Here’s a dark meditation to begin your day: imagine that everything you experience upon waking is an illusion. The pale light slipping through the window, the roasted aroma of brewing coffee, the birds chirping in the distance—these sensations are the very means by which you take yourself to be awake. And yet, what if they are nothing more than a carefully staged deception, orchestrated by a malicious demon?
How could you prove otherwise?
This unsettling possibility—that you are merely a puppet being manipulated by unseen forces—has cast a long shadow over philosophy. It was René Descartes—the 17th-century philosopher, scientist, and mathematician—who gave the problem its most provocative form, and thinkers have been grappling with it ever since.
Born in 1596 in La Haye, France, Descartes earned his reputation as the “father of modern philosophy” by breaking decisively with Aristotle and forcing philosophy to confront skepticism head-on. At the same time, he reshaped mathematics and science—pioneering analytic geometry, refining mathematical notation, and advancing the study of optics—while championing a radically new, mechanical picture of nature and the human body.
But Descartes’ ambitions ran deeper. He sought to establish an unshakable foundation for scientific knowledge and, as a devout Catholic, to prove the existence of God. These two tasks—we will argue—proved incompatible; in trying to lay the groundwork for the future of science, Descartes inadvertently undermined the substructure of religious faith—the authority of revelation.
Let’s see how Descartes demolished the very thing he was striving to defend.
Descartes’ Search for Certain Knowledge
Taking the time to rigorously examine our deepest convictions is not something most of us are inclined to do. If anything, it runs directly against the grain of human nature. But Descartes knew that, if he were to establish anything “firm and lasting in the sciences,” he would have to “raze everything to the ground and begin again from the original foundations.”
The method he employed is strikingly simple—accessible to anyone with an open mind and some free time to spare. It asks us to question what we normally take for granted, to scrutinize “common sense” assumptions that many people have never seriously examined. Yet in doing so, it becomes a deeply instructive exercise.
It proceeds like this: reject as false any belief that admits even the slightest doubt, then take whatever happens to survive as a secure foundation—like the axioms of geometry—upon which the entire edifice of knowledge can be rebuilt. This was Descartes’ bold and ambitious project. To appreciate its force, let’s retrace his steps and see where they lead.
Notice first that Descartes did not attempt the impossible task of examining each belief one by one. Instead, he turned his attention to the foundations on which those beliefs rest, reasoning that if the base could be called into question, everything built upon it would be subject to doubt.
So he started with the senses themselves. He noted how they can sometimes deceive us: sticks submerged in water appear bent, phantom pain is felt from a limb that was previously amputated, the taste of food is altered when you have a cold. The list of perceptual illusions is seemingly endless. In fact, there’s an entire branch of psychology devoted to the many ways that the mind misconstrues reality. The question, Descartes realized, is whether or not it always does.
“I have noticed that the senses are sometimes deceptive,” Descartes wrote, “and it is a mark of prudence never to place our complete trust in those who have deceived us even once.” Just as you cannot trust a habitual liar, you likewise cannot depend on the senses to furnish accurate information about the world. If the senses are unreliable some of the time, perhaps they are all of the time.
While it’s true that perceptual illusions exist only in relation to some standard—and that we expose illusions according to this baseline—it is still within the realm of possibility that we’re simply replacing one illusion with another, and that all sense experience is, to some degree, a deviation from an underlying reality that is beyond our reach.
This is a major roadblock, of course, considering that virtually all of our knowledge is arrived at with or through the senses—even the perception of our own bodies. In fact, according to Descartes, we can’t even be sure we have bodies at all. They could be illusory, or we could be dreaming, or we could be characters in a computer simulation or even brains in a vat.
The senses, then, are out as infallible guides to knowledge. But what about things like numbers? “For whether I am awake or asleep,” Descartes proclaimed, “two plus three make five, and a square does not have more than four sides. It does not seem possible that such obvious truths should be subject to the suspicion of being false.”
And yet, after further reflection, Descartes knew that they could. Not even the precision of mathematics can escape Descartes’ razing method of doubt. It is here that Descartes introduces his malicious demon:
I will suppose not a supremely good God, the source of truth, but rather an evil genius, supremely powerful and clever, who has directed his entire effort at deceiving me. I will regard the heavens, the air, the earth, colors, shapes, sounds, and all external things as nothing but the bedeviling hoaxes of my dreams, with which he lays snares for my credulity. I will regard myself as not having hands, or eyes, or flesh, or blood, or any senses, but as nevertheless falsely believing that I possess all these things.
The possibility of this demon implies an even more disturbing thought: the axioms of mathematics seem unquestionably certain—but what if that certainty itself is an illusion? What if a malicious demon is not merely deceiving your senses, but shaping your very sense of logical necessity? In that case, the threat of deception reaches even deeper into the structure of thought itself. Even the feeling of absolute certainty—the last true refuge of knowledge—can no longer be trusted as a foundation for anything at all.
What began as an ambitious and hopeful project has turned into something rather bleak. Everywhere we look for certain knowledge, we find a dead end—even in the theorems of mathematics. “But I have persuaded myself that there is absolutely nothing in the world, no sky, no earth, no minds, no bodies,” Descartes bemoans. “Is it then the case that I too do not exist?”
But wait a minute. How could Descartes deny his own existence? Even if he’s deceived about everything he thinks he knows, there must still be a him for that deception to occur. To be misled, he must exist. His existence, at least as a thinking thing, withstands even the most radical doubt—it is the one point that cannot be denied without contradiction. Existence, it turns out, is our true bedrock—even in the presence of the malicious demon.
“Let [the malicious demon] do his best at deception,” Descartes declares, “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.”
This is the big payoff: “Cogito, ergo sum”—I think, therefore I am. At last, Descartes had acquired the certain knowledge he labored so hard to find. And yet, after the triumph wears off, one can’t help but feel a little underwhelmed. The brute fact of our own existence doesn’t leave us much to work with. How are we supposed to go about acquiring knowledge of the world when the only tools at our disposal—our senses and reason—are by nature unreliable and open to doubt?
Descartes had an answer. He would use the uplifting power of God to pull himself out of the epistemological hole he dug for himself. Deploying a variation of the ontological argument, Descartes tried to show that a perfectly good God—who’s perfection necessitated His own existence—would not deceive us. From this foundation, Descartes rebuilt the entire world, one extensible object at a time.
Except it’s not convincing, even on Descartes’ own terms. Recall that, if the malicious demon can make us think 1 + 1 = 3 with absolute certainty—as Descartes claimed he could—what’s stopping this demon from making us also think that there is a perfectly good God that would never deceive us when, in fact, there isn’t? Descartes had already eliminated the subjective feeling of absolute certainty as a criterion of truth. Yet he invokes it later to preserve his Catholic beliefs.
There’s another problem: Descartes claims that clear and distinct ideas can be trusted as true because of the existence of a non-deceptive God, but he then proves the existence of this God using clear and distinct ideas. Clearly, he’s begging the question—a common criticism known as the Cartesian Circle.
It’s hard to see how any of this is anything more than a case of special pleading. Descartes refused to accept that 2 + 3 = 5 with absolute certainty yet fully embraces the “clear and distinct” idea of a perfectly good God. Most philosophers, unsurprisingly, reject this solution, and we will swiftly follow suit.
But where does that leave us? Can we be certain about anything beyond our own existence and ability to think? The answer seems to be yes, and no.
The Saving Grace of Science
The gravity of the problem Descartes introduced cannot be overstated. We truly have no means of conclusively proving that our senses faithfully represent an external reality, rather than merely projecting a convincing illusion. There is no vantage point beyond the confines of our own nervous system—no place from which to compare appearance with reality itself. And if that’s the case, then we have little choice but to abandon the idea that true epistemic certainty—beyond our existence as thinking beings—can ever be achieved.
Yet this is no cause for despair; it is precisely where dogma ends that scientific progress begins. It is true that, individually, we are severely compromised by Cartesian doubt. Evolution itself may have blinded us to our own biases. But collectively, we’ve discovered the means by which we can circumvent the distorting effects of our own fallibility. The institutions of science—complete with peer review, rigorous testing, and higher standards of evidence—have rendered Cartesian doubt irrelevant in practice, and what we’ve lost in absolute certainty we’ve more than gained in our expanding mastery of the environment.
Scientific claims, of course, are held only provisionally, and are always open to revision in light of new evidence. Yet they are strengthened through repeated testing and attempted refutation. In this way, our beliefs, though fallible, can achieve a kind of institutional credibility when they withstand rigorous scrutiny. The ideas that survive this process converge, gradually, on increasingly reliable accounts of reality. This is not perfection, but it is progress. And by this standard, the achievements of science are extraordinary.
Cartesian doubt, then, did not spell the end of knowledge, nor did it obstruct scientific progress. Science, in fact, brilliantly works around it. But what Cartesian doubt did demolish—unbeknownst to Descartes at the time—is the authority of religion. Unlike science, religions are founded on personal revelation—and therefore on the unreliability of the individual human mind.
Cartesian Doubt and the Collapse of Faith
Saul of Tarsus—before becoming Saint Paul—was a devout Jew and early opponent of Christianity. On his way to Damascus to arrest more of Jesus’s followers, he was struck by a blinding light and fell to the ground. A voice then said, “Saul, Saul, why do you persecute me?” When Saul asked who was speaking, the voice said, “I am Jesus, whom you are persecuting.” From that point forward, Paul would be a committed follower of Jesus, and did arguably more than any other historical figure to spread the Christian faith.
For the committed Christian, this well-known and instructive tale of religious conversion helps to solidify one’s faith. But considering everything we’ve just seen, how trustworthy should you take this story to be? How trustworthy should Saul have taken his experience to be?
Let’s push this inquiry a bit further and ask: What if Saul had adopted Descartes’ method of doubt? Where might this have led him?
First, Saul might have noted the unreliability of his senses. His “vision” may have simply been the result of a seizure or bout of epilepsy, or the consequences of a sunstroke or migraine aura. Or perhaps he was suffering from guilt-induced psychological crisis or from some other form of mental illness. Alternatively, had Saul been fasting, perhaps he was experiencing a starvation-induced hallucination. The deeper question is this: by what means could Saul have ruled out these competing natural explanations?
Suppose, however, we grant that Saul did experience something genuinely supernatural. A deeper problem immediately emerges: how could he identify the source? From his own perspective, what grounds would justify the conclusion that the voice was truly Jesus Christ? By what reliable method could he distinguish divine revelation from deception (or self-deception)? Absent such a method, the possibility remains that he was not encountering Christ at all, but Descartes’ malicious demon.
The religious implications of this for the rest of us are unsettling. From our vantage point, if Descartes’ demon can deceive us regarding the existence of our own body—and even of the theorems of mathematics—then it can surely deceive us regarding the content and significance of our personal religious experiences and revelations.
In science, remember, the problem of the malicious demon does not disappear, but it’s made to be more manageable. For all we know, we are all being deceived about the deepest laws of motion and gravity. But collective deception is still collective knowledge, and whether we are being deceived or not, we’re still able to use this knowledge, for instance, to dependably launch orbiting satellites into space and receive back reliable signals. If this is not an accurate depiction of some ultimate reality, it remains a faithful account of the reality presented to us—one we can reliably understand, manipulate, and control.
But religion is a different beast. Revelation is personal. It cannot be institutionally verified or supported. If you hear a voice in your head—and no one else can hear it—there is no legitimate means for you to be able to distinguish this voice as emanating from the God of your chosen religion rather than from Descartes’ deceptive demon—or from mental illness, or from some other perceptual illusion.
And—crucially—faith cannot save you. Faith is based on a feeling of absolute certainty, and Descartes had already shown us that our feelings cannot be trusted as infallible guides to knowledge. Descartes, then, unwittingly turned faith—belief without sufficient evidence—into unwarranted gullibility. After Descartes, there can no longer be any refuge in the inner subjectivity of religious faith.
Descartes seems to have stopped short of the full implications of his own method, and ultimately retreated back into the familiar comfort of his Catholic beliefs. But for the rest of us, faith now sits uncomfortably close to the very illusions his demon was designed to reveal.
References
Descartes, René. Meditations on First Philosophy. Translated by Donald A. Cress. 3rd ed. Indianapolis: Hackett Publishing Company, 1993.
Hatfield, Gary. “René Descartes.” The Stanford Encyclopedia of Philosophy, edited by Edward N. Zalta and Uri Nodelman. Last modified October 23, 2023. https://plato.stanford.edu/entries/descartes/.