There were important postmedieval philosophers before Descartes—there were Renaissance thinkers and thinkers of the early Protestant Reformation that were very important for the development of modern thought—but Descartes, in the early 17th century, made such a characteristic break with medieval thought, and laid down certain patterns that have bedeviled philosophy ever since, that he is usually considered the starting point of modern philosophy.

Descartes was a model of the 17th-century philosopher: a mathematician who made important contributions to analytic geometry, scientist, and a philosopher all at the same time. After a Jesuit education, and having been trained as an army engineer, he traveled, especially through the intellectual center of Europe at the time, Paris, where he became one of the key members of the generation of scientist-philosophers, along with Gassendi, Hobbes, Mersenne, and others who attacked Scholasticism in light of the new Copernican-Galilean science. In fact, Descartes’ most famous book was published with comments by several of these thinkers. Today we’ll examine the argument of that famous little book of his, Descartes’ Meditations on First Philosophy. We’ll see how he begins with radical doubt, discovers what he thinks is certain, and proves the existence of God and the world. But further, we’ll see that the way he did these things changed philosophy forever.

The question Descartes is trying to answer is, what can we know with certainty? We have, on the one hand, our Scholastic tradition, which includes the thought of Aristotle and all the medieval Christian thinkers; on the other hand, we have this new Galilean-Copernican science. How can we know which is right? The way to do this is to nothing less than seek a new foundation, or what some call an Archimedean point for all knowledge. That wasn’t Descartes’ own phrase; the notion was that the ancient Greek mathematician and engineer Archimedes discovered the principle of the lever, and he once said, “If you gave me the right place to stand and a lever long enough, I could move the whole Earth.” Whether he could or couldn’t have done that, ever since then, the notion of an Archimedean point, or a foundation point, for all knowledge, has been attractive to some thinkers— and to no one more attractive than to Descartes. What Descartes hopes is to find some absolute crux of certainty, a foundation of certain knowledge, on which he can base everything else.

The way to do this, he decides, is to try to doubt everything—literally, to try to doubt every belief that’s in his head—and then see if any belief can’t be doubted. Sitting in his study, he proceeded to try to examine all classes of his knowledge or beliefs. This is no easy task: You can’t list all your beliefs. What Descartes tried to do was to look at whole categories of beliefs and decide whether each category of belief was absolutely certain or not. Remember, the criterion, the bar that he’s using to judge a belief, is very, very high: It’s absolute certainty, because he wants to see if there’s a certain foundation for knowledge.

The first thing he does is ask himself, “I have, of course, many beliefs, an uncountable number of beliefs, about the world around me based on my sense experience. Is my sense experience always right, or is it dubitable?” Here, the answer comes back philosophically very fast: Certainly sometimes my sense experience is wrong. If we take a yardstick and stand at the edge of a pond and put it in the pond, it appears to be bent. If we pull it back out, we see it wasn’t bent. How do I know that in other cases, my sense experience isn’t equally misleading? He decides, “Everything I know from my senses is going to be put aside as dubitable.” Again, he’s not insane; he’s not saying, “Everything I learned from my senses is definitely wrong.” He’s saying, “I can’t be absolutely sure it’s right. And that’s the standard, of course, that he’s seeking.

Then he turns to more general facts about the world; in other words, not questions of “Is there a lectern next to me at this point?” (that’s a thing about sense experience), but claims like “Fire is hot” and “Heavy objects dropped near the Earth’s gravitational field fall,” and general facts about the world. Mustn’t they be right, even if my everyday sense experience might be wrong? But then he comes up with a deeper reason for doubting them. He asks, what if I’m dreaming? After all, tonight I may suddenly, after lying down in bed, decide, “Ah, I’m on the Left Bank of Paris and sipping café au lait in a Parisian café.” But I’m not really there; I’m home in my bed

sleeping. Every night, I have the experience of believing a host of general facts that aren’t true at all; so general facts also can be doubted.

Lastly, he comes to what should be the trump card—and has been ever since Plato—for the idea of certain knowledge. He says, what about logic and mathematics? By “logical truths,” he means logical arguments, like: If Socrates is a man, and all men are mortal, then Socrates, too, must be mortal. Doesn’t that have to be true if the premises are true? What about two plus two is four, or all triangles have three sides, or the sum of all the angles in a triangle is 180 degrees? Isn’t it the case, he asks himself, that these must absolutely be true, even if I’m wrong about everything else? But then he comes up with yet another awful thought: What if there’s an evil genius [that is, an all-powerful being] who’s putting illusions in my mind? In more contemporary terms we might ask, how do I know I’m not in the Matrix? How do I know I’m not on a strange planet where all my experiences are programmed into me by some alien presence? If that were true, couldn’t it deceive me about everything, including the alleged truths of mathematics?

Having gone through these examples, Descartes says, it now seems that I can doubt everything. I can doubt mathematics, physics, the sciences, my memories, history, my perceptions; what can’t I doubt? It’s at this point that Descartes comes to his foundation. As he put it in another book, he says, “Cogito ergo sum”: “I think, therefore I exist.” (In the little book the Meditations, his phrase is, “I think, I exist”; and he writes, “I think, therefore I exist” in two other works, once in French and once in Latin.) The point is even if my mind is being deceived about everything, it must be that I have a mind. If my mind is doubting everything, it must be the case that I have a mind that is doubting. It may be that all the knowledge in my mind is false, but that still means I have a mind. To say, “I doubt my sense experience, I doubt memories, I doubt everything I’ve known before” presumes the truth of “I doubt.” Therefore, I must exist: I must be doubting.

This is his famous starting point, and it leads immediately to a short list of indubitable claims that come with it. First of all, I must exist; he’s just decided that. What does the “I” refer to? I must exist as a mental, nonphysical thinking substance—a res cogitans in Latin, which just means a thing that thinks. He’s taking Aristotle’s notion of substance here but changing it.

My mind exists. That doesn’t mean my brain exists (my brain is a physical object); that doesn’t mean my body exists (my body is a physical object)— all I know so far is that my mind exists. But there is a second step we can take: I also know that all the ideas in my mind are indubitable as ideas—not in terms of what they represent about the world, but simply as ideas in my mind. In other words, I think I’m experiencing this lectern. I can doubt the lectern’s existence, I can doubt that it’s real, but I can’t doubt that I think I’m seeing it. Another way to put this is when someone asks you, “Is a dream real?” in one sense the dream is real—you’re having a dream; you’re having images in your mind—what isn’t real is that the images in the mind of the dreamer are not representing a true reality outside that mind.

We can learn something else. When we examine this proof he’s just given me, this proof that he’s explained—I think, therefore I exist; my mind must exist—we can learn a little something about the very process of gaining certainty. What makes this notion, “I think, therefore I exist,” certain is that, Descartes says, the ideas in it are perfectly clear and distinct. It’s logically perspicuous, which means we can see it by the light of nature, the “natural light” of reason. In other words, in the same sense as if you asked me, is two the same as two? I would have to say, of course it is. In that same sense, I recognize if I’m thinking, I must exist in order to be thinking; it’s a logical necessity that’s shown by the natural light of reason.

Descartes further reasons that the essence of material substance, if it exists, is res extensa, or spatial extension. This is actually quite an interesting claim. What he’s saying is the difference between matter and mind—and remember, matter is still in doubt; he’s not assuming matter exists—if matter exists, if material substance exists, what is it? It’s something that takes up space. Mind does not take up space; your thought does not take up space. A thought has no space; an idea has no volume, no width or breadth. Spatial extension is the criterion of material things; material things are res extensa.

If Descartes were to stop right here, he would be a solipsist. In philosophy, that means he would have only his own mind and his mental contents, and he would have no idea of whether the contents of his mind represent true realities that exist independent of his mind. He certainly doesn’t want to stay with solipsism. He wants to prove that the material world exists; he wants to build up from his foundation. But his way of doing this is rather unusual (or it might seem so to us): He doesn’t attempt directly to prove from the existence of his mind that matter exists. First, he must prove God exists. Notice: If he’s going to prove the existence of God—and proving the existence of God, I should say, is nothing new in the history of philosophy; there are many, many proofs of the existence of God, especially in medieval philosophy. The question is, are any of them valid? There are many such attempted proofs—his proofs have to be very particular: They may not use as premises anything that presumes the existence of something outside his mind. In other words, he has to prove God from the existence of his mind and nothing else. He actually offers two proofs of God’s existence—the second one, we’re just going to briefly mention—but first, let’s go into a little detail about the first proof.

He’s already told us that certain things are clear by the “light of nature.” He says, by the light of nature, or what is rationally obvious, the cause of a thing must be at least as great as its effect. He thinks that must be true, because otherwise something would come from nothing. The effect of something is much greater than its cause; where did that greatness come from, if not from the cause? It comes from nothing. The cause of a thing must be at least as great as the effect. As he correctly says, I have in my mind the idea of God as an infinite, perfect being. I—a mind existing by itself; I don’t know about the existence of anything else—have in my mind the idea of God. That doesn’t prove that God exists; I just have the idea of God in my mind, just like I have the idea of this lectern in my mind.

But about the idea of God in my mind, we can ask a very special question: How in the world did it get there? Why is that a problem? Perhaps it came from my experience, but nothing in my experience is infinite or perfect. Given that a cause must be at least as great as its effect, how in the world can an experience of something finite and imperfect lead to the idea of something infinite and perfect? This is one of the oldest strategies of epistemic rationalism: that I have ideas in my mind—certain special ideas like infinity or perfection—which, the claim of the rationalists is, couldn’t have come from my experience. Therefore, I have some knowledge that comes from beyond experience. My mind is not infinite, nor perfect either; thus you might say I imagined or created the idea of God. But that won’t work either, because my mind isn’t infinite or perfect, and so couldn’t cause the idea of God, which is infinite and perfect. What option is left? It must be the case that the idea of God is innate in my mind—meaning it was placed in my mind when my mind was created—and it could only be caused (hence, only be placed in my mind) by a being that really is infinite and perfect. Namely, there must be a God—an infinite, perfect being in reality, independent of my mind—in order to put in my mind the idea of God. And that’s his proof.

He gives a second proof, which again, we don’t have time to go into. Essentially, it’s a rehearsal of the famous ontological argument for God from Saint Anselm in the Middle Ages. Just to tell you what that is very briefly, Descartes argues that we take the idea of God: God is by definition the being with all possible perfections. He then argues that existing in reality is more perfect than not existing; therefore, God must exist by definition. This is a fascinating little proof. Many people have written about it, and it essentially comes from Anselm.

But we’re more interested in how he moves on to prove the existence of matter. How does he do this? Still, at this point, I know God exists, I know my mind exists, but nothing else. God has made me in such a way, Descartes says, has made my mental substance in such a way, that I have no faculty by which I could know that matter does not exist if it does not. Indeed, I’ve been made in such a way that I have a very great inclination to believe in the existence of matter. In other words, my mind had to be created by God. God is an infinite mental substance; my mind is a finite mental substance. God made my mind and put the idea of God himself in my mind as an innate idea. God is responsible for the way my mind is built. I am built in such a way that if I really am in the Matrix—that is, if all of this is just an illusion being put in my mind—there’s no way I could figure that out. But that would make God a deceiver. If I make an error or misjudge things and the cause of my misjudgment is my own fault, then this doesn’t make God a deceiver. But if, so to speak, the situation I am in taxes me so greatly that the best possible use of the reason God gave me still cannot figure out the truth, then God is responsible for my mistakes, and that makes God a deceiver. But God is perfect, and deceit is an imperfection: God cannot deceive. To save God from the charge of being a deceiver, material substances must exist.

This is the bare skeleton of his argument, but what we now want to look at is the metaphysical picture that it establishes. Let’s look at the metaphysics this leaves us with. First of all, reality is composed of finite material substances, the essence of which is to take up space. All physical objects that exist are material substances; they take up space. In addition, there are finite mental substances: These are human minds, and for Descartes, they are also the human soul (as we’ll see in a moment). Lastly, there is one infinite mental substance: God. Mind and body (And remember, body includes the brain. The brain is not the mind; the brain is a physical object. I could hold my brain in my hand—well, I couldn’t, but I could hold yours in my hand.) are utterly distinct kinds of metaphysical things. Body is matter, without qualitative or substantial differences. In other words, the old Aristotelian doctrines of final causes and substantial forms are going to have no role in distinguishing and understanding material substances. Material substances are going to be understood by the new science of Galileo and Copernicus as matter in motion.

Minds are nonspatial thinking things, without parts. Something without parts cannot come apart; it cannot decay. Descartes naturally identifies minds with immortal souls: My soul and my mind are the same thing; they are my mental substance. My soul has no parts; my mind has no parts: It can’t come apart, hence it is immortal. Descartes has thus provided a means, a powerful means, of separating physical material reality that will now be the province of the new science, the new materialist, mechanical science of nature that comes especially from Galileo. Descartes has departed that from the mind— the mind, which is also the soul, the seat of emotions, which is also the seat of my own free will and hence responsible for ethics. Descartes has given to science what is science’s, and to religion and ethics what is religion’s and ethic’s; it belongs to these fields and can’t be studied by science.

Descartes also showed that whenever we use the best cognitive method God has given us, we must be right; otherwise, God is a deceiver. That best method is multiple: the clarity and distinctness of logic, the light of nature that he used earlier, and hence all logic and mathematics. But it’s also the scientific method, the new scientific method of people like Bacon, Galileo, and eventually Newton. These are the methods which, if we use them, we must be right, or else God is a deceiver. Notice this argument doesn’t apply to just anything. If I think my keys are on the night table, and in fact, they’re on the kitchen table, that doesn’t mean God’s a deceiver; it just means I failed to properly apply the rational methods that God gave me to figure out where my keys really are. But if I used the best methods, I must be right, or else God would be a deceiver.

Scientific method, then, guarantees that our beliefs are true, meaning they correspond or are adequate to their objects. As we already saw, Descartes supported epistemic rationalism. Rationalism holds that besides sensory experience, we have another source of ideas and knowledge: For Descartes, those were the innate ideas. These are the most famous basic features of his approach.

However, there are more important things about Descartes’ approach to philosophy, especially his metaphysics, that we need to discuss a bit further. One of those is this: Descartes, in a momentous move, claimed that individual consciousness, the field of evidence presented to my individual mind, is the foundation of all evidence for any claim. This was new; it’s not Aristotelian or Scholasticist at all. Nobody made this claim before Descartes. Everything starts with my mind, or, that is to say, with my consciousness. The guarantee that clarity and distinctness and the light of nature, hence sheer intuitive logical perspicuity, must grant truth is that God cannot be a deceiver. But we must presume the former to get the proof of God. That is to say, thinkers noticed later on that part of Descartes’ argument in the Meditations is circular. He needs to believe in the light of nature, and certain ideas, like a cause must be greater than its effect, in order to prove the existence of God; and it’s only the proof of the existence of God that shows there cannot be a deceiver who is deceiving me about all that I believe. That then justifies that the light of nature must be true. In other words, he’s allowing A to prove B and B to prove A; that’s a circle. Many people noticed this, but Descartes’ view was so useful in other ways that it wasn’t held against him.

Now notice a very important thing—a very important and perhaps perplexing consequence of Descartes’ view. Descartes is a metaphysical dualist. There are only two boxes in which to put any reality—I’m exempting God for the moment; God is a third thing, an infinite mental substance. But regarding anything else that exists, either it’s matter in motion, extended substance, a physical object like a rock; or it’s a mind, a human mind and a soul. This means nonhuman living things have no soul or mind; they are purely physical, exactly like a machine. For example, for Descartes, if I take my pet rabbit and put it in a blender and turn on the blender—which I probably won’t do, but if I were to do that—the scream that the rabbit emits is not an indication that it feels pain. The rabbit feels no pain; it feels no more pain than my fan belt does, which also screams if it needs replacing or if it’s very loose.

How in the world could he say such a thing, that animals have no feelings, can’t feel pain, etc.? He absolutely must say it; it’s implicit in his entire scheme. Why? There are only two options: If the rabbit were to have feelings, it would have to have a mental substance or mind; only mental substances can have feelings. Furthermore, mental substances are identical to souls; the soul and mental substance is the same thing. Only humans are supposed to have souls. If Descartes were to allow animals or plants, but especially animals, to have anything like feelings, anything metaphysically different than what a material object like a machine or a car can have, then the only option, since he’s a dualist, would be to grant them human souls and say that my little pet rabbit’s soul is going to rabbit heaven. He can’t do that. He’s a Christian, and so he’s stuck; animals are nothing but machines.

I should say, by the way, there are later philosophers who have argued— and this is partly because Descartes’ influence is so enormous—that this is characteristic of much of modern philosophy: the failure to recognize that, so to speak, there’s anything between the consciousnesses of the human mind and sheer material existence, like the kind of existence a machine has; in other words, a failure to take, for example, biology seriously. But Descartes is doing this; he’s creating this gap. Why? Because he needs to get rid of Aristotle. Aristotle had a set of gradations between material objects: Plants had a lower level of soul; animals had a higher level of soul than humans. That kind of gradation will not fit with Descartes’ new philosophical motivations, because Descartes wants to give all physical reality to Galileo and mechanical science to understand.

Descartes clearly practiced and was the best example of what we call in philosophy foundationalism: the belief that philosophy’s job is to discover the foundation or ground of all realist, objective knowledge and thereby answer skepticism. That’s the core task of philosophy, Descartes says. Descartes did not invent that quest, but he gave it a canonical formulation; and for more than 300 years, essentially until the 20th century, it went unquestioned.

But lastly, the biggest problem that Descartes leaves us with—he’s left us with many, but the biggest problem for people who take his distinction between mental and physical substance seriously—is this: How can mind and body, thus understood, interact? And interact they must: It must be necessary that my mind influences my body, and my body influences my mind. External events in the world presumably causally impact my senses, which cause perceptions; perceptions and ideas in my mind are supposed to represent external events; and my thoughts and decisions in my mind are supposed to affect my bodily actions, if there’s anything like free will. But how can any of this happen if a mental event doesn’t take up space and a physical object—which is equivalent to a rock, just a piece of matter in space—how can these two things causally interact with each other?

In closing, Descartes forged one of the most successful, and probably the most influential, solutions to the basic problem of the 17th century: of how to put together the new science with other notions of psychology, ethics, and religion—probably one of the most successful and influential solutions. But we have been struggling with that solution ever since.