Descartes’ Cogito

The expression “I think, therefore I am” is basically philosophy’s motto. Before I studied any academic philosophy, I had of course heard of it and thought it was just ancient Greek wisdom, because everyone knows how much the ancient Greeks loved to think about thinking. So I was embarrassed to learn shortly into my philosophical ventures that the expression rather comes from Descartes, a French mathematician and scientist of the 1600’s, and that it marks a modern turn in philosophy away from scholasticism.

The expression—called the cogito because Descartes also formulated the expression, besides in French, as the Latin “Cogito, ergo sum”—is actually a compact argument (the ‘therefore’ should be an obvious indicator to anyone who studies arguments), one which factors into a larger argument about knowledge that is certain. Because of this, the cogito is also understood as the larger argument around the expression about certain knowledge. So what is certain knowledge, and what is Descartes argument for a particular kind of certain knowledge that we can have? Let’s answer these questions.

In our context, a person who has certain knowledge is a person who has a belief that cannot be doubted. To unpack this a little more, beliefs are what a person accepts as true. What we accept as true is often expressed in propositions, or sentences that are either true or false. For example, you may accept as true that water is wet, or that Wednesday is the day after Tuesday. Now, someone may reject a proposition for any or no reason at all. A proposition that is rejected for no reason is also perhaps understood to be doubted by that person. However, this is merely a psychological understanding of doubt. Another understanding, the epistemological understanding, is one where doubting is done for good reasons. Here, good reasons encompass evidence against or counterexamples to the belief. So, for example, if I believe my keys are in some spot that I think I remember recently putting them, then a good reason to doubt this belief is by the counterexample of looking in that spot and not seeing them there. Here, my belief is properly doubted. Improperly, I might start not believing I have ever had a key, and that the government is responsible for opening my door when I want to drive, because I am insane. And so, here I now doubt my previous belief that my keys are in that spot, but not for evidence or good counter-examples (what constitutes good vs. not so good counterexamples will not matter in what follows).

Descartes proceeded to certain knowledge through his method of doubt. Descartes’ method of doubt involved him examining his beliefs, entire classes of them at a time, and finding whether or not any of them could withstand the fiercest doubts possible; that is, if there was any possible evidence or counterexample to a kind of belief, no matter how far-fetched, he would reject all of that kind as if it were built from the flimsiest evidence, until he was left with either no more beliefs or else some belief that is certain, one which cannot be doubted.

And this was done in the extreme. With basically the waving of his armchair-philosopher’s hand, Descartes discounted all of the knowledge of his senses about his environment, because he might be dreaming. The candle near his fireplace or the men walking around outside might not be anything at all, just a figment of his imagination. Even his own leg, arm, face, and so on—his body—was not exempt from doubt here. Being a mathematician, you might think that Descartes would consider a mathematical proposition such as “3 + 2 = 5” incapable of being doubted. Well, you would be mistaken. Descartes’ counterexample to mathematical knowledge (or any demonstrable knowledge) is the possibility that some very powerful demon was tasked with deceiving Descartes as he calculated numbers, such that ‘5’ was merely implanted in his mind whenever he put 3 and 2 together, and that he never knew the true answer to the problem (his teachers were maybe also subpar).

So what could survive Descartes’ method of doubt? In doubting even his own thoughts as something a god-like being was implanting (like the government), Descartes wondered if he himself could be doubted to exist all, because, after all, he doubted that he even had a body. Answering this, Descartes says, “Then . . . there is no doubt that I exist, if [the god-like being] is deceiving me. And let him do his best at deception, he will never bring it about that I am nothing so long as I shall think that I am something. Thus, after everything has been most carefully weighed, it must finally be established that this pronouncement ‘I am, I exist’ is necessarily true every time I utter it or conceive it in my mind” (1).

That is, Descartes’ current state of taking himself to be radically deceived is proof positive that he is something or other, that he exists. If not, who is being deceived? If no one, but we are already supposing that someone is deceiving him. So still, Descartes is. If, alternatively, Descartes is now convinced that he is deceived and that he doesn’t exist, still he is doubting that he exists. Therefore he is thinking he exists (Descartes includes doubting as part of thinking, and this technically isn’t required for Descartes’ conclusion). Therefore, Descartes concludes, “I exist.” This latter way of proceeding with the proof is often called the dubito (“I doubt”, in Latin). It goes: “I doubt; therefore, I think; therefore, I am.” And so, anyway you spin the cogito, the certain conclusion is that the one who thinks through it exists.

There are some rebuttals to the cogito that center around questioning what this “I” is that Descartes invokes. However, Descartes has a fairly certain way of responding to such doubts. He uses the cogito as his guide for what the I is, whatever else it may be besides. He says, “But what then am I? A thing that thinks. What is that? A thing that doubts, understands, affirms, denies, wills, refuses, and that also imagines and senses.” That the thinker or thing who thinks the cogito constitutes at least one of these things—it seems to me—is impossible to doubt. And even if I am wrong, well, it still seems that way to me. And then what am I? A thing for which things seem, it is only certain to conclude.

Descartes had found certain knowledge through his method of doubt. His purpose of acquiring such certain knowledge was no less than building up more knowledge “from the original foundations” in careful succession in order to “establish anything firm and lasting in the sciences” (1). Much of Descartes philosophy’ is set to the task of giving us back our knowledge of math, the external world, and the sciences generally from this foundational starting point. If you would like to learn more about how he takes up this task, I would recommend Descartes’ Meditations on First Philosophy, which is a succinct and fairly clear read. Until next time, happy existing!


  1. Descartes, Meditations of First Philosophy, trans. Cress, Donald A (Indianapolis/Cambridge: Hackett Publishing, 1993), 20.

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