In his third mediation, Descartes is primarily concerned with God and His potential existence. However, his thoughts leading up to these more theological arguments and theories are quite important in themselves, as they address his thoughts regarding truth and the nature of the mind. He begins by looking at what he has already established as truth, namely the fact that he exists as a thinking thing. At this stage, he maintains that he can doubt all else. He then asks himself whether he can use this one truth to find other truths. After all, logic dictates that if he has managed to find one true piece of information, it must be possible for him to establish what it is that makes this information so indubitable (a criterion or rule of sorts), and then use this to build on his knowledge. In other words, he has a foundation, and should now be able to start erecting a building on top of it.
After some reflection, he realizes what it is about this one fact – that he exists – that makes it so certain. This is that he can perceive it with utter clarity and distinction, more so than he ever has of anything else in the past. Accordingly, he argues that anything he perceives clearly and distinctly must be true. The main limitation of this argument lies in the fact that he does not describe precisely how one is meant to know that something is being perceived clearly and distinctly. He does say that the mind or will should be “compelled” to agree with the perception’s validity, but there are various problems associated with this. After all, how can we be sure that our mind is indeed compelled to agree with something – we might just convince ourselves that it is because we want to believe it. Also, I might perceive something in the wrong way and think it’s clear and distinct because of this, when in reality it isn’t. I might, for instance, see a magnified image of an insect and – not realizing that it has been magnified – have a very clear and distinct perception of an enormous monster, when the invertebrate in question is actually quite small.
At this point Descartes looks back at the problem of the malignant demon theory, with specific reference to his previous uncertainty regarding arithmetic. If the malignant demon hypothesis is true, he may well be deceived about even the most basic mathematics. However, he finds that he can liken his understanding of such simple maths to his understanding of his existence. It seems almost absurd to think that two plus two might not actually equal four. He perceives this very clearly and very distinctly. And yet, as long as the evil genius theory remains valid – which it does – he can still doubt this. He realizes that this is a problem he has to address before he can add anything to his knowledge, and chooses to do so by attempting to prove the existence of an omnibenevolent God (who he argues would not wish to deceive him constantly, unlike the aforementioned demon). He opts to do this by examining his mind and thoughts, and it’s his findings in this area that I intend to discuss here.
He begins by establishing three categories of thought: ideas (images, such as the image of a candle), feelings (subjective, often emotional responses to images, such as “I like candles”), and judgements (decisions made about things, such as “that candle’s about to go out”). He argues that the first two are necessarily true. If he imagines a candle it’s true that he’s imagining a candle. If he feels that he likes candles, it’s true that he likes candles – no one can dispute that. This all relates to one of Descartes most controversial (at the time) opinions; that subjective knowledge is more certain than objective knowledge. Judgements, however, can be wrong according to Descartes. He says that he has often made false judgements in the past and so he has good cause to doubt these. So he knows that some thoughts can be seen as true whereas others can be false. He then moves on to describing three subdivisions of ideas: innate (ideas like that of a perfect circle), adventitious or acquired (ideas that are inspired by external stimuli), and imagined or fabricated (ideas such as that of a dragon or nymph). He goes on to say that he can’t be sure there is a distinction between these. It may turn out that all ideas are innate, or that they’re all adventitious somehow.
He asks himself how he can be sure that adventitious ideas do have an external source – that is, whether or not they are adventitious – and (if they do) whether he can be sure that they accurately reflect these objects. He says that he feels a natural inclination to believe that they do, and the fact that he can’t control external objects with his mind seems telling. The problem with the former is that he has been fooled by natural inclinations in the past, so he can doubt them. The problem with the latter is that his inability to control something doesn’t mean it exists. He can’t control objects in his dreams, after all. And, of course, it seems clear that his adventitious ideas, even if they are inspired by external things, don’t always give him an accurate conception of them. Take his idea of the sun, for instance; the image of it that he has derived from his senses is of something tiny, when the sun is in fact very large.
However, though he can’t be sure of where his ideas come from, they do differ in certain ways. His ideas of things such as size, shape and motion appear to be more definite – subjectively – then things like scent and colour. The size and shape of a candle can be very clear, after all, whereas its smell can be indistinct, and after a while this may seem to fade away. He says that these things (size, shape, etc.) have more “intentional reality”. It is on this basis – this foundation – that he then goes on to argue God’s existence.