In the meditations, Descartes aims to provide a sound basis for science, and to vindicate rationalism by proving that true source of scientific knowledge lies in the mind and not the senses. In order to prove that the mind should be the true source of such knowledge, Descartes subjects all knowledge derived from the senses to doubt. He argues that as a child, he accept a large number of falsehoods and has subsequently built a ‘whole edifice’ on them of a ‘highly doubtful nature’. He argues that such childhood prejudices arise naturally through immersion in the senses. He uses the example of optical illusions such as a stick in water appearing bent to show the unreliable nature of the information gathered through the senses, and argues that we should reject such information as a result. Here, Descartes leaves it solely to the individual to assess the reliability of knowledge. It could be argued that the reliability of sense data could in fact be improved though corroboration with others, and it should not simply be rejected.
In order to provide a truly solid foundation for knowledge Descartes feels the need to subject all knowledge to doubt, and uses two thought experiments to do so. He says that we cannot doubt the things we see up close and in good light, equating such doubt with madness and dismissing the possibility that he is mad. In order to doubt even these perceptions, Descartes employs the dream argument, arguing that in dreams we appear to see things up close and in good light when in actual fact we do not. However, the dream argument has been objected to on the grounds that it is an asymmetric argument: simply because we are unsure we are asleep when we are dreaming, it does not necessarily follow that we do not know that we are awake when we are awake.
Descartes does not stop at a posteriori knowledge – he also feels the need to challenge the validity of mathematics and a priori knowledge. In order to do so he proposes the idea of an omnipotent being with the capability of deceiving us of even this knowledge. Descartes rejects the idea of a deceiver God, as God is by definition ‘good’ and would not deceive us, and instead proposes an omnipotent evil demon. However, the very idea of ‘deception’ relies upon the idea that we are not deceived all the time. As a result, it seems that the omnipotent demon could not in fact deceive us about all a priori knowledge as if this was the case there would be no such thing as ‘deception’. In this way, Descartes seems to fail in doubting all a priori knowledge.
By using these sceptical arguments, Descartes claims to have demolished all existing opinions and allowed himself to start afresh and find a solid foundation for knowledge. However, through this very claim Descartes reveals that he has not infact doubted everything, as he continues to assume the truth of foundationalism and believes that it is possible to find a foundation of knowledge, when in fact it may not be. Furthermore, he fails to doubt logic, memory and language – all tools used by Descartes throughout the Meditations, indicating that his doubt is in fact not ‘universal’. Alternatively, it could be suggested that Descartes’ method of doubt is in fact too extreme. By wanting all knowledge to have the certainty of maths and by discounting all sense data, it could be argued that Descartes simply sets the bar for knowledge too high.
After Meditation One, Descartes’ greatest challenge seems to be to overcome his evil demon. It is in challenging his own existence with the evil demon hypothesis that Descartes claims to find a piece of certain knowledge, which will form the foundation for all other knowledge. He argues that the evil demon cannot deceive nothing into thinking that it exists when it does not, and therefore when he conceives that he is something he cannot be nothing. Descartes argues that the Cogito is indubitable, because if he is being deceived by an evil demon, he must exist. If true, the Cogito is of central importance to Descartes’ whole strategy – it provides an ideal starting point from which he can build further truths. It provides certain knowledge not only about our ideas, but a substantive existential truth, not born of experiment or observation, but of thought alone.
However, the Cogito has been criticised for its apparent use a suppressed premise – namely, ‘all thinking things exist’. This premise is highly questionable, because it is not clear that the existence of thoughts necessarily imply a thinker. Hume argued that we have no right to assume this, and the Buddhist teaching advocates that the supposed ‘self’ is impermanent. Alternatively, perhaps Descartes should have said ‘there is thinking going on therefore there are thoughts’, rather than making the inference between thinking and the existence of a thinker. Furthermore, Descartes strays from rationalism here, since ‘thinking things exist’ seems to be an a posteriori observation. In this way, it is unlikely that the Cogito actually proves the existence of a self.
In response to this, it has been argued that the Cogito does not use a suppressed premise and is in fact an analytic truth, with the concept of my present existence contained within that of my thinking, just as the concept ‘female fox’ is contained within that of ‘vixen’. However, if the cogito truly was analytic, nothing substantial about the world could follow from it and it could not form the foundation of knowledge as analytic statements tell us about concepts, rather than reality. Furthermore, as argued by Kant and Russell, existence does not seem to be a property amongst other properties that can be assigned to things, but instead the condition of possibility for having properties at all. This is shown through the example of fictional characters – we can talk of characters having thoughts, and in this way we can talk of non-existent thinkers whereas it is not possible to talk of male vixens. This suggests that there is no conceptual entailment between existence and thought.
Russell also argued that the Cogito is a circular argument as it assumes what it sets out to prove, using the word ‘I’ in an argument that looks to prove the existence of said ‘I’. Lichtenberg argued that this ‘I’ is merely a linguistic convenience, similar to the ‘it’ in ‘it is raining’, and that it does not actually refer to anything. In his failure to analyse these fundamental concepts, Descartes’ project could be seen as not radical enough, with empiricists arguing that it is subject to rationalist prejudices by regarding such concepts as innate rather than derived from experience.
Descartes’ claim to have overcome the evil demon with the Cogito is also highly questionable. There does not seem to be any reason why an omnipotent being capable of deceiving Descartes regarding the logic of mathematics would be unable to deceive him regarding the logic and reasoning he has used in the Cogito. Indeed, by failing to employ truly universal doubt in meditation one, and doubt his own logic and reasoning, Descartes seems to have failed in his aim of providing a certain and indubitable foundation in the Cogito.
Having apparently established the existence of his own existence through the Cogito, Descartes then attempts to rebuild knowledge and prove the existence of things beyond his own consciousness. In Meditation Three, Descartes assesses the knowledge he has so far acquired in order to find some distinguishing features of it that may help him to recognize other truths. He argues that what makes the Cogito certain is that he has a ‘clear and distinct’ understanding of it, and therefore other things he understands clearly and distinctly may also be true; he uses the Cogito as a benchmark against which all other propositions can be measured. Clear and distinct ideas are those perceptions which are so self-evident that, while they are held in the mind, they cannot be logically doubted. By ‘clear’ Descartes means those ideas that are present to the attentive mind and by ‘distinct’ he means ideas that are not confused with anything that is not clear.
The inductive leap between the Cogito as a piece of certain knowledge that is known clearly and distinctly and all clear and distinct knowledge being certain seems immediately problematic. Simply because the Cogito is known clearly and distinctly, it does not necessarily follow that all things perceived clearly and distinctly are true. Furthermore, an evil demon could easily deceive us regarding apparent clear and distinct perceptions. Descartes himself acknowledges the need dispel remaining doubts concerning clear and distinct beliefs. To do so, Descartes uses the Trademark Argument to prove the existence of God, the guarantor of clear and distinct beliefs. The argument is as follows:
P1) I have an idea of God
P2) In every cause there must be at least as much reality as in the effect
P3) Since I am imperfect I cannot be responsible for this idea of perfection
P4) Whatever caused the idea of perfection must be perfect
C) Therefore, God exists
Descartes argues that the existence of this perfect being guarantees the truth of clear and distinct ideas as, by its very definition, a perfect being would not deceive him. In this way, Descartes seems to overcome the deceiving creator of Meditation One and establish faith in a priori reasoning. However, it could be argued that the benevolence of a perfect God is necessarily incompatible with deception. An example of such a combination would be that of a loving parent deceiving her child about the existence of the tooth fairy.
The greatest criticism of the trademark argument is that Descartes presupposes what he sets out to prove, employing a circular argument known as the Cartesian Circle. Where Descartes attempts to use the trademark argument to guarantee the truth of clear and distinct ideas, he seems to use many of these ideas within the argument itself. God’s existence is necessary in order to ensure that clear and distinct ideas are reliable, but clear and distinct ideas are what enable Descartes to know that God exists. The very idea of God as well as the idea of causation are supposedly clear and distinct ideas, both of which are used in the trademark argument to prove God’s existence. Furthermore, even if the truth of clear and distinct ideas can be ensured, they only seem to offer subjective truths to those who conceive them.