If you are reading this, then you are probably looking at a screen or a piece of paper. Think to yourself: “I have some paper in my hand,” “I am in front of a computer” or whatever fits.
Is your belief here certain? Is there any way that you could believe this, yet your belief be false? Is there any possibility that you are mistaken about this belief? René Descartes (1596-1650) argues you could: this belief, and almost all other beliefs, are not certain.
Descartes argues that there is one clear exception, however: “I think, therefore I am.” He claims to have discovered a belief that is certain and irrefutable. Perhaps there is no saying more famous in philosophy than this phrase, often known as the “Cogito” after its Latin phrasing, cogito ergo sum.
This essay explores the meaning of the Cogito, its importance to Descartes, and its legacy for philosophy up to the present day.
1. Doubt and Skepticism
The phrase “I think, therefore I am” first appears in Discourse on the Method (1637). But Descartes changes the wording to “I am, I exist” in his most famous (1641) work, Meditations on First Philosophy (called the Meditations for short).
In the Meditations, Descartes reflects on the fact that he has had many false beliefs, and he sets out to address that problem, with the hope of finding a way to ensure he only has true beliefs and even that scientific research yields only truths as well.
His strategy is to doubt, or not believe, any claim that is false or could be false. He recognizes that his senses might be deceiving him now, since they have deceived him before; he might also be reasoning erroneously now, since he has reasoned badly before. He thereby doubts all beliefs from his senses and from his faculty of reasoning, since those beliefs could be false.
Descartes then considers the most extreme reason for doubt: there may exist an evil demon (sometimes translated ‘genius,’ ‘genie,’ or ‘spirit’) who has the power to control all of his thoughts, tricking him into believing anything. Descartes cannot prove that this demon does not exist. So he acknowledges that it’s possible that all his beliefs about the world external to his own mind are illusions caused by the demon, corresponding to nothing at all, and so all his beliefs about the external world are false.
Descartes is usually thought of as considering skepticism, the view that we lack knowledge or justified belief. Here skepticism is considered because we lack certainty: what we believe might be false, so our beliefs aren’t knowledge. As we will see, Descartes argues that the Cogito enables him to defeat skepticism and show that we have knowledge, with certainty.
2. The Cogito and Certainty
After considering the evil demon, Descartes soon discovers the Cogito. He realizes that thinking “I am, I exist,” withstands the evil-demon test! Even if all the beliefs and types of beliefs that Descartes reviews are false, or could be false, at the least, he must exist to be deceived. Even if one doubts one’s own existence, one must exist at that moment, since there must be something, or someone, doing the doubting. Doubting is a way of thinking, and one’s existence is required to doubt or think in the first place: it is impossible to doubt and yet not exist.
So, the “I think” element in the Cogito implies the direct, immediate, certain knowledge of one’s own existence. Thought requires a thinker and this is known with certainty, since not even the demon could deceive someone who doesn’t exist. Descartes thereby found what he was looking for: some certain, indubitable, irrefutable knowledge.
3. Defeating Skepticism
Once the Cogito is discovered, Descartes argues it can serve as a foundation for how to find other truths that are certain.
Descartes proposes that the Cogito is undeniably true because it is clear and distinct. About clarity, Descartes explains, “Some perceptions are so transparent and at the same time so simple we can never think without believing them to be true…” When something is distinct, the mind has an unclouded vision of what is most essential about that object. These qualities become the standard against which all other beliefs can be evaluated.
Descartes argues that the clarity and distinctness rule, derived from the Cogito, can justify our beliefs about the external world. But what verifies the clarity-and-distinctness rule? God’s existence, Descartes argues. By reflecting on his idea of God, he argues that God exists. Descartes then argues that a truthful, good God would not allow us to be deceived when we understand objects clearly and distinctly, and so God would not allow us to routinely have false beliefs.
The Cogito then serves as the foundation for a series of claims that build upon each other. According to Descartes, his reasoning establishes that, what he originally doubted, he actually knows, with certainty. He thereby defeats the skeptical concerns that he considered earlier.
4. Conclusion: Knowledge without Certainty
Descartes was impressed by the Cogito because he had found a belief that is certain and so, when believed, cannot be false. He thought that certainty was necessary for a belief to be known. While he argued that, fortunately, we can ultimately be certain of much of what we think we know, most philosophers following him have denied that.
Contemporary theorists of knowledge tend to deny that knowledge requires certainty: they tend to be “falliblists,” arguing that we can know some claim, yet not be certain that it is true. The problem with Descartes’ standard for knowledge is that almost no beliefs meet it. Descartes thought he could show how our ordinary knowledge claims are ultimately based on the Cogito, but most philosophers have not been convinced by his case.
The epistemic lesson of the Cogito is that if certainty is a necessary requirement for knowledge, we are left with very little knowledge indeed. The challenge, however, is that if certainty is not required for knowledge, what is?
 It should be noted that although the idea expressed in cogito ergo sum is usually attributed to and associated with Descartes, it was not an entirely new idea. For instance, over a 1000 years earlier, St. Augustine, in The City of God (Book XI, 26), wrote “ergo sum si fallor,” which is often paraphrased as fallor ergo sum: “I make mistakes, therefore I am.”
 Cottingham, John, Robert Stoothoff, and Dugald Murdoch. The Philosophical Writings of Descartes: Volume 1. Cambridge University Press (2012) (AT VI 32: CSM I 127).
 It’s instructive to consider why Descartes changes the wording from the Discourse on the Method to the Meditations. Unlike in the Discourse, Descartes employs strict tests of doubt in the Meditations, where even simple inferences are put in question. In other words, in setting the stage for the Cogito, the meditator is unsure that logic is reliable, and so cannot legitimately argue from premises to a conclusion that she exists. Another way to account for the missing ergo in the Cogito of the Meditations is to point out that Descartes seeks a foundational belief upon which to provide justification for other beliefs and therefore ground knowledge, and that for a belief to be properly foundational it is in no need of justification itself.
 Cottingham, John, Robert Stoothoff, and Dugald Murdoch. The Philosophical Writings of Descartes: Volume 2. Cambridge University Press (2012) (AT VII 25: CSM II 17).
 The 1999 science-fiction film The Matrix is an update on this idea: one’s beliefs may be caused by the Matrix, not the physical world, and so they are false: e.g., someone “plugged into” the Matrix believing she is riding a bicycle is not actually riding a bicycle, so that belief is false.
 There are different kinds of skeptics. Some skeptics are “global” skeptics, who deny that we have any knowledge at all, about anything: Descartes seems close to a global skeptic, at least before he reaches the Cogito. Other types of skepticism are more limited: e.g., someone might be a skeptic about knowledge claims about the future (“Nobody really knows what will happen in the future”), or a skeptic about claims to religious knowledge, or moral knowledge, or skeptics about knowledge claims based on testimony, and more.
 What follows from the certainty of the Cogito is the nature of Descartes himself: he must be a thing that thinks. The Cogito does not prove that Descartes has a body or a brain, or even that other minds exist: these can all be doubted. Only thought is certain: Descartes says, “I am, I exist, that is certain. But for how long? For as long as I think. It might perhaps even happen that if I stopped thinking, I should at once altogether stop being.” Cottingham, John, Robert Stoothoff, and Dugald Murdoch. The Philosophical Writings of Descartes: Volume 2. Cambridge University Press (2012) (AT VII 27: CSM II 18).
 Cottingham, John, Robert Stoothoff, and Dugald Murdoch. The Philosophical Writings of Descartes: Volume 2. Cambridge University Press (2012) (AT 7, 145-6, CMS 2, 104).
 Descartes argues that his idea of God is such that it could only have been caused by God: Descartes couldn’t have created that idea on his own or from any of his own experiences. Descartes also offers a distinct ontological argument for God’s existence: see The Ontological Argument for the Existence of God by Andrew Chapman
 Thus, Descartes is clearly a foundationalist. See e.g. Hasan, Ali, “Foundationalist Theories of Epistemic Justification,” in E. N. Zalta (ed.), The Stanford Encyclopedia of Philosophy, Fall 2018 edition.
 Something unclear is whether each individual would have to go through the thought processes that Descartes engaged in to have knowledge, or else they lack knowledge, or whether Descartes (or anyone’s, or enough people’s) engaging in these meditations would contribute to everyone’s having knowledge.
Descartes’ Meditations 1-3 by Marc Bobro
Descartes’ Meditations 4-6 by Marc Bobro
Epistemology, or Theory of Knowledge by Thomas Metcalf
External World Skepticism by Andrew Chapman
The Gettier Problem by Andrew Chapman.
The Ontological Argument for the Existence of God by Andrew Chapman
Modal Epistemology: Knowledge of Possibility & Necessity by Bob Fischer
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The editors are grateful to Marc Bobro for his review of this essay.
About the Author
Charles Miceli teaches philosophy in New Jersey and in Asia. He received an MA from Fordham University and reads novels in his spare time. micelicharles @ yahoo.com