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Though Descartes lived most of his adult life as a recluse, the history of his day did come to bear on his life in one very practical way. The early 17th century was a time of intense struggle between science and religion, and Descartes, as one of the leading proponents of the new mechanistic science was heavily influenced by this struggle.
By the time Descartes reached maturity, a scientific revolution was already underway. Thinkers such as Nicolas Copernicus, Johannes Kepler, and Galileo Galilei, had posited a new sort of worldview, one which contradicted the biblical accounts of the universe and of our central place within it. The Church reacted by banning certain books and forbidding offending hypotheses to be stated as theories of fact. This tension came to a head in 1633, when the Church condemned Galileo, placing him under arrest for asserting his radical cosmological findings as fact rather than fiction.
When news of the condemnation reached Descartes, he had just finished his own book on cosmology, The World, in which he too attempted to establish the heliocentric system as fact rather than as useful fiction. Terrified by Galileo's treatment, he suppressed the work. It was not published until after his death. Descartes himself was a deeply religious man, and so in addition to fearing for his well-being, he probably also had a simple desire for the approval of the religious establishment.
After Galileo's condemnation, Descartes tread lightly on all scientific ground. Though he included his cosmology in Principles of Philosophy, it is modified considerably, so that the earth itself remains static. In addition, he took to adding lengthy and cagey caveats to all of his works, admitting that his theories could not contradict anything that God himself had revealed, while at the same time claiming that God himself could be used to guarantee the utter truth of his arguments. Several of Descartes' more controversial arguments, such as his proof that man's body is a machine, are stated in a purposely obscure manner, in order to avoid religious wrath.
The 17th century saw a dramatic rise in mechanistic and mathematical explanations in science, descriptions of the natural world that referred only to the motion of matter (often in the form of mathematical formulas) in order to account for all observable phenomena. Descartes was not the first scientist to develop a mechanistic, mathematical science, though he was influential in its development and perhaps was the most ambitious scientist in terms of his scope. He was, however, the first to give a thorough and comprehensive philosophical response to the demands raised by this new way of viewing the world. His writings initiated a dramatic revision of philosophical method and concerns.
Descartes explains in the preface to Principles of Philosophy why he felt the need to give a philosophical response to the new science in the first place. As he writes there, he viewed all of human knowledge as a tree, each part relying heavily on the others for vitality. The trunk of the tree he compared to physics, and the branches to the applied sciences of medicine, mechanics, and morals. The roots, giving support and nourishment to the entire system, he claimed, was metaphysics, the philosophical study of the nature of God, the world, and everything in it. Principles of Philosophy was intended as a coherent picture of the entire tree, his magnum opus, which he hoped would serve as a textbook, should his work ever be taught at the universities.
In order to understand why Descartes felt that a new metaphysics was needed to ground his new physics, it is important to have a sense of the worldview he was reacting to. Both Descartes's philosophy and his physics are best viewed as a response to the Aristotelian-influenced Scholastics, who had dominated the intellectual scene for almost 2000 years. According to the Scholastic view, all of natural philosophy reduced to the study of change. Explanations relied heavily on the obscure metaphysical notions of "essence", the characteristic that makes something the sort of thing that it is, "matter," the thing that remains constant through change, and "form," the thing that changes when change occurs. Also crucial to these accounts of change were the four elements: earth, air, fire, and water. The most basic units of existence of this view, substances, are all various mixtures of these four elements.
Descartes believed that the obscure metaphysical notions of matter, form, and the elements needlessly complicated the picture of the world. More specifically, the inclusion of such concepts made it impossible to give explanations purely in terms of the motion of matter (which is precisely what the new mechanistic physics sought to do). In order to clear the way for a new scientific outlook, Descartes had to dramatically simplify the metaphysical picture. Where the Scholastics had posited numerous types of substances, each with their own essence and each requiring their own type of explanation in terms of earth, air, fire, and water, Descartes argued that there were only two types of substances in the world. There was mental substance, whose essence was thinking, and there was physical substance, whose essence was extension. Since the entire observable world thus reduced to a single sort of substance (i.e. physical substance or body), all natural phenomena could be explained by relying on just a small number of principles, based entirely on the property of extension. Physics conveniently collapsed into geometry, the study of extended body.
Given his mechanistic picture of the world, on which all explanation could be given in terms of the extension of physical substance, Descartes also needed a new epistemology, or theory of cognition, to complement his new physics and metaphysics. Scholastic philosophers, following Aristotle, believed that all human knowledge comes through the senses. That is to say, they were empiricists. However, their empiricism was of a very naïve form; they believed that our senses are incapable of systematically deceiving us about the kinds of things that are in the world. If the senses tell us that there are colors, then there are colors. If the senses tell us that there are enduring objects, such as tables and chairs, then there are enduring objects. The trustworthiness of the senses was built into the conception of how perception operated: the one perceiving, on this view, took on the form of the thing perceived, became, in a very obscure sense, like the object of perception. Yet in Descartes' picture of the world, there was no such thing as color, sound, odor, taste, heat. There was only extension and the properties that arose from it, such as size, shape, and motion. In order to defend his physics and metaphysics, therefore, Descartes was forced to come up with a new understanding of where human knowledge comes from. Knowledge could not come from our senses, because our senses tell us that we live in a colorful, loud, odorous, tasty, hot, cold world.
In order to rid knowledge of sensory influence, Descartes' freed the intellect from the senses altogether. Where the Scholastics had claimed that nothing got into the intellect except through the senses, in Descartes' theory of cognition, certain concepts are present in the intellect at birth. According to Descartes, human beings are born with certain innate concepts, concepts such as "God," "extension," "triangle," and "something cannot come from nothing." Using these innate concepts, and our faculty of reason, we can trace chains of logical connections and unravel all the possible knowledge in the world.
Both Descartes's metaphysics and his epistemology have been hugely influential in the history of philosophy. In fact, Descartes is largely responsible for setting the modern philosophical conversation in motion. John Locke, Baruch Spinoza, G.W. Leibniz, George Berkeley, and Immanuel Kant, all modeled their metaphysical positions on the Cartesian picture, presenting their own radically modified versions of Descartes' view. Even today, Descartes' theory of the nature of mind, and of mind's relation to body, continues to play a central role in philosophical debates. In epistemology, Descartes' terminology and his conception of a purely intellectual faculty found their way into the writings of John Locke, Blaise Pascal, Baruch Spinoza, and G.W. Leibniz. His concern with the limitations of human reason in its pursuit of knowledge was picked up by an even wider circle.
Descartes' theory of knowledge also gave rise to the most famous split in the history of modern philosophy, the divide between the rationalists and the empiricists. The rationalists (Nicolas Malebrance, Baruch Spinoza, and G.W. Leibniz) accepted the Cartesian idea that humans have a purely intellectual faculty that can serve as a reliable source of substantive knowledge about the world. The empiricists (most famously, John Locke, Thomas Reid, George Berkeley, and David Hume) also believed in the existence of Descartes' purely intellectual faculty, but they were dubious that this faculty could tell us anything, except tautological truths, without the help of the senses. This debate, too, rages on even today, with the two sides gaining and losing respectability at one another's expense, on a decades-long cycle.