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.