in every possible way. Everything I believe in – ‘the sky, the earth and all external things’ – might be illusions that the demon has devised in order to trick me. Yet this very extremity of doubt, when pushed as far as it will go, yields the first indubitable truth in the Cartesian quest for knowledge – the existence of the thinking subject. ‘Let the demon deceive me as much as he may, he can never bring it about that I am nothing, so long as I think I am something. . . . I am, I exist, is certain, as often as it is put forward by me or conceived in the mind.’ Elsewhere, Descartes expresses this cogito argument in the famous phrase ‘Cogito ergo sum’ (‘I am thinking, therefore I exist’). Having established his own existence, Descartes proceeds in the Third Meditation to make an inventory of the ideas he finds within him, among which he identifies the idea of a supremely perfect being. In a much criticized causal argument he reasons that the representational content (or ‘objective reality’) of this idea is so great that it cannot have originated from inside his own (imperfect) mind, but must have been planted in him by an actual perfect being – God. The importance of God in the Cartesian system can scarcely be overstressed. Once the deity’s existence is established, Descartes can proceed to reinstate his belief in the world around him: since God is perfect, and hence would not systematically deceive, the strong propensity he has given us to believe that many of our ideas come from external objects must, in general, be sound; and hence the external world exists (Sixth Meditation). More important still, Descartes uses the deity to set up a reliable method for the pursuit of truth. Human beings, since they are finite and imperfect, often go wrong; in particular, the data supplied by the senses is often, as Descartes puts it, ‘obscure and confused.’ But each of us can nonetheless avoid error, provided we remember to withhold judgment in such doubtful cases and confine ourselves to the ‘clear and distinct’ perceptions of the pure intellect. A reliable intellect was God’s gift to man, and if we use it with the greatest possible care, we can be sure of avoiding error (Fourth Meditation).
In this central part of his philosophy, Descartes follows in a long tradition going back to Augustine (with its ultimate roots in Plato) that in the first place is skeptical about the evidence of the senses as against the more reliable abstract perceptions of the intellect, and in the second place sees such intellectual knowledge as a kind of illumination derived from a higher source than man’s own mind. Descartes frequently uses the ancient metaphor of the ‘natural light’ or ‘light of reason’ to convey this notion that the fundamental intuitions of the intellect are inherently reliable. The label ‘rationalist’, which is often applied to Descartes in this connection, can be misleading, since he certainly does not rely on reason alone: in the development of his scientific theories he allows a considerable role to empirical observation in the testing of hypotheses and in the understanding of the mechanisms of nature (his ‘vortex theory’ of planetary revolutions is based on observations of the behavior of whirlpools).
What is true, nonetheless, is that the fundamental building blocks of Cartesian science are the innate ideas (chiefly those of mathematics) whose reliability Descartes takes as guaranteed by their having been implanted in the mind by God. But this in turn gives rise to a major problem for the Cartesian system, which was first underlined by some of Descartes’s contemporaries (notably Mersenne and Arnauld), and which has come to be known as the Cartesian circle. If the reliability of the clear and distinct perceptions of the intellect depends on our knowledge of God, then how can that knowledge be established in the first place? If the answer is that we can prove God’s existence from premises that we clearly and distinctly perceive, then this seems circular; for how are we entitled, at this stage, to assume that our clear and distinct perceptions are reliable? Descartes’s attempts to deal with this problem are not entirely satisfactory, but his general answer seems to be that there are some propositions that are so simple and transparent that, so long as we focus on them, we can be sure of their truth even without a divine guarantee. Cartesian science and dualism. The scientific system that Descartes had worked on before he wrote the Meditations and that he elaborated in his later work, the Principles of Philosophy, attempts wherever possible to reduce natural phenomena to the quantitative descriptions of arithmetic and geometry: ‘my consideration of matter in corporeal things,’ he says in the Principles, ‘involves absolutely nothing apart from divisions, shapes and motions.’ This connects with his metaphysical commitment to relying only on clear and distinct ideas. In place of the elaborate apparatus of the Scholastics, with its plethora of ‘substantial forms’ and ‘real qualities,’ Descartes proposes to mathematicize science. The material world is simply an indefinite series of variations in the shape, size, and motion of the single, simple, homogeneous matter that he terms res extensa (‘extended substance’). Under this category he includes all physical and biological events, even complex animal behavior, which he regards as simply the result of purely mechanical processes (for non-human animals as mechanical automata, see Discourse, Part V). But there is one class of phenomena that cannot, on Descartes’s view, be handled in this way, namely conscious experience. Thought, he frequently asserts, is completely alien to, and incompatible with, extension: it occupies no space, is unextended and indivisible. Hence Descartes puts forward a dualistic theory of substance: in addition to the res extensa that makes up the material universe, there is res cogitans, or thinking substance, which is entirely independent of matter. And each conscious individual is a unique thinking substance: ‘This ‘I’ – that is, the soul, by which I am what I am, is entirely distinct from the body, and would not fail to be what it is even if the body did not exist.’ Descartes’s arguments for the incorporeality of the soul were challenged by his contemporaries and have been heavily criticized by subsequent commentators. In the Discourse and the Second Meditation, he lays great stress on his ability to form a conception of himself as an existing subject, while at the same time doubting the existence of any physical thing; but this, as the critics pointed out, seems inadequate to establish the conclusion that he is a res cogitans – a being whose whole essence consists simply in thought. I may be able to imagine myself without a body, but this hardly proves that I could in reality exist without one (see further the Synopsis to the Meditations). A further problem is that our everyday experience testifies to the fact that we are not incorporeal beings, but very much creatures of flesh and blood. ‘Nature teaches me by the sensations of pain, hunger, thirst and so on,’ Descartes admits in the Sixth Meditation, ‘that I am not merely present in my body as a sailor is present in a ship, but that I am very closely joined and as it were intermingled with it.’ Yet how can an incorporeal soul interact with the body in this way? In his later writings, Descartes speaks of the ‘union of soul and body’ as a ‘primitive notion’ (see letters to Elizabeth of May 21 and June 28, 1643); by this he seems to have meant that, just as there are properties (such as length) that belong to body alone, and properties (such as understanding ) that belong to mind alone, so there are items such as sensations that are irreducibly psychophysical, and that belong to me insofar as I am an embodied consciousness. The explanation of such psychophysical events was the task Descartes set himself in his last work, The Passions of the Soul; here he developed his theory that the pineal gland in the brain was the ‘seat of the soul,’ where data from the senses were received (via the nervous system), and where bodily movements were initiated. But despite the wealth of physiological detail Descartes provides, the central philosophical problems associated with his dualistic account of humans as hybrid entities made up of physical body and immaterial soul are, by common consent, not properly sorted out.
Influence. Despite the philosophical difficulties that beset the Cartesian system, Descartes’s vision of a unified understanding of reality has retained a powerful hold on scientists and philosophers ever since. His insistence that the path to progress in science lay in the direction of quantitative explanations has been substantially vindicated. His attempt to construct a system of knowledge by starting from the subjective awareness of the conscious self has been equally important, if only because so much of the epistemology of our own time has been a reaction against the autocentric perspective from which Descartes starts out. As for the Cartesian theory of the mind, it is probably fair to say that the dualistic approach is now widely regarded as raising more problems than it solves. But Descartes’s insistence that the phenomena of conscious experience are recalcitrant to explanation in purely physical terms remains deeply influential, and the cluster of profound problems that he raised about the nature of the human mind and its relation to the material world are still very far from being adequately resolved.
See also COGITO ERGO SUM, FOUNDATION- ALISM , PHILOSOPHY OF MIND ,