Philosopher and scientist. He was educated in the Jesuit College of La Flèche. Having spent a few years in military service, he later traveled extensively through Europe, and finally, in 1629, settled in Holland, where he devoted himself to the study of philosophy. His philosophical works include the (1637), (1641), and (1644). He is called “The Father of Intellectualism”, and contributed to the advancement of mathematics and physics, being the founder of analytical geometry.
The outstanding feature of his system is universal methodic doubt. Contrasting the evidence and certainty of mathematics with the uncertainties and errors of philosophy, he sought to construct a new system of philosophy which would be as evident and convincing as the mathematical sciences. He began by calling into doubt whatever knowledge he had previously acquired, and by seeking a truth so evident that it could not be doubted. This truth he found in the intuition of his own thought and existence. Hence the starting point of his system is the one truth about which he could not doubt: “I think, therefore I am.” He laid down the general principle that the clearness of an idea is the criterion of its truth. From the idea of an infinitely perfect being he inferred the existence of God, because existence must belong to the Infinite and because only God could cause such an idea. The reliability of man’s faculties is deduced from the perfection of God, who cannot deceive. The essence of the soul is thought, and the essence of bodies is extension. The union of body and soul in man is not substantial, but merely mechanical. The soul, located in the pineal gland, moves the organism as a machine, and receives from it external impressions.
The doctrines of Descartes have had a profound influence and, together with those of Francis Bacon, have determined the course of the development of modern philosophy. He himself confined his method strictly to matters of philosophy, at no time calling into doubt truths that belong to theology, and striving always to reconcile his doctrines with the dogmas of Catholic Faith. He is a strange instance of a man who, steadfast in faith and devotion to the Church, and correct in morals, could still by error of judgment propagate views which perverted reason and made both faith and morality unreasonable. Mahony in his traces to Descartes among other errors those of: Occasionalism, that disowns free will for man; Ontologism, denying that we perceive ideas within our soul, or objects directly in themselves; Panilieism, as advocated by Spinoza. Maritain says that the sin of Descartes was Angelism, by regarding man as if he were a pure spirit without a body; as if thought must be intuitive, not deductive; innate, not abstracted from reality external to us; and independent of things but evolved from our own consciousness.