The dominant philosophy of the last half of the 17th century was that of René Descartes. A crucial figure in the history of philosophy, Descartes combined (however unconsciously or even unwillingly) the influences of the past into a synthesis that was striking in its originality and yet congenial to the scientific temper of the age. In the minds of all later historians, he counts as the progenitor of the modern spirit of philosophy.

From the past there seeped into the Cartesian synthesis doctrines about God from Anselm and Aquinas, a theory of the will from Augustine, a deep sympathy with the Stoicism of the Romans, and a skeptical method taken indirectly from Pyrrho and Sextus Empiricus. But Descartes was also a great mathematician—he invented analytic geometry—and the author of many important physical and anatomical experiments. He knew and profoundly respected the work of Galileo; indeed, he withdrew from publication his own cosmological treatise, The World, after Galileo’s condemnation by the Inquisition in 1633.

Each of the maxims of Leonardo, which constitute the Renaissance worldview, found its place in Descartes: empiricism in the physiological researches described in the Discourse on Method (1637), a mechanistic interpretation of the physical world and of human action in the Principles of Philosophy (1644) and The Passions of the Soul (1649), and a mathematical bias that dominates the theory of method in Rules for the Direction of the Mind (1701) and the metaphysics of the Meditations on the First Philosophy (1642). But it is the mathematical theme that clearly predominates in Descartes’s philosophy.

Bacon and Descartes, the founders of modern empiricism and rationalism, respectively, both subscribed to two pervasive tenets of the Renaissance: an enormous enthusiasm for physical science and the belief that knowledge means power—that the ultimate purpose of theoretical science is to serve the practical needs of human beings.

In his Principles, Descartes defined philosophy as “the study of wisdom” or “the perfect knowledge of all one can know.” Its chief utility is “for the conduct of life” (morals), “the conservation of health” (medicine), and “the invention of all the arts” (mechanics). He expressed the relation of philosophy to practical endeavours in the famous metaphor of the “tree”: the roots are metaphysics, the trunk is physics, and the branches are morals, medicine, and mechanics. The metaphor is revealing, for it indicates that for Descartes—as for Bacon and Galileo—the most important part of the tree was the trunk. In other words, Descartes busied himself with metaphysics only in order to provide a firm foundation for physics. Thus, the Discourse on Method, which provides a synoptic view of the Cartesian philosophy, shows it to be not a metaphysics founded upon physics (as was the case with Aristotle) but rather a physics founded upon metaphysics.

Descartes’s mathematical bias was reflected in his determination to ground natural science not in sensation and probability (as did Bacon) but in premises that could be known with absolute certainty. Thus his metaphysics in essence consisted of three principles:

1. To employ the procedure of complete and systematic doubt to eliminate every belief that does not pass the test of indubitability (skepticism).
2. To accept no idea as certain that is not clear, distinct, and free of contradiction (mathematicism).
3. To found all knowledge upon the bedrock certainty of self-consciousness, so that “I think, therefore I am” becomes the only innate idea unshakable by doubt (subjectivism).

From the indubitability of the self, Descartes inferred the existence of a perfect God; and, from the fact that a perfect being is incapable of falsification or deception, he concluded that the ideas about the physical world that God has implanted in human beings must be true. The achievement of certainty about the natural world was thus guaranteed by the perfection of God and by the “clear and distinct” ideas that are his gift.

Cartesian metaphysics is the fountainhead of rationalism in modern philosophy, for it suggests that the mathematical criteria of clarity, distinctness, and logical consistency are the ultimate test of meaningfulness and truth. This stance is profoundly antiempirical. Bacon, who remarked that “reasoners resemble spiders who make cobwebs out of their own substance,” might well have said the same of Descartes, for the Cartesian self is just such a substance. Yet for Descartes the understanding is vastly superior to the senses, and only reason can ultimately decide what constitutes truth in science.

Cartesianism dominated the intellectual life of continental Europe until the end of the 17th century. It was a fashionable philosophy, appealing to learned gentlemen and highborn ladies alike, and it was one of the few philosophical alternatives to the Scholasticism still being taught in the universities. Precisely for this reason it constituted a serious threat to established religious authority. In 1663 the Roman Catholic Church placed Descartes’s works on the Index Librorum Prohibitorum (“Index of Forbidden Books”), and the University of Oxford forbade the teaching of his doctrines. Only in the liberal Dutch universities, such as those of Groningen and Utrecht, did Cartesianism make serious headway.

Certain features of Cartesian philosophy made it an important starting point for subsequent philosophical speculation. As a kind of meeting point for medieval and modern worldviews, it accepted the doctrines of Renaissance science while attempting to ground them metaphysically in medieval notions of God and the human mind. Thus, a certain dualism between God the Creator and the mechanistic world of his creation, between mind as a spiritual principle and matter as mere spatial extension, was inherent in the Cartesian position. An entire generation of Cartesians—among them Arnold Geulincx, Nicolas Malebranche, and Pierre Bayle—wrestled with the resulting problem of how interaction between two such radically different entities is possible.