The Great Events by Famous Historians [Volumes 1-14]

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.

There is, in truth, another half of Descartes' system, equally important, or nearly so: we mean the deductive method. His eminence as a mathematician is universally recognized. He was the first to make the grand discovery of the application of algebra to geometry; and he made this at the age of twenty-three. The discovery that geometrical curves might be expressed by algebraical numbers, though highly important in the history of mathematics, only interests us here by leading us to trace his philosophical development. He was deeply engrossed in mathematics; he saw that mathematics were capable of a still further simplification and a far more extended application. Struck as he was with the certitude of mathematical reasoning, he began applying the principles of mathematical reasoning to the subject of metaphysics. His great object was, amid the scepticism and anarchy of his contemporaries, to found a system which should be solid and convincing. He first wished to find a basis of certitude—a starting-point: this he found in consciousness. He next wished to find a method of certitude: this he found in mathematics.

"Those long chains of reasoning," he tells us, "all simple and easy, which geometers use to arrive at their most difficult demonstrations, suggested to me that all things which came within human knowledge must follow each other in a similar chain; and that provided we abstain from admitting anything as true which is not so, and that we always preserve in them the order necessary to deduce one from the other, there can be none so remote to which we cannot finally attain, nor so obscure but that we may discover them." From these glimpses of the twofold nature of Descartes' method, it will be easy to see into his whole system: consciousness being the only ground of certitude, mathematics the only method of certitude.