He who argues thus seems quite reasonable; but strangely enough, the advocate of the third principle brings equally strong arguments against his two adversaries. How is it known, he asks, that the principle of contradiction is true? Only because we see in the idea of being the impossibility of its being and not being at the same time, and vice versa; therefore, we are sure of the principle of contradiction only from the application of the principle: whatever is contained in the clear and distinct idea of anything, may he affirmed of it with all certainty. If nothing can be sustained without relying upon the principle of contradiction,—and this is based upon our principle,—ours is the foundation of them all.
162. They are all three right, and all three wrong. They are right in asserting that the denial of their respective principles is the ruin of the others. They are all wrong in pretending that the denial of the others is not the ruin of their own. Whence then the dispute? From the confusion of ideas, by which they compare principles of very different orders, all indeed very true, but not to be compared with each other for the same reason that we cannot compare the white and the warm, and dispute whether a thing has more degrees of heat or whiteness. Comparison requires not only opposition in the extremes, but also something in common; if things are totally unlike, comparison is impossible.
Descartes' principle is the enunciation of a simple fact of consciousness; that of contradiction is a truth known by evidence; and that of the Cartesians is an assertion that the criterion of evidence is valid, and that it is a truth of reflection expressing the intellectual impulse by which we are borne to believe the truth of what we know by evidence.
The importance of this question requires a special examination of each of the three principles, which we shall make in the next chapters.[(16)]
[CHAPTER XVII.]
THOUGHT AND EXISTENCE.—DESCARTES' PRINCIPLE.
163. Am I certain that I exist? Yes. Can I prove it? No. Proof supposes reasoning; there is no solid reasoning without a firm principle on which to rest it; and there is no firm principle unless we suppose the existence of the reasoning being.
In effect, if he who reasons is not certain of his own existence, he cannot be certain of his own reasoning, since there will be no reasoning if there be no one to reason. Therefore there are, unless we suppose this, no principles on which to rest; there is nothing but illusion, or rather there is neither any illusion, for there can be none where there is no one illuded.
Our existence cannot be demonstrated: we have so clear and strong a consciousness of it that it leaves us no uncertainty; but it is impossible to prove it by reasoning.