How Do We Know: Sense Perception (Part I: Descartes)

The majority of all our thoughts, actions, and beliefs are concerned with sensible information: what we see, hear, touch, etc. Our senses provide us, as it were, a kind of portal or window through which to access the “real” or external world. Imagine, then, the utter impossibility of a coherently functioning life in which one either had no sense perceptions at all, or could never be sure his senses gave him any sort of reliable information about the world.

And yet, since the start of the modern philosophical project, it has been precisely the reliability of the senses that has been rigorously questioned and doubted by many of the great and leading thinkers. Why should we trust our senses? How do we really know that they give us accurate information about the world? What reasons can we give to support or ground our trust in sense perceptions?

Perhaps these seem like frivolous and asinine questions, which no sane or serious-minded person would ever need to consider. But the impact they have had on Western thought in the past few centuries has been immense; and, as C. S. Lewis once quipped, good philosophy must exist, if for no other reason than that bad philosophy needs to be answered.

As Aristotle put it, “The least initial deviation from the truth is multiplied later a thousandfold” [1]. Saint Thomas would paraphrase him: “An error small in the beginning is great in the end” [2]. And, because “our natural knowledge begins from sense” [3], it follows that it is of great significance whether we can in fact trust the senses. 

The modern project of calling the senses into doubt has its start with the Meditations of Descartes. Descartes notes that we hold many beliefs, and often we discover that some of these beliefs are false. From this it follows that among the set of our current beliefs, it is entirely possible and perhaps even likely that a number are false. The problem is that, at present, you do not know which are true and which are false. Descartes’ solution is a radical systematic doubt: for any belief about which any possible reason for doubt can be given, assent to that belief ought to be withheld. He writes:

“Reason now leads me to think that I should hold back my assent from opinions which are not completely certain and indubitable just as carefully as I do from those which are patently false. So, for the purpose of rejecting all my opinions, it will be enough if I find in each of them at least some reason for doubt” [4].

 The only beliefs, Descartes reasons, that must be free from even the theoretical possibility of falsehood, are those beliefs about which we have an absolute certainty, which we could not even conceive of doubting in principle. Of course, it would be practically impossible to go through the entire set of one’s beliefs one by one, examining to see if there is any possible reason for doubt. Instead, Descartes suggests that since many beliefs rest upon more fundamental beliefs, we only need to examine  the foundational beliefs, such that if they are rejected, all beliefs dependent upon them must likewise be rejected.

As we’ve already noted, a vary large number of our beliefs are derived from sensory perceptions. Thus, it is here that Descartes begins his project of epistemological deconstruction:

“Whatever I have up till now accepted as most true I have acquired either from the senses or through the senses. But from time to time I have found that the senses deceive, and it is prudent never to trust completely those who have deceived us even once” [5].

Descartes’ argument here could be formulated something like this:

  1. In the past, my senses have deceived me
  2. If my senses have deceived me in the past, it is possible that they are deceiving me now
  3. If it is possible that my senses are deceiving me know, then I have at least some possible reason to doubt the veracity of my senses
  4. Therefore, I have some possible reason to doubt the veracity of my senses   

Since Descartes’ method is to withhold assent from and reject as though false every belief which is not absolutely and indubitably certain — and since, for him, any belief which has even some reason to doubt is by definition not indubitable — Descartes concludes that all of his beliefs based upon the veracity of sensory perception ought to be rejected.

How true is it, however, that in the past our senses have deceived us? The typical instances of sense deception are things such as mistaking the color of some object at a very great distance away, or various other simple optical illusions. Most of us, however, don’t have very many experiences of instances wherein we were mistaken about something we see right in front of our faces.

But Descartes does not leave it here. Perhaps past deception is not a sufficient reason to doubt very clear sensory experiences. But there are other occasions where we are deceived about our senses: namely, in dreams. Most of us have experienced fairly realistic dreams, during which we had no idea that we were actually asleep, and instead believed that what we were seeing and hearing were truly real. So Descartes formulates the following argument:

  1. In the past, I have had sensory experiences while dreaming which I took to be real
  2. If this has been the case in the past, it is possible that it is the case right now
  3. If it is possible that my current sensory experiences are actually dream induced rather than real, then I have at least some possible reason to doubt the veracity of my senses
  4. Therefore, I have some possible reason to doubt the veracity of my senses

And, once again, we reach the same conclusion. But, one might respond, even if it is the case that we are currently dreaming, we might be deceived about the specific content of our sensory experiences; but the actual sensory material must come from somewhere. When we dream, we experience a various assortment of visual memories that have been stored in the brain. The dream stems from actual sensory experiences we have had. So, even if we might doubt whether or not we’re dreaming right now, this shouldn’t be reason to doubt the entirety of our sensory information. At least some of it must be legitimate.

So Descartes continues with one final formulation of the argument:

  1. It is theoretically possible that some supremely powerful, malicious demon is deceiving me by causing my mind to have all its beliefs
  2. If this is possible, then I have at least some possible reason to doubt the veracity of my senses
  3. Therefore, I have some possible reason to doubt the veracity of my senses

Premise one here departs from the starting points of the previous two arguments. After all, simple optical illusions and realistic dreams are experiences we’ve all had before. Being deceived by a powerful, malicious demon, not so much. But the fact is, such a state of affairs is at least logically possible; and, contends Descartes, there’s no real way to know whether or not it is actually the case. There might not be any good reason to think it is the case, but its very possibility means that there is at least some reason, however implausible, that we could have doubts about our sensory beliefs. And if there is even some theoretical possibility of doubt, such beliefs are by definition not indubitable, and hence not absolutely certain.

Perhaps you can agree with Descartes that it is at least theoretically possible for us to have some reason to doubt our senses, and hence that our sensory beliefs are not absolutely and indubitably certain. But how do we get from this admittance, to the position that we ought to doubt our senses, or cannot trust them?

It depends in large part on what one takes “knowledge” to be. Descartes’ argument as a whole can be stated something like this:

  1. Whatever can be doubted is not absolutely and indubitably certain
  2. Whatever is not absolutely and indubitably certain could be false
  3. If a belief could be false, there is no way to know whether or not it is actually false or true
  4. One ought not to believe what cannot be known to be either true or false
  5. The senses can be doubted
  6. Therefore one ought not to believe the senses

This is, admittedly, an extremely simplistic summary; and there’s a lot underlying each of the premises that we have not yet gone over. The really crucial premises are three and four, because they set a standard for knowledge and belief which is almost impossible to reach. Indeed, in the end Descartes arrives at only a single belief which he is absolutely unable to even in principle doubt: that he exists.

This is Descartes’ infamous cogito argument. Cogito ergo sum: I think, therefore I am. He writes:

“So what remains true? Perhaps just the one fact that nothing is certain . . . . Does it follow now that I too do not exist? No: if I convinced myself of something then I certainly existed. But there is a deceiver of supreme power and cunning who is deliberately and constantly deceiving me. In that case I too undoubtedly exist, if he is deceiving me; and let him deceive me as much as he can, he will never bring it about that I am nothing so long as I think that I am something. So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind” [6].

Is it possible to doubt that one exists? For Descartes, it is possible to doubt that one’s body exists, that an external world exists, that other people exist, that what one is perceiving is true; but it is not, Descartes suggests, even in principle possible to doubt that one’s self actually exists. For doubting is a cognitive function, and whatever has functions, exists. A doubting thing is an existing thing. So as soon as one attempts to doubt their own existence, their own existence is thereby proved absolutely certain.

From this new foundation, Descartes next attempts to reconstruct all his prior knowledge. Whatever beliefs he is going to assent to must follow absolutely and necessarily from this starting point; otherwise they might be doubted. Despite the overwhelming immensity of such a task, Descartes considers that he can arrive at the following conclusions, starting from the single premise of his own existence:

  1. I think, so I exist
  2. If I exist, God exists as my Creator
  3. If God exists, God is all powerful and all good
  4. If God is all powerful and all good, then God could and would make my faculties of perception such that they work properly and are not deceived
  5. Therefore, my faculties work properly and are not deceived

Again, this is an extraordinarily oversimplified presentation, but it is sufficient for our purposes. On the basis of this argument, Descartes concludes that he can trust his sense perceptions. Not all of his sense perceptions, however. Just those that are “clear and distinct.”

So what should we think of Descartes’ overall project? First, Descartes’ project of reconstruction is, for all intents and purposes, a failure. He offers two arguments for the existence of God from the basis of his own existence, but neither are successful (I, of course, think that God exists and can be demonstrated to exist; but I do not think Descartes does so, within his own framework). Furthermore, premise four is quite questionable; it does not at all have the kind of certainty behind it which Descartes needs for it to in order to work.

Second, even if the project did not fail, I think Descartes still runs into some pretty significant problems. Descartes is trying to provide certitude for his beliefs by grounding them in unshakable foundations. So, whenever Descartes sees a tree a few feet in front of him, he cannot believe that the tree is there simply because he sees it. Instead, his belief that the tree is really there must be grounded in the conjunction both of the experience of seeing the tree, and of his knowledge of the previous laid out argument, the conclusion of which is that he is justified in trusting such sensory experiences.

In other words, it would seem that every single time Descartes has some sensory experience, in order to justify believing that it is true, he must go through the entire process of the previous argument once more. Descartes thinks he can get around this problem in the following way:

“Now, however, I have perceived that God exists, and at the same time I have understood that everything else depends on him, and that he is no deceiver; and I have drawn the conclusion that everything which I clearly and distinctly perceive is of necessity true. Accordingly, even if I am no longer attending to the arguments which led me to judge that this is true, as long as I remember that I clearly and distinctly perceived it, there are no counter-arguments which can be adduced to make me doubt it, but on the contrary I have true and certain knowledge of it” [7].

So, says Descartes, it is not the case that I must always be conscious of the whole argument; I must rather simply remember that I was at some point convinced by such an argument, and that will be enough to justify my trust in my current sensory perceptions. But the problem is that Descartes’ “remembering” of the argument is itself a kind of experience which he cannot justify trusting.

For most people, the process of coming to believe in a tree they see right in front of them is relatively simple: first they have a sensory experience of the tree, and then the belief in the tree’s reality is automatically formed in them. Descartes contends that we ought not accept the latter belief as true unless we can justify it with an absolute certainty, which his whole argument was an attempt to do. So, for Descartes, the process must rather look something like this: first there is a sensory experience of the tree, then there is the recalling that one has before been convinced by an argument to justify the belief in such sensory experiences, then the belief in the tree’s reality is assented to. But, once again, the recalling of the memory is itself a kind of experience. Why should he trust that experience? After all, his memories could be mistaken. The malicious demon could be deceiving him into believing that at some time in the past he had been convinced by such an argument, when in fact he had not. The only escape, then, is that every single time Descartes has any sort of experience, he must repeat the process of going through the entire argument, consciously in his mind, before he can believe in the veracity of the experience. This, of course, is a practical impossibility.

In Descartes, then, we find the roots of modern skepticism (and various other significant modern philosophical problems). For those who would come after Descartes were taken with the questions he raised, but remained unconvinced at his attempted answers. By the time of Hume, as we shall see, even Descartes’ foundational premise of knowledge, that “I exist,” is thrown into doubt.




[1]. McKeon, Richard, editor. The Basic Works of Aristotle. New York: Random House, Inc, 1941. Print, De Caelo 1.5, 271b9-10.

[2]. My own translation. Latin taken from: Aquinas, Thomas. De Ente et Essentia. Translated as Aquinas on Being and Essence:
A Translation and Interpretation. 1965. Adapted and html-edited by Joseph Kenny, O.P. < >.

[3]. Thomas Aquinas. Summa Theologiae. Translated by Fr. Laurence Shapcote, O.P. The Aquinas Institute for the Study of Sacred Doctrine, 2012. I, Q. 12, Art. 12.

[4]. Descartes. Meditations on First Philosophy: With Selections from the Objections and Replies. Revised edition. Edited by John Cottingham. Cambridge: Cambridge University Press, 1996. Page 12.

[5]. Ibid.

[6]. Ibid. 16-17.

[7]. Ibid. 48.

3 thoughts on “How Do We Know: Sense Perception (Part I: Descartes)

  1. Harrison, the third premise of one of our arguments reads:

    If it is possible that my senses are deceiving me know…

    Of course, it should read, “…my senses are deceiving me now…”


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