Descartes, RENÉ (Latinised into RENATUS CARTESIUS), generally regarded as the father of modern philosophy, was born March 31, 1596, at the village of La Haye, near Tours. He was sent at the age of eight to the Jesuit College at La Flèche, where he soon became distinguished for his keenness of intellect, and made great and rapid progress in languages, mathematics, and astronomy. It was not long, however, before he became dissatisfied with the doctrines and method of scholasticism, and felt it impossible to acquiesce in what had hitherto been regarded as knowledge. The first thing that he did after leaving college, as we are informed in his treatise on Method, was to abandon books, and endeavour to efface from his mind all that he had hitherto been taught, that it might be free to receive the impressions of truth, whencesoever they should come. In pursuance of his plan, he resolved to travel, and soon entered the army as a volunteer, serving successively in Holland, Bohemia, and Hungary. It was while he was in winter-quarters at Neuburg on the Danube (St Martin's Eve, 1619, as he himself informs us) that there first dawned upon him the principles of the new method which he afterwards applied in philosophy and for the reorganisation of knowledge generally. So in 1621 he left the army, but continued to travel as a private gentleman, with occasional residences in Paris, till 1629. In that year he retired to Holland, where he lived in great seclusion for twenty years, devoted to the elaboration of his discoveries and the publication of his chief works. In time his doctrines attracted many disciples in the land of his adoption, but involved him at the same time in bitter controversy with the Dutch theologians. These unpleasant encounters had probably their share in inducing him to accept an invitation to go to Sweden, addressed to him by Queen Christina, who desired his learned intercourse and instruction. He left Holland in September 1648, but his constitution was not able to stand the rigour of the northern winter and the exposure involved in waiting upon the queen at five every morning for an hour's philosophical instruction. He died at Stockholm of inflammation of the lungs on February 11, 1650. Sixteen years later, his body was brought to Paris, and buried in the church of St Geneviève-du-Mont. In 1819 his remains were again transferred to St Germain-des-Prés, where they now rest.
The grand object towards which Descartes directed his endeavours was the attainment of philosophical certainty. The way whereby he sought to attain this end is explained in the Discourse on Method (Discours de la Méthode, published in 1637). This small but extremely interesting and important treatise contains a history of the inner life of the author, tracing the progress of his mental development from its commencement in early years, to the point where it resulted in his resolution to hold nothing for true until he had ascertained the grounds of certitude. The author also, in the same treatise, lays down the methodical rules by which he resolved to guide his inquiries, and by the observance of which he hoped to arrive at absolute certainty, if indeed it were at all attainable. The results of his inquiries are indicated in the Discourse, but are exhibited more particularly in his Meditations de Première Philosophie (Amst. 1641), and the Principia Philosophiæ (Amst. 1644). Doubt, according to Descartes, is the philosophic starting-point, the solvent which we must bring to bear upon all our inherited beliefs and opinions. This doubt is to be distinguished from scepticism, which is a permanent state of mind and involves despair of truth. It is to be regarded simply as an instrument of philosophical inquiry, and as such has received the name of Cartesian doubt. Applying this test relentlessly to all that had hitherto borne the name of knowledge, Descartes found one, and only one proposition that seemed to him to stand firm, and of which the truth could not possibly be doubted: that proposition was that he existed, which he inferred from the fact of his possessing consciousness. He could not doubt that he felt and thought, and therefore he could not doubt that he, the feeler, the thinker, existed. This relation between consciousness and existence he expressed by the memorable words: Cogito, ergo sum. On inquiring further into the ground of his certitude in the case of this proposition, Descartes was able to assign no further reason than that he saw very clearly that in order to think, it is necessary to exist.
He formulated, therefore, the following criterion of certainty—that whatever is clearly and distinctly thought, must be true. Amongst these clear and distinct thoughts he soon recognised the idea of God as the absolutely Perfect Being. This idea, he reasoned, could not be formed in our minds by ourselves, for the imperfect can never originate the perfect; it must be connate—i.e. part of the original structure of our understanding, and implanted there by the Perfect Being himself. Hence, from the existence of the idea of perfection, Descartes inferred the existence of God as the originator of it; he inferred it also from the mere nature of the idea, because the idea of perfection involves existence. The latter is known as the ontological argument, which Descartes thus reintroduced into modern philosophy. But if God exist, then we have a guarantee, according to Descartes, of the previously determined ground of certitude, for God the Perfect Being cannot deceive, and therefore whatever our consciousness clearly testifies may be implicitly believed.
The most general fundamental principle of the philosophical system of Descartes is the essential difference or dualism of spirit and matter—the thinking and the extended substances—a difference so great, according to Descartes, that they can exert no influence upon each other. Mind or spirit is pure consciousness, and matter is mere extension; these attributes are mutually exclusive, and hence these two 'created substances' can be united (as we find them, for example, in the human being) only through the intervention of the infinite substance or God. This doctrine led directly to the system called occasionalism, the principle of which was that body and mind do not really affect each other, God being always the true cause of the apparent influence of one on the other. A volition on our part is merely the occasion of God's producing a corresponding bodily motion, and similarly the material processes in the nerves and brain are only the occasion, not the cause, of the ensuing mental result. This doctrine was further developed by Leibnitz in connection with his doctrine of pre-established harmony (see LEIBNITZ). The human body being thus a mere machine accidentally united to a rational soul, it was an immediate consequence of the Cartesian doctrine that animals (in which the rational soul is absent) are literally automata; their cries no more imply feeling than the creaking of a machine. This ruthless product of logical consistency unfortunately led to cruelty in practice.
Descartes did not confine his attention to mental philosophy. His philosophy is in fact only the introduction to an elaborate system of physics. In this department it is noteworthy that he completely discarded final causes and proposed to himself the explanation of all physical phenomena from matter and motion. His celebrated theory of vortices, devised to explain the motions of the heavenly bodies, held the field till it was superseded, after a stubborn resistance, by the Newtonian theory of gravitation. It was in mathematics, however, that Descartes achieved the greatest and most lasting results. It was Descartes who first recognised the true meaning of the negative roots of equations; and we owe to him the theorem, which is called by his name, that an equation may have as many positive roots as there are changes of sign in passing from term to term, and as many negative roots as there are continuations of sign, and not more of either kind. He gave a new and ingenious solution of equations of the fourth degree; and first introduced exponents, and thereby laid the foundation for calculating with powers. He showed, moreover, how to draw tangents and normals at every point of a geometrical curve, with the exception of mechanical or transcendental curves; and what perhaps was his highest merit, he showed how to express the nature and the properties of every curve, by an equation between two variable co-ordinates; thus, in fact, originating Analytical Geometry, which has led to the brightest discoveries. Editions of Descartes' collected works were published in Latin in 1697 and 1713, and in French by Cousin in 1824-26. His chief philosophical works have been translated into English by Professor Veitch. See Millet, Descartes, sa Vie, ses Travaux, ses Découvertes (2 vols. 1867-71); Kuno Fischer, Descartes and his School (Eng. trans. 1887); and English works by W. Cunningham (1877), Lowndes (1878), Mahaffy (1880), and Martineau (1885).