Rene Descartes and the Search for Certain Knowledge

We all hold innumerable beliefs with varying degrees of certainty, but few of us have challenged the veracity of those beliefs to the degree that Rene Descartes did in the Meditations on First Philosophy, published in 1641. Descartes wrote:

“Some years ago I was struck by how many false things I had believed, and by how doubtful was the structure of beliefs that I had based on them. I realized that if I wanted to establish anything in the sciences that was stable and likely to last, I needed – just once in my life – to demolish everything completely and start again from the foundations…I am here quite alone, and at last I will devote myself, sincerely and without holding back, to demolishing my opinions.”

Descartes, the French philosopher, mathematician, and scientist often referred to as the father of modern philosophy, proceeded to challenge all of his beliefs, rejecting propositions that were not only obviously false but that were also not completely certain, writing, “all I need, for the purpose of rejecting all my opinions, is to find in each of them at least some reason for doubt.” 

Descartes didn’t scrutinize every one of his beliefs; he simply challenged the foundation on which all of his other beliefs were built—the senses. He noted two things about the senses (sight, sound, smell, taste, and touch):

  1. The senses sometimes deceive us, and it is unwise to trust completely that which has deceived us even once (think optical or auditory illusions, for example). 
  2. It is sometimes impossible to distinguish dreams from reality in the moment when one is dreaming, and the realization that one was dreaming comes only after waking. The present moment of a dream can be subjectively indistinguishable from the present moment of waking life; therefore, there is no reliable method to prove that you’re not dreaming. 

Realizing that all of his knowledge of the world—of matter, extension, size, color, shape, quantity, place, and time—depended on input from the senses, and also realizing that the senses cannot be trusted entirely, Descartes was forced, based on his stated project, to reject all of his beliefs, as they all were subject to at least some degree of doubt. 

As for knowledge that seems to be independent of the senses, such as, for example, the basic truths of arithmetic, Descartes addressed next. Surely, he proposed, it cannot be doubted that two plus three equals five. But even here, after further reflection, Descartes realized that, just as the senses can deceive us, and that we could be dreaming at any moment, we could also be subject to systematic manipulation by someone or something that could create the false impression of certainty of even basic mathematical truths. Descartes wrote:

“I shall suppose that some malicious, powerful, cunning demon has done all he can to deceive me…I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely dreams that the demon has contrived as traps for my judgment. I shall consider myself as having no hands or eyes, or flesh, or blood or senses, but as having falsely believed that I had all these things.”

As implausible as the systematic manipulation of all our senses and beliefs by a cunning demon seems, there appears to be no way of refuting it, which is quite unnerving for all those who give it serious consideration. Modern versions of Descartes’ demon include the idea that we are all living inside of a computer simulation created by an advanced civilization, or that we are all “brains in a vat,” where our neurons are stimulated by scientists or aliens to create the impression of sensation and waking experience. 

Again, the implausibility of the scenario alone does not show that it is not, or cannot, be true, and at the very least elucidates the point that what we take to represent experience is based on our senses, which may or may not accurately represent reality. There is, in fact, no way to know, no way to get outside of our own minds to assess things “objectively” or independently from our minds’ perceptual filters. 

Think about it this way: if we were all born with green-tinted glasses that could never be removed (or were forced to see only green by Descartes’ demon), all we would see is the color green. But this wouldn’t be an accurate representation of the properties of objects, it would simply be a representation of our own sensory mechanisms. If you apply this reasoning to all of our senses regarding all properties of all objects you will see the depth of the problem Descartes uncovered. 

Can anything be known with absolute certainty?

Descartes’ project was to reject all beliefs that introduced even the slightest degree of doubt, which forced him to discard all of his beliefs derived from the senses. It even forced him to reject all abstract knowledge that could be subject to mental manipulation and a false sense of certainty. 

This could easily lead one to conclude that no knowledge is certain and that everything is open to doubt, as it did for the ancient Greek Phyrronian Skeptics. In fact, it is worth pointing out that Descartes is not the first philosopher to scrutinize his beliefs and to note the unreliability of the senses. That distinction belongs to the ancient Greek Skeptics, including Sextus Empiricus.

But Descartes did not follow the Skeptics in suspending all belief; in meditating on the problem of knowledge, Descartes had a realization. While he conceded to the possibility that a “supremely powerful and cunning deceiver” could constantly deceive him regarding every belief at every moment, he noted:

“Even then, if he is deceiving me I undoubtedly exist: let him deceive me all he can, he will never bring it about that I am nothing while I think I am something. So after thoroughly thinking the matter through I conclude that this proposition, I am, I exist, must be true whenever I assert it or think it.

Descartes found the certain knowledge he was searching for, which appears to be beyond doubt. You may be deceived about everything, and all of your beliefs may be false, but the fact remains that you are thinking the false beliefs, or that you exist in the first place to be deceived. To deny your own existence—your own feelings, experiences, and thoughts—presupposes the very thing you are denying. 

Whether you contain a soul or are simply the emergent properties of your neurons, you are still a thinking being, which is as certain as can be. As Descartes wrote, “Well, then, what am I? A thing that thinks. What is that? A thing that doubts, understands, affirms, denies, wants, refuses, and also imagines and senses.”

Regardless of your thoughts on the the mind-body interaction, if you try to convince yourself that you are not a thinking being, the very attempt proves the point.  

Admittedly, this is not much to work with. The recognition of your own existence is a flimsy foundation on which to build the remainder of your knowledge—you’re still faced with the possibility of being wrong about everything else

You can try to work around the problem by assigning probabilities to your beliefs, but there will always be an underlying, deep level of doubt in regard to any belief that is not related to your own existence as a thinking being. And, the further you move away from this certainty, the more uncertain your beliefs become. The more you rely on your senses or the senses of others, and the more complex the issue in question becomes, the less certain you should be that 1) your beliefs are correct, or 2) that knowledge is even possible.

I think, therefore I am; now what?

The problem Descartes uncovered cannot be ignored because it casts doubt on everything we think we know (other than our own existence). Unless we can answer the question of what can be known, we have to face the possibility that knowledge of anything is impossible. To avoid this radical skepticism, we must answer this question: what can we know with any degree of certainty, beyond our own existence as thinking beings? 

There are three possible answers: (1) Deny the possibility of any knowledge beyond one’s own existence, (2) ground knowledge in something supernatural, or (3) accept that certain knowledge cannot be attained while recognizing that knowledge does not require certainty. Let’s take a closer look at each option.

1. Pyrrhonian Skepticism

In my previous post titled Sextus Empiricus and the Search for Intellectual Tranquility, I noted how the ancient Greek Skeptics were the first group of thinkers to develop sophisticated arguments against the reliability of the senses and therefore against the possibility of attaining certain knowledge. While Descartes’ method was subtly different, the ancient Skeptics had ultimately come to the same conclusion—that everything we believe could be false and that there is no way to confirm our beliefs one way or another.

Unlike Descartes, however, the Skeptics stopped there. To them, there is no way out of the skeptical challenge, and for the sake of intellectual tranquility, the Skeptics chose to suspend all judgment. 

While Descartes found certain knowledge in his own existence, it’s interesting to note that, beyond this, he could have followed the lead of the ancient Skeptics. If Descartes had rejected the possibility of all knowledge beyond one’s own existence, he would have simply been remembered as a particularly innovative Phyrronist. But Decartes, as we’ll see next, was unwilling to go down that road.

2. God as the foundation of knowledge

Descartes instead attempted to answer the skeptical challenge by demonstrating (unconvincingly) that God exists, using a version of the ontological argument in which he asserted that “God’s existence is inferred directly from the fact that necessary existence is contained in the clear and distinct idea of a supremely perfect being.” And since a supremely perfect being would not deceive His creation, we can know that our “clear and distinct” ideas are true. 

If this sounds like St. Anselm’s ontological argument to you, that’s because it essentially is (although slightly different). Remember that Descartes cannot offer empirical proof for God’s existence because, consistent with Descartes’ philosophy, all empirical evidence, received by the senses, is subject to doubt and is therefore uncertain. He is therefore forced to prove God’s existence a priori using a version of the ontological argument, which states that existence is a necessary characteristic of any “perfect being.” 

There are many problems with the ontological argument, the main one being that it is not possible to wish, think, or imagine something into reality (see Gaunilo’s Perfect Island argument). Further, as Immanuel Kant pointed out, existence doesn’t function as a predicate. The instantiation of predicates (omniscience, omnipotence, omnibenevolence) in a subject (God) simply assumes that the subject first exists (circular reasoning). Existence isn’t a property; it’s a metaphysically necessary condition for the instantiation of any properties.

Leibniz also pointed out that the idea of a supremely perfect being is incoherent in itself. For example, consider the paradox of omnipotence: Can God create a stone heavy enough so that even He cannot lift it? However you answer, this seems to place restrictions on God’s power.

There are further problems with and paradoxes of omniscience, omnibenevolence, and the existence of evil. For all of these reasons (and more), it is difficult to accept Descartes’ proposed solution based on a “supremely perfect being.” (See my previous post covering objections to the ontological argument.) 

Remember also that reasoning God into existence is highly problematic according to Descartes’ own project. If Descartes is claiming that you cannot even trust your own judgment that 2+3=5, then how can you trust your judgment concerning the existence of a perfect being? This could just be the evil demon trying to convince you—with a false sense of certainty—that God exists. The fact that it appears certain to you that God exists—just like it appears certain that 2+3=5—could simply be an illusion, and since Descartes does not establish the existence of God until after pointing out this possibility, the invoking of God simply does not work as a plausible solution. 

Ultimately, if you find the ontological argument persuasive, you can then build your knowledge on the presumption of the existence and perfection of God and the idea that a perfect God would not deceive you in your knowledge of the world. However, if you are unconvinced by a priori arguments for God’s existence—as most philosophers are—then you would need to find another way to establish legitimate grounds for knowledge. 

3. Critical rationalism and evolutionary epistemology

In a previous post, I noted how the answer to the skeptical challenge posed by the Phyrronian Skeptics was the recognition that knowledge does not require certainty. We may never achieve certain knowledge or final truth—and all beliefs may be shrouded in some degree of doubt—but this does not imply that all beliefs are equally likely to be true or useful, or that we can’t build on our knowledge and achieve progress through constant criticism and inquiry. 

We can at the same time accept an underlying level of uncertainty regarding all beliefs and still proportion our beliefs according to the best evidence as presented to us. Using this approach, we remain skeptical of assigning to our intuitions, perceptions, and memories a level of reliability that they simply do not possess. Instead, we can provisionally accept conclusions based on the available evidence with the understanding that any of our beliefs can be potentially falsified. This is the spirit of science and the critical rationalist approach advocated by Karl Popper. 

On a deeper level—if in fact we accept the critical rationalist approach that recognizes the possibility of acquiring probable, but ultimately uncertain, scientific knowledge—we may look to evolutionary epistemology (EE) as a potential foundation on which to base our knowledge. EE is a complex branch of the philosophy of knowledge with several schools of thought and innumerable, subtle distinctions; but for our purposes we can recognize that all approaches to EE adhere to the following basic propositions:

  1. According to our best evidence and explanations, human beings, like all forms of life, evolved over time from common ancestors via the process of natural selection.
  2. The human brain, as part of the body, has also evolved, and though we don’t fully understand the mind-brain interaction, our best explanation is that the mind emerges out of the activity of the brain and cannot exist without the brain. The mind is also predictably impaired by damage to the brain. 
  3. The brain, evolving within nature, has evolved to react and respond to nature. Therefore, there is no reason to suppose that an organ evolving within nature in response to nature is attuned to anything “beyond” nature or has access to anything not directly involved in the process of its shaping. The brain, as an evolved organ, should also be expected to provide fairly accurate representations of the natural world in which it was shaped, otherwise it would not have aided in the survival of its host organism.  
  4. As in all evolutionary adaptations, perfection is not expected; random mutation and environment all work to craft mental processes that help us to survive but that predispose us to error, which we find in our perceptual and cognitive limitations. 

We can accept the fact of our own existence (as thinking beings) as beyond doubt, and we can further suppose (without complete certainty but according to our best knowledge) that our minds are the products of evolution. While all beliefs have inherent uncertainty, we should expect our minds, evolving within nature, to be able to discover useful truths about nature that are, while prone to error and distortion, generally useful. 

While we can’t establish any knowledge of the world with complete certainty, we can expect to attain useful knowledge of the natural world that is subject to error and constant refinement, of which science, and the scientific method, is best suited to address. 

This line of thinking aligns with the fact that we have undoubtedly achieved scientific and technological progress, indicating that we must, in fact, know some things with a fair amount of certainty. 

It can be said, however, that evolution, as an empirical fact, even if a near-certain empirical fact, is not completely certain in the Cartesian sense. The evil demon could have manipulated the evidence to point to the false conclusion of evolution, in which case evolution is ultimately an illusion. 

We have no way to refute this possibility, so critical rationalism and evolutionary epistemology do not conclusively solve Descartes’ skeptical challenge. However, unless we are prepared to deny the possibility of all knowledge, or else resort to unconvincing versions of ontological arguments, critical rationalism is simply the best we can do.


In essence, the doubt Descartes introduced is theoretically unresolvable unless you ground knowledge in something reliable. For Descartes and others, the existence of God, proven a priori, provides this foundation. For those unconvinced, naturalistic epistemology appears to be the most promising route, as it has the advantage of being consistent with both science and evolutionary theory. Since science has proven to be our most reliable method of acquiring cumulative gains in knowledge, this approach appears most promising, or at least most pragmatic. 

But what you gain in consistency and reliability you sacrifice in scope and certainty, as evolutionary epistemology confines all knowledge—and all possibility of knowledge—to the natural world—in which there are only material/physical truths and psychological truths. This is a conclusion many, including Descartes, can’t or won’t accept. 

Question for readers: How will you answer the skeptical challenge? Will you follow the ancient Skeptics and deny the possibility of any knowledge? Will you follow Descartes and ground your knowledge in the supposed perfection of God? Or will you accept the certainty of your own existence and hold all other beliefs provisionally? Or perhaps something else entirely?

Further reading

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