Cartesian Foundationalism and Doubt in Introspective Processes

by Luke Gavin

The epistemology of Descartes is a paradigmatic example of classical foundationalism. Comparing our system of knowledge to a building, Descartes proposes that in order for beliefs to have validity, they must either be foundational beliefs that are absolutely certain and indubitable or be deduced, at least indirectly, from such beliefs.[i] The first principle that Descartes arrives at as absolutely certain is the fact of his own existence, which in his view is incorrigible and self-evident, as are the facts of his own mental states (he is certain that he is thinking at this moment, doubting at that moment, and so on).[ii]

In this essay, I will argue that Descartes’ selection of the testimony of one’s own consciousness as certain is arbitrary in comparison to knowledge of the sort which he regards as dubitable (e.g. the data of sense perception), for the primary reason that reasonable doubt can be cast on the testimonies of one’s own consciousness. I will further argue that this entails that the Cartesian must not only adopt a  more inclusive set of first principles, which includes the reliability of sense perception, but must also either abandon foundationalism (and the method of radical doubt ) or embrace global scepticism. To achieve this, I will first consider two examples that I believe challenge the veracity of Descartes’ claims. I will then respond to a further class of objections—those which insist on the greater certainty of internal states in comparison to facts of the external world, on the basis of immediate acquaintance—which I shall reject by demonstrating that they are both logically and practically inconsistent.

If I were to ask you to prove that you are thinking, you would be unable to (short of rubbing your temple and furrowing your brow…). In the words of Thomas Reid, “[h]ow do you know that your consciousness cannot deceive you?”[iii] For any of your own mental states: doubting, believing, desiring, and so on, it seems that we are forced to take their existence for granted, without needing further explication. In other words, these mental states are forced on us in some sense “irresistibly” (or in Descartes’ terms, they are “clear and distinct”).[iv] Still, why shouldn’t we take sense perception for granted in the same way? Because, says Descartes, we can be deceived on the matter, as in the case of a dream, hallucination, or an evil demon tricking us with the illusion of reality. Not so with mental states, apparently. 

Since Descartes requires only conceivable doubt and not reasonable doubt[v] in order to consign a source of beliefs to the dustbin, it will be sufficient for me to show that our beliefs about our own mental states can go wrong in an analogous way to sense perception, either by presenting things in such a way that the representation is not how the thing really is (what philosophers call, “illusion”), or by presenting something that is not really there at all (i.e. “hallucination”).

To take an example, it is a truism that our own emotions can be opaque to us at times, and we might make erroneous conclusions regarding them based on introspective difficulties embedded in the process of deciphering them. Consider the case of a woman trapped in a loveless marriage: due to factors of social pressure, relative comfort, stability, etc., when she consults her emotions as to whether she loves her husband, they reply affirmatively (if uncertainly), and she continues to believe she loves her husband. Only after the divorce does she realize how she had been deceiving herself: she did not love her husband. So, it seems that the introspective process by which we access our emotions can sometimes lead us to beliefs about them which are in fact distorted, or false, with respect to our broader mental schemas. Consider this then as an argument for a further kind of illusion, namely, introspective illusion.

Sensations can be even more deceptive: as I type there is a prickling feeling in my lower back. I know from experience that this is nerve pain, but were I bereft of this experience, I might believe it to be an itch, or something similar. So, sensations might also be examples of introspective illusion.

Simple cognitions such as knowing that I am thinking (that is, knowing that I am thinking at all) are, intuitively speaking, more difficult to doubt. However, to borrow an argument from William Alston, it is not inconceivable that disciplines such as neurophysiology, for example, could progress to the point where we have evidence that a person’s higher-level thoughts such as these can plausibly be different from what they are believed to be.[vi] Consider a scenario in which a scientist has implanted electrodes in the areas of your brain which correspond to belief formation, and imagine that the scientist now stimulates these parts of your brain in such a way that you believe that you have just had a particular thought, T1, without actually having had the original thought.  While it is true that you would have had a thought in the process of this belief formation, it is not the particular thought in question T1, but rather a separate thought, T2, which merely refers to T1. On the other hand, the thought  T1 has never actually occurred, because your brain has simply misled you about your own thoughts: it has led you into the wake of an introspective hallucination. And while it might be objected that this is a case about remembered thoughts, and not the mere awareness of thinking in the moment (to which Descartes himself refers), there does not seem to be any prima facie reason why the scientist should not be able to also electrically stimulate an artificial awareness of the thinking act itself, which if possible would lead to one being indefinitely trapped in a (theoretically) infinite chain of hallucinatory thoughts, where a hallucinatory thought is followed by a subsequent hallucinatory awarenesses of those thoughts—itself just another hallucinatory thought, leading to further hallucinatory thoughts; and so on, and so on, ad infinitum. The mere possibility that this may be the case provides conceivable doubt for the claim, made by Descartes, that “I am thinking,” and hence the Cartesian is arguably not able to lay claim to knowledge in this regard.

At this stage, however, there may seem to be a problem with my argument; not only does the neuro-physical thought experiment remain to be demonstrated empirically, but it might also be construed by my reader as being somewhat, as it were, elaborate. Yet this charge of sophistry is easily averted.  Recall that Descartes justifies his doubting of the senses, firstly, on the basis of the possibility that he might be dreaming, and secondly, with respect to the possibility of an “Evil Demon”. Further, recall that in both these thought experiments, in order for them to serve as legitimate grounds for doubt, the mere possibility of the “dreaming” (or “evil demon”) was sufficient. Yet I believe the thought experiments which I have given to demonstrate why we cannot be certain of the existence of introspective thoughts, work in exactly the same way as the Cartesian’s account of the doubtfulness of the senses. I have given two examples where introspective processes mislead, establishing nothing more, but also nothing less, than the possibility of being misled with respect to these more holistic and introspective kinds of cognition, such as the thought that I am thinking at all. The Cartesian at this juncture has a number of objections available: first, to squabble over whether the argument through possibility I have outlined is sufficient grounds for conceivable doubt; second, to throw her hands up in the air and commit to radical scepticism; and third, to admit that, however small, some uncertainty is in fact acceptable in our beliefs (perhaps adding the caveat that we can still be somewhat more certain about introspective beliefs than those regarding the external world— perhaps drawing on the idea of acquaintance, which I will elaborate on shortly—while all the while maintaining we can be absolutely certain about neither). Since the third option seems to be the most promising, I will now consider it in more detail.

Having been forced into accepting that no belief can ever fully satisfy the stringent requirements of Descartes, the Cartesian may reply along the following lines:  “My acquaintance with my mental states,” says the Cartesian, “is more immediate than my acquaintance with the external world, as it is a process which requires only two “points of contact” as it were, between myself and my thought, whereas my perception of an object requires three “points of contact”, that of the object, an idea of the object in my mind, and myself.[vii] Since it is fairly clear that the former is less problematic than the latter, there is less possibility for error in a two-term relation than there is in a three-term one. So, even if I must abandon infallibilism, I still have good reason to prefer introspective processes of belief formation over perceptual ones.” In brief, what the Cartesian fails to consider here is that despite what one is tempted to think, there is no sense in which perception and introspection come from a “different shop”, as it were.[viii] Why then, should we believe in the dictates of consciousness over those of perception? In the words of Thomas Reid, if both faculties were “made by the same artist,” then what should hinder him, in both introspection and perception, from putting, as it were, “false ware into my hands” (or, indeed, my mind)?[ix] In simpler terms, if one of our faculties is unreliable, and the other faculties came from the same place, why should we preferentially doubt one over the other?

The Cartesian’s best and only reply is that this is because of the aforementioned inequality in reliability between so-called “two-term” and “three-term” relations. However, there still seems to be problems with this account. Consider, by analogy, an engineer who must choose between applying a mechanical device with x moving parts, and a mechanical device with x + 1 moving parts, in order to solve a set of problems in construction. Assume, further, that the engineer knows that the machines came from the same manufacturer, and are identical to each other in all other respects; they also have the option of abandoning the whole problem altogether, if it seems sufficiently hopeless (that is, they are not forced into making one decision over the other). While it may be the case that it is the safest choice to first make use of the former device over the latter, it would be strange, upon success with this device[x], to abstain from use of the latter device simply because it has one more point of failure. The engineer has already put their trust in the manufacturer, and their trust in the manufacturer has been vindicated. If the engineer wanted the safest option, they would have simply not used either device at all: so what is stopping them from putting their trust in the latter device as well? At the stage they have now reached, the trustworthiness of the manufacturer outweighs considerations regarding the trustworthiness of the device itself, and thus the mere fact the latter device has more parts does not provide a good reason to doubt its reliability. In this case, using one but not both of these devices is inconsistent.

In this essay, the possibility of doubt in introspective processes was discussed and affirmed. This possibility was then asserted to be problematic to the Cartesian project of ascertaining certain and indubitable beliefs. The Cartesian was then forced into accepting a less rigid and fallibilistic epistemology. The subsequent objection, that introspective processes are still more certain than perceptual ones on the basis of immediate acquaintance, was considered and rejected, based on the fact that both faculties must ultimately arise from the same epistemic origins. Hence, I conclude, it is arbitrary to treat one as being more reliable than the other.

Luke Gavin is studying a Bachelor of Science/Arts (Computer Science & History/Philosophy). They have an interest in metaethics and analytic philosophy. They also enjoy powerlifting and judo in their spare time. 


[i]René Descartes, The Philosophical Writings of Descartes, volume 2, ed. John Cottingham (Cambridge University Press, 1984), 12.

[ii]Descartes, 16, 19.

[iii]Thomas Reid, The Works of Thomas Reid, D.D.: Now Fully Collected, with Selections from his Unpublished Letters, ed. William Hamilton (Maclachlan & Stewart, 1846), 463a.

[iv]Reid, 463a.

[v]Since Descartes uses the example of the Evil Demon, one can only imagine this is what he means, since it is not really a reasonable doubt that an Evil Demon is deceiving me, as Descartes has presented no arguments except that it is possible.

[vi]W.P. Alston, The Reliability of Sense Perception (Cornell University Press, 1993), 128.

[vii]Philip de Bary, Thomas Reid and Scepticism: His Reliabilist Response (Routledge, 2002), 27.

[viii]Thomas Reid, An Inquiry into the Human Mind on the Principles of Common Sense: A Critical Edition, ed. Derek R. Brookes (Penn State Press, 2000), 169.

[ix]Reid, 169.

[x]I am assuming here, with the Cartesian, that introspection is successful in its goals.


Alston, William P. The Reliability of Sense Perception. Cornell University Press, 1993.

Bary, Philip de. Thomas Reid and Scepticism: His Reliabilist Response. Routledge, 2002.

Descartes, René. The Philosophical Writings of Descartes: Volume 2. Edited by John Cottingham. Cambridge University Press, 1984.

Reid, Thomas. An Inquiry into the Human Mind on the Principles of Common Sense: A Critical Edition. Edited by Derek R. Brookes. Penn State Press, 2000.

Reid, Thomas. The Works of Thomas Reid, D.D.: Now Fully Collected, with Selections from His Unpublished Letters. Edited by William Hamilton. Maclachlan & Stewart, 1846.

Featured photo by Jezael Melgoza on Unsplash


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