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.
165. And it is to be remarked that indemonstrability does not belong solely to certain premises; it is found, in some measure, in every argument by its very nature, abstracting the propositions which compose it. We know that the premises A and B are certain; from them we infer the proposition C. By what right? Because we see that C is connected with A and B. But how do we know this? If by immediate evidence, by intuition, here is something else that cannot be demonstrated, the connection of the conclusion with the premises. If by argument, ratiocination, establishing ourselves on the art of reasoning, there are two considerations, both tending to demonstrate indemonstrability. I. If the principles of the art are indemonstrable, we have at once something indemonstrable; if they are demonstrable, we must make use of others which serve as their basis, and at last either come to one which does not admit of demonstration, or else proceed to infinity. II. How do we know the principles of reasoning to be applicable to this case? By another act of reasoning? Then we shall encounter the same difficulty as in the other case. Is it because we see that it is so? because it is immediately evident? Then here again we have an indemonstrable point. These reflections will clearly show that to demand proof of everything is to demand what is impossible.
166. A being which does not think has no consciousness of itself: the stone exists, but does not know that it exists, neither would man himself in a similar case, were all his intellectual and sensible faculties in complete inaction. We easily conceive the difference of these two states by calling to mind what occurs, when from waking we pass into a profound sleep, and again when we awake from it. The first starting-point of our cognitions is this intimate presence of our internal acts, abstraction made of the questions which may be raised upon their nature. If every thing existed as at present, and there also existed, besides the world which we see, infinite other worlds, not even then would any thing exist for us, had we not those internal acts of which we are speaking. We should be like an insensible body placed in the immensity of space, which would suffer no mutation were every thing around it to disappear, and would perceive no change even if it were itself to sink into the abyss of nothing. On the other hand, if we suppose every thing to be annihilated except this being within us which feels, thinks, and wills, there still remains a point whereon to base the edifice of human cognitions: this being, though alone in immensity, would render itself an account of its own acts to the extent of its ability, and might go into numberless combinations having for their object the possible though not the real.
167. The famous principle of Descartes, I think, therefore I am, has been often attacked, and justly and conclusively so, if this philosopher really understood his principle in the sense which the schools are accustomed to give it. If Descartes presented it as a true argument, as an enthymema with an antecedent and a consequent, the argument was clearly defective in its foundation. For when he said, "I am going to prove my existence with this enthymema: I think, therefore I am:" this objection might have been made; your enthymema is equivalent to a syllogism in this form: whatever thinks, exists; but I think; therefore, I exist. This syllogism, in the supposition of a universal doubt, excluding even the supposition of existence itself, is inadmissable in its propositions and in their connection. In the first place, how do you know that whatever thinks exists? Because nothing can think without existing. How do you know that? Because what does not exist, does not act. But how in its turn do you know this? Supposing every thing to be doubted, nothing to be known, these principles are not known; otherwise we fall short of the supposition of universal doubt, and consequently go out of the question. If any one of these principles must be admitted without proof, it is just as well to admit your own existence and save yourself the trouble of proving it with an enthymema.
In the second place, how do you know that you think? Your argument may be retorted, as dialecticians say, in the following manner: nothing can think without existing; but your existence is doubtful, for you are trying to prove it; therefore you are not sure that you think.
168. Manifestly, then, Descartes' principle, taken as a true argument, cannot be defended; and it is so easy to see its defect, that it seems impossible for so clear and penetrating an intellect to have overlooked it. It is therefore probable that Descartes understood his principle in a very different sense; and we will now briefly show what meaning, in our judgment, the illustrious philosopher must have given to it.
Supposing himself for a moment in universal doubt, without accepting for certain anything that is known, he concentrated himself on himself, and in the depth of his soul sought a point whereon to base the edifice of human cognitions. Although we abstract all around us, we clearly cannot abstract ourselves, our mind, which is present to its own eyes, only the more lucidly, the greater the abstraction in which we place ourselves with respect to eternal objects. Now in this concentration, this collection of himself within himself, this withdrawal from every thing for fear of error, and asking himself if there be any thing certain, if there be any foundation and starting-point in the career of knowledge; first of all is presented to him the consciousness of thought, the very presence of the acts of his mind. If we mistake not, this was Descartes' thought: I wish to doubt of every thing; I refrain from affirming as from denying any thing; I isolate myself from whatever surrounds me, because I know not if it be any thing more than an illusion. But in this very isolation, I meet with the intimate sense of my internal acts, with the presence of my mind; I think, therefore I am; this I feel in a manner that leaves no room for doubt or uncertainty; therefore, I am; that is to say, this sense of my thought makes me know my existence.
169. This explains why Descartes did not present his principle as a mere enthymema, as an ordinary argument, but as determining a fact presented to him and first in the order of facts: even if he inferred existence from thought, it was not by deduction, properly so called, but as one fact contained in another, or rather identified with it.
We say identified, because it really is so in Descartes' opinion; and this confirms what we have already advanced, that this philosopher did not offer an argument, but laid down a fact. According to him, the essence of the soul consists in thought; and as other schools of philosophy distinguish between substance and its acts, considering the mind in the first class, and thought in the second, so Descartes held that there was no distinction between mind and thought, that they were the same thing, that thought constituted the essence of the soul. "Although one attribute," he says, "suffices to make us know the substance, there is, nevertheless, in every substance one attribute,
