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. 164. It is a prejudice and a fatal error to believe ourselves able to prove everything by the use of reason; the principles on which it is founded are prior to its use; the existence of reason, and that of the being that reasons, are prior to both. Not only are not all things demonstrated, but it may even be demonstrated that some things are indemonstrable. Demonstration is a ratiocination in which we infer from evident propositions, a proposition evidently connected with them. If the premises are of themselves evident, they do not admit of demonstration; if we suppose them in their turn demonstrable, we shall have the same difficulty with respect to those on which the new demonstration is founded; therefore we must either stop at an indemonstrable point, or proceed to infinity, which would be never to finish the demonstration.