Rene Descartes (_Renatus Cartesius_) was born in 1596, at La Haye in Torraine. Possessing an independent property, he volunteered as a soldier in his twenty-first year, and served in the wars with the Dutch, the Bavarians, and the Imperialists. After this he travelled a good deal, and then abode a considerable time in Paris. In 1629 he left his native land, and betook himself to Holland, that he might there, undisturbed and unknown, devote himself to philosophy, and elaborate his scientific ideas. He spent twenty years in Holland, enduring much vexatious treatment from fanatical theologians, till in 1649 he accepted an invitation from Queen Christina of Sweden, to visit Stockholm, where he died in the following year. The chief content of the Cartesian system may be seen condensed in the following epitome. 1. If science would have any thing fixed and abiding, it must begin with the primal ground of things; every presupposition which we may have cherished from infancy must be abandoned; in a word, we must doubt at every point to which the least uncertainty is attached. We must therefore doubt not only the existence of the objects of sense, since the senses so frequently deceive, but also the truths of mathematics and geometry—for, however evident the proposition may appear that two and three make five, or that the square has four sides, yet we cannot know but what God may have designedly formed us for erroneous judgments. It is therefore advisable to doubt every thing, in fact to deny every thing, to posit every thing as false. 2. But though we posit every thing as false to which the slightest doubt may be attached, yet we cannot deny one thing, viz., the truth that we, who so think, do exist. But rather from the very fact that I posit every thing as false, that I doubt every thing, is it manifest that I, the doubter, exist. Hence the proposition: I think, therefore I am (_cogito ergo sum_), is the first and most certain position which offers itself to every one attempting to philosophize. Upon this the most certain of all propositions, the certainty of all other knowledge depends. The objection of _Gassendi_ that the truth of existence follows from any other activity of man as well as from thinking, that I might just as well say: I go to walk, therefore I exist,—has no weight; for, of all my actions, I can be absolutely certain only of my thinking.