KNOWLEDGE But surely even dreams are made up of elements drawn from reality: Suppose I am dreaming, and these particulars, that I open my eyes, shake my head, put out my hand, are incorrect; suppose even that I have no such hand, no such body; at any rate it has to be admitted that the things that appear in sleep are like painted representations, which cannot have been formed except in the likeness of real objects So at least these general kinds of things, eyes, head, hands, body must not be imaginary but real objects (AT VII.20; CSMK II.14) Perhaps these, in their turn, are imaginary complexes; but then the simpler elements out of which these bodies are composed—extension, shape, size, number, place, time—must surely be real And if so we can trust the sciences of arithmetic and geometry which deal with these objects ‘Whether I am awake or asleep, two and three add up to Wve, and a square has only four sides; and it seems impossible for such obvious truths to fall under a suspicion of being false’ (ibid.) Even mathematics, however, is not immune to Cartesian doubt It is not just that mathematicians sometimes make mistakes: it may be that the whole discipline itself is a delusion God is omnipotent, and for all we know he can make us go wrong whenever we add two and three, or count the sides of a square But surely a good God would not that! Well, then: I will suppose not that there is a supremely good God, the source of truth; but that there is an evil spirit, who is supremely powerful and intelligent, and does his utmost to deceive me I will suppose that sky, air, earth, colours, shapes, sounds, and all external objects are mere delusive dreams, by means of which he lays snares for my credulity I will consider myself as having no hands, no eyes, no Xesh, no blood, no senses, but just having a false belief that I have all these things (AT VII.23; CSMK II.15) The second Meditation brings these doubts to an end by producing the Cogito, the famous argument by which Descartes proves his own existence However the evil genius may deceive him, he cannot trick him into thinking he exists when he does not: Undoubtedly I exist if he deceives me; let him deceive me as much as he can, he will never bring it about that I am nothing while I am thinking that I am something The thought ‘I exist’ cannot but be true when I think it; but I cannot doubt it without thinking of it Hence, it is not only true but indubitable, because whenever I try to doubt it I see its truth The Cogito is the rock on which Descartes’ epistemology is built From his day to ours, critics have questioned whether it is as solid as it looks ‘I am 120