The seventeenth 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 the Principles 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. The Principles 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' 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' 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.
The union between mind and body that Descartes posits at the end of Part IV, raises two big worries: (1) what can it mean for two distinct substances to form a union and (2) how can an immaterial substance causally interact with a material one? Many people still consider these worries the biggest obstacle to Descartes' dualistic theory (and thus, in a sense, to his entire metaphysics and physics). Luckily, Descartes' contemporary critics with their modern counterparts and pressed him on these questions in their correspondences. From these correspondences we can arrive at Descartes' answer to these puzzling problems. In his response, Descartes seems to merge these two questions, and to answer them both with an intuition that has been validated by later advances in science and philosophy.
First let us see why Descartes merges the two questions. In order to understand why Descartes does this, it is important to see how Descartes explains the union between mind and body. The best way to describe the union, Descartes claims in IV, as well as in Meditation VI, and in a letter to his friend Regius, is by appealing to the fact that we sense the actions done to body, rather than intellectually perceiving them. When someone else's hand gets burned, we perceive that fact in a very different way than we perceive the burning of our own hand. This is because mind and body are unified.
This way of describing the union seems to imply that the connection between mind and body is causal. To say that the mind and body form a union, it seems, is to say that there is a dense network of causal interactions between mind and body; whenever something is done to body, something happens to mind. In order to answer the first challenge—what can it mean for two distinct substances to form a union, Descartes must answer the second—how can an immaterial substance causally interact with a material one. Descartes answers this question in a correspondence with Princess Elizabeth. There he attempts to challenge the supposition that the only kind of comprehensible interaction is contact interaction, i.e. an interaction in which two material substances come into physical contact, thereby affecting each other. It is perfectly obvious, he rightly claims, that mind and body do interact; we observe this interaction constantly. And because he takes himself to have proven incontrovertibly that mind is immaterial, he believes it must follow that immaterial substances can interact with material substances. The only hurdle to this inference is the flawed supposition that all interaction is contact interaction.
In order to invalidate this supposition, he appeals to a commonly held view regarding gravity. Most people, he claims, implicitly conceive of gravity (which, being pre-Newtonian, he refers to as heaviness) as something distinct from bodies, something that can exist on its own in the absence of body. This, however, involves a conception of a non-extended substance causally interacting with bodies. Though this conception is mistaken (remember that, according to Descartes, gravity is merely a property of body), the intuition present in this mistake—that something immaterial can act on something material—is exactly what is needed in order to defeat the view that only contact interaction is conceivable. We can, therefore, conceive of an immaterial mind acting on a material body and vice versa.
Descartes' intuition—that not all interaction need be contact interaction and that immaterial-material interaction is no more mysterious that material- material interaction—seems to have been largely validated by later advances in science and philosophy. David Hume showed that material-material interactions are not the obvious, well-understood phenomena we take them to be. The evidence we are left with for material interactions is no more than the evidence Descartes insists on for mind-body interaction: we just constantly see it happen. So contact interaction is just as mysterious as material-immaterial interaction. And, in fact, according to modern science, there is absolutely no interaction involving contact between bodies; one body acts on another by way of an electromagnetic field. Descartes' biggest problem, then, is not a problem specifically for his philosophy at all.
Only the most die-hard Descartes scholars pay much attention to his scientific explanations of heavenly phenomena, since they are all consistently off target. (This explains the scarcity of English translations; there is currently only one full translation). However, Descartes' accomplishment should not be underestimated simply because he was dead wrong. It is fascinating to read through the explanations in Parts III and IV and see how each principle that he established in Parts I and II plays a role in some way.
Just to run through the more prominent examples from this section: First, why does Descartes' claim that the heavens are a fluid? This claim rests on two previous principles: the proof that space is a plenum and the proof of the nature of fluid bodies. Since there is no such thing as empty space, the plenums have to be some sort of body. In order to determine what sort of body they are, we must consider their behavior. One thing that can certainly be said about the heavens is that they are penetrable—they easily yield their place to other bodies. Comets zip through them, and planets take their place in their midst, etc. Looking back to the definition of II.54, we see that this means that the heavens must be fluid, since it is fluid bodies, composed of quickly moving particles, that are easily displaced by other bodies.
Next, we must ask, why does this heavenly fluid move in vortices, like a whirlpool. This claim rests on principle II.33, in which Descartes proves that all motion must be in the form of a continuous circuit. Since all of space is a plenum, in order for A to move to B's spot, B must move to C's spot and C to D's spot and so on. As each tiny body takes the other's spot in the quickly moving heavenly fluid, a vortex, swirling around the center, is formed.
Descartes is then able to use the physics of vortices to explain certain other observable phenomena, particularly the relative motion of the planets. Those planets that are positioned toward the center of the vortex naturally move around the sun the quickest, since in a vortex the middle swirls most rapidly. (Just imagine a whirlpool to understand why this is true).
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