in what sense it exists. I can never read off the kind of existence from the process itself, for I can discover it only when I consider the process in its relation to other things. But this, again, yields me no knowledge beyond just its relation to other things. My inquiry touches firm ground only when I find an object, the reason of the existence of which I can gather from itself. Such an object I am myself in so far as I think, for I qualify my existence by the determinate and self-contained content of my thought-activity. From here I can go on to ask whether other things exist in the same or in some other sense.