Had Descartes done no more than point out this fact he would have no claim to notice here; and we are surprised to find many writers looking upon this "Cogito, ergo sum" as constituting the great idea in his system. Surely it is only a statement of universal experience—an epigrammatic form given to the common-sense view of the matter. Any clown would have told him that the assurance of his existence was his consciousness of it; but the clown would not have stated it so well. He would have said, "I know I exist, because I feel that I exist."
Descartes therefore made no discovery in pointing out this fact as an irreversible certainty. The part it plays in his system is only that of a starting-point. It makes consciousness the basis of all truth. There is none other possible. Interrogate consciousness, and its clear replies will be science. Here we have a new basis and a new philosophy introduced. It was indeed but another shape of the old formula, "Know thyself," so differently interpreted by Thales, Socrates, and the Alexandrians; but it gave that formula a precise signification, a thing it had before always wanted. Of little use could it be to tell man to know himself. How is he to know himself? By looking inward? We all do that. By examining the nature of his thoughts? That had been done without success. By examining the process of his thoughts? That, too, had been accomplished, and the logic of Aristotle was the result.
The formula needed a precise interpretation; and that interpretation Descartes gave. Consciousness, said he, is the basis of all knowledge; it is the only ground of absolute certainty. Whatever it distinctly proclaims must be true. The process, then, is simple: examine your consciousness, and its clear replies. Hence the vital portion of his system lies in this axiom: All clear ideas are true: whatever is clearly and distinctly conceived is true. This axiom he calls the foundation of all science, the rule and measure of truth.
The next step to be taken was to determine the rules for the proper detection of these ideas; and these rules he has laid down as follows:
1. Never accept anything as true but what is evidently so; to admit nothing but what so clearly and distinctly presents itself as true that there can be no reason to doubt it.
2. To divide every question into as many separate questions as possible; that each part being more easily conceived, the whole may be more intelligible—(Analysis).
3. To conduct the examination with order, beginning by that of objects the most simple, and therefore the easiest to be known, and ascending little by little up to knowledge of the most complex—(Synthesis).
4. To make such exact calculations and such circumspections as to be confident that nothing essential has been omitted.
Consciousness, being the ground of all certainty, everything of which you are clearly and distinctly conscious must be true; everything which you clearly and distinctively conceive exists, if the idea of it involves existence.
In the four rules, and in this view of consciousness, we have only half of Descartes' system; the psychological half. It was owing to the exclusive consideration of this half that Dugald Stewart was led—in controverting Condorcet's assertion that Descartes had done more than either Galileo or Bacon toward experimental philosophy—to say that Condorcet would have been nearer the truth if he had pointed him out as the "Father of the Experimental Philosophy of the Mind." Perhaps the title is just; but Condorcet's praise, though exaggerated, was not without good foundation.