René Descartes (1596–1650) is usually considered the founder of modern philosophy, and, I think, rightly. He is the first man of high philosophic capacity whose outlook is profoundly affected by the new physics and astronomy. While it is true that he retains much of scholasticism, he does not accept foundations laid by predecessors, but endeavours to construct a complete philosophic edifice de novo. This had not happened since Aristotle, and is a sign of the new self-confidence that resulted from the progress of science. There is a freshness about his work that is not to be found in any eminent previous philosopher since Plato. All the intermediate philosophers were teachers, with the professional superiority belonging to that avocation. Descartes writes, not as a teacher, but as a discoverer and explorer, anxious to communicate what he has found. His style is easy and unpedantic, addressed to intelligent men of the world rather than to pupils. It is, moreover, an extraordinarily excellent style. It is very fortunate for modern philosophy that the pioneer had such admirable literary sense. His successors, both on the Continent and in England, until Kant, retain his unprofessional character, and several of them retain something of his stylistic merit.

Descartes's father was a councillor of the Parlement of Brittany and possessed a moderate amount of landed property. When Descartes inherited, at his father's death, he sold his estates, and invested the money, obtaining an income of six or seven thousand francs a year. He was educated, from 1604 to 1612, at the Jesuit college of La Flèche, which seems to have given him a much better grounding in modern mathematics than he could have got at most universities at that time. In 1612 he went to Paris, where he found social life boring, and retired to a secluded retreat in the Faubourg St Germain, in which he worked at geometry. Friends nosed him out, however, so, to secure more complete quiet, he enlisted in the Dutch army (1617). As Holland was at peace at the time, he seems to have enjoyed two years of undisturbed meditation. However, the coming of the Thirty Years' War led him to enlist in the Bavarian army (1619). It was in Bavaria, during the winter in 1619–20, that he had the experience he describes in the Discours de la Méthode. The weather being cold, he got into a stove1 in the morning, and stayed there all day meditating; by his own account, his philosophy was half finished when he came out, but this need not be accepted too literally. Socrates used to meditate all day in the snow, but Descartes's mind only worked when he was warm.

In 1621 he gave up fighting; after a visit to Italy, he settled in Paris in 1625. But again friends would call on him before he was up (he seldom got up before midday), so in 1628 he joined the army which was besieging La Rochelle, the Huguenot stronghold. When this episode was finished, he decided to live in Holland, probably to escape the risk of persecution. He was a timid man, a practising Catholic, but he shared Galileo's heresies. Some think that he had heard of the first (secret) condemnation of Galileo, which had taken place in 1616. However that may be, he decided not to publish a great book, Le Monde, upon which he had been engaged. His reason was that it maintained two heretical doctrines: the earth's rotation and the infinity of the universe. (This book was never published in its entirety, but fragments of it were published after his death.)

He lived in Holland for twenty years (1629–49), except for a few brief visits to France and one to England, all on business. It is impossible to exaggerate the importance of Holland in the seventeenth century, as the one country where there was freedom of speculation. Hobbes had to have his books printed there; Locke took refuge there during the five worst years of reaction in England before 1688; Bayle (of the Dictionary) found it necessary to live there; and Spinoza would hardly have been allowed to do his work in any other country.

I said that Descartes was a timid man, but perhaps it would be kinder to say that he wished to be left in peace so as to do his work undisturbed. He always courted ecclesiastics, especially Jesuits—not only while he was in their power, but after his emigration to Holland. His psychology is obscure, but I incline to think that he was a sincere Catholic, and wished to persuade the Church—in its own interests as well as in his—to be less hostile to modern science than it showed itself in the case of Galileo. There are those who think that his orthodoxy was merely politic, but though this is a possible view I do not think it the most probable.

Even in Holland he was subject to vexatious attacks, not by the Roman Church, but by Protestant bigots. It was said that his views led to atheism, and he would have been prosecuted but for the intervention of the French ambassador and the Prince of Orange. This attack having failed, another, less direct, was made a few years later by the authorities of the University of Leyden, which forbade all mention of him, whether favourable or unfavourable. Again the Prince of Orange intervened, and told the university not to be silly. This illustrates the gain to Protestant countries from the subordination of the Church to the State, and from the comparative weakness of Churches that were not international.

Unfortunately, through Chanut, the French ambassador at Stockholm, Descartes got into correspondence with Queen Christina of Sweden, a passionate and learned lady who thought that, as a sovereign, she had a right to waste the time of great men. He sent her a treatise on love, a subject which until then he had somewhat neglected. He also sent her a work on the passions of the soul, which he had originally composed for Princess Elizabeth, daughter of the Elector Palatine. These writings led her to request his presence at her court; he at last agreed, and she sent a warship to fetch him (September, 1649). It turned out that she wanted daily lessons from him, but could not spare the time except at five in the morning. This unaccustomed early rising, in the cold of a Scandinavian winter, was not the best thing for a delicate man. Moreover, Chanut became dangerously ill, and Descartes looked after him. The ambassador recovered, but Descartes fell ill and died in February, 1650.

Descartes never married, but he had a natural daughter who died at the age of five; this was, he said, the greatest sorrow of his life. He always was well dressed, and wore a sword. He was not industrious; he worked short hours, and read little. When he went to Holland he took few books with him, but among them were the Bible and Thomas Aquinas. His work seems to have been done with great concentration during short periods; but perhaps, to keep up the appearance of a gentlemanly amateur, he may have pretended to work less than in fact he did, for otherwise his achievements seem scarcely credible.

Descartes was a philosopher, a mathematician, and a man of science. In philosophy and mathematics, his work was of supreme importance; in science, though creditable, it was not so good as that of some of his contemporaries.

His great contribution to geometry was the invention of co-ordinate geometry, though not quite in its final form. He used the analytic method, which supposes a problem solved, and examines the consequences of the supposition; and he applied algebra to geometry. In both of these he had had predecessors—as regards the former, even among the ancients. What was original in him was the use of co-ordinates, i.e. the determination of the position of a point in a plane by its distance from two fixed lines. He did not himself discover all the power of this method, but he did enough to make further progress easy. This was by no means his sole contribution to mathematics, but it was his most important.

The book in which he set forth most of his scientific theories was Principia Philosophiae, published in 1644. There were, however, some other books of importance: Essais philosophiques (1637) deals with optics as well as geometry, and one of his books is called De la formation du foetus. He welcomed Harvey's discovery of the circulation of the blood, and was always hoping (though in vain) to make some discovery of importance in medicine. He regarded the bodies of men and animals as machines; animals he regarded as automata, governed entirely by the laws of physics, and devoid of feeling or consciousness. Men are different: they have a soul, which resides in the pineal gland. There the soul comes in contact with the 'vital spirits', and through this contact there is interaction between soul and body. The total quantity of motion in the universe is constant, and therefore the soul cannot affect it; but it can alter the direction of motion of the vital spirits, and hence, indirectly, of other parts of the body.

This part of his theory was abandoned by his school—first by his Dutch disciple Geulincx, and later by Malebranche and Spinoza. The physicists discovered the conservation of momentum, according to which the total quantity of motion in the world in any given direction is constant. This showed that the sort of action of mind on matter that Descartes imagined is impossible. Assuming—as was very generally assumed in the Cartesian school—that all physical action is of the nature of impact, dynamical laws suffice to determine the motions of matter, and there is no room for any influence of mind. But this raises a difficulty. My arm moves when I will that it shall move, but my will is a mental phenomenon and the motion of my arm a physical phenomenon. Why then, if mind and matter cannot interact, does my body behave as if my mind controlled it? To this Geulincx invented an answer, known as the theory of the 'two clocks'. Suppose you have two clocks which both keep perfect time: whenever one points to the hour, the other will strike, so that if you saw one and heard the other, you would think the one caused the other to strike. So it is with mind and body. Each is wound up by God to keep time with the other, so that, on occasion of my volition, purely physical laws cause my arm to move, although my will has not really acted on my body.

There were of course difficulties in this theory. In the first place, it was very odd; in the second place, since the physical series was rigidly determined by natural laws, the mental series, which ran parallel to it, must be equally deterministic. If the theory was valid there should be a sort of possible dictionary, in which each cerebral occurrence would be translated into the corresponding mental occurrence. An ideal calculator could calculate the cerebral occurrence by the laws of dynamics, and infer the concomitant mental occurrence by means of the 'dictionary'. Even without the 'dictionary', the calculator could infer words and actions, since these are bodily movements. This view would be difficult to reconcile with Christian ethics and the punishment of sin.

These consequences, however, were not at once apparent. The theory appeared to have two merits. The first was that it made the soul, in a sense, wholly independent of the body, since it was never acted on by the body. The second was that it allowed the general principle: 'one substance cannot act on another'. There were two substances, mind and matter, and they were so dissimilar that an interaction seemed inconceivable. Geulincx's theory explained the appearance of interaction while denying its reality.

In mechanics, Descartes accepts the first law of motion, according to which a body left to itself will move with constant velocity in a straight line. But there is no action at a distance, as later in Newton's theory of gravitation. There is no such thing as a vacuum, and there are no atoms; yet all interaction is of the nature of impact. If we knew enough, we should be able to reduce chemistry and biology to mechanics; the process by which a seed develops into an animal or a plant is purely mechanical. There is no need of Aristotle's three souls; only one of them, the rational soul, exists, and that only in man.

With due caution to avoid theological censure, Descartes develops a cosmogony not unlike those of some pre-Platonic philosophers. We know, he says, that the world was created as in Genesis, but it is interesting to see how it might have grown naturally. He works out a theory of the formation of vortices: round the sun there is an immense vortex in the plenum, which carries the planets round with it. The theory is ingenious, but cannot explain why planetary orbits are elliptical, not circular. It was generally accepted in France, where it was only gradually ousted by the Newtonian theory. Cotes, the editor of the first English edition of Newton's Principia, argues eloquently that the vortex theory leads to atheism, while Newton's requires God to set the planets in motion in a direction not towards the sun. On this ground, he thinks, Newton is to be preferred.

I come now to Descartes's two most important books, so far as pure philosophy is concerned. These are the Discourse on Method (1637) and the Meditations (1642). They largely overlap, and it is not necessary to keep them apart.

In these books Descartes begins by explaining the method of 'Cartesian doubt', as it has come to be called. In order to have a firm basis for his philosophy, he resolves to make himself doubt everything that he can manage to doubt. As he foresees that the process may take some time, he resolves, in the meanwhile, to regulate his conduct by commonly received rules; this will leave his mind unhampered by the possible consequences of his doubts in relation to practice.

He begins with scepticism in regard to the senses. Can I doubt, he says, that I am sitting here by the fire in a dressing-gown? Yes, for sometimes I have dreamt that I was here when in fact I was naked in bed. (Pyjamas, and even nightshirts, had not yet been invented.) Moreover madmen sometimes have hallucinations, so it is possible that I may be in like case.

Dreams, however, like painters, present us with copies of real things, at least as regards their elements. (You may dream of a winged horse, but only because you have seen horses and wings.) Therefore corporeal nature in general, involving such matters as extension, magnitude, and number, is less easy to question than beliefs about particular things. Arithmetic and geometry, which are not concerned with particular things, are therefore more certain than physics and astronomy; they are true even of dream objects, which do not differ from real ones as regards number and extension. Even in regard to arithmetic and geometry, however, doubt is possible. It may be that God causes me to make mistakes whenever I try to count the sides of a square or add 2 to 3. Perhaps it is wrong, even in imagination, to attribute such unkindness to God, but there might be an evil demon, no less cunning and deceitful than powerful, employing all his industry in misleading me. If there be such a demon, it may be that all the things I see are only illusions of which he makes use as traps for my credulity.

There remains, however, something that I cannot doubt: no demon, however cunning, could deceive me if I did not exist. I may have no body: this might be an illusion. But thought is different. 'While I wanted to think everything false, it must necessarily be that I who thought was something; and remarking that this truth, I think, therefore I am, was so solid and so certain that all the most extravagant suppositions of the sceptics were incapable of upsetting it, I judged that I could receive it without scruple as the first principle of the philosophy that I sought.'2

This passage is the kernel of Descartes's theory of knowledge, and contains what is most important in his philosophy. Most philosophers since Descartes have attached importance to the theory of knowledge, and their doing so is largely due to him. 'I think, therefore I am' makes mind more certain than matter, and my mind (for me) more certain than the minds of others. There is thus, in all philosophy derived from Descartes, a tendency to subjectivism, and to regarding matter as something only knowable, if at all, by inference from what is known of mind. These two tendencies exist both in Continental idealism and in British empiricism—in the former triumphantly, in the latter

regretfully. There has been, in quite recent times, an attempt to escape from this subjectivism by the philosophy known as instrumentalism, but of this I will not speak at present. With this exception, modern philosophy has very largely accepted the formulation of its problems from Descartes, while not accepting his solutions.

The reader will remember that St Augustine advanced an argument closely similar to the cogito. He did not, however, give prominence to it, and the problem which it is intended to solve occupied only a small part of his thoughts. Descartes's originality, therefore, should be admitted, though it consists less in inventing the argument than in perceiving its importance.

Having now secured a firm foundation, Descartes sets to work to rebuild the edifice of knowledge. The I that has been proved to exist has been inferred from the fact that I think, therefore I exist while I think, and only then. If I ceased to think, there would be no evidence of my existence. I am a thing that thinks, a substance of which the whole nature or essence consists in thinking, and which needs no place or material thing for its existence. The soul, therefore, is wholly distinct from the body and easier to know than the body; it would be what it is even if there were no body.

Descartes next asks himself: why is the cogito so evident? He concludes that it is only because it is clear and distinct. He therefore adopts as a general rule the principle: All things that we conceive very clearly and very distinctly are true. He admits, however, that there is sometimes difficulty in knowing which these things are.

'Thinking' is used by Descartes in a very wide sense. A thing that thinks, he says, is one that doubts, understands, conceives, affirms, denies, wills, imagines, and feels—for feeling, as it occurs in dreams, is a form of thinking. Since thought is the essence of mind, the mind must always think, even during deep sleep.

Descartes now resumes the question of our knowledge of bodies. He takes as an example a piece of wax from the honeycomb. Certain things are apparent to the senses: it tastes of honey, it smells of flowers, it has a certain sensible colour, size and shape, it is hard and cold, and if struck it emits a sound. But if you put it near the fire, these qualities change, although the wax persists, therefore what appeared to the senses was not the wax itself. The wax itself is constituted by extension, flexibility, and motion, which are understood by the mind, not by the imagination. The thingthat is the wax cannot itself be sensible, since it is equally involved in all the appearances of the wax to the various senses. The perception of the wax 'is not a vision or touch or imagination, but an inspection of the mind'. I do not see the wax, any more than I see men in the street when I see hats and coats. 'I understand by the sole power of judgment, which resides in my mind, what I thought I saw with my eyes.' Knowledge by the senses is confused, and shared with animals; but now I have stripped the wax of its clothes, and mentally perceive it naked. From my sensibly seeing the wax, my own existence follows with certainty, but not that of the wax. Knowledge of external things must be by the mind, not by the senses.

This leads to a consideration of different kinds of ideas. The commonest of errors, Descartes says, is to think that my ideas are like outside things. (The word 'idea' includes sense-perceptions, as used by Descartes.) Ideas seem to be of three sorts: (1) those that are innate, (2) those that are foreign and come from without, (3) those that are invented by me. The second kind of ideas, we naturally suppose, are like outside objects. We suppose this, partly because nature teaches us to think so, partly because such ideas come independently of the will (i.e. through sensation), and it therefore seems reasonable to suppose that a foreign thing imprints its likeness on me. But are these good reasons? When I speak of being 'taught by nature' in this connection, I only mean that I have a certain inclination to believe it, not that I see it by a natural light. What is seen by a natural light cannot be denied, but a mere inclination may be towards what is false. And as for ideas of sense being involuntary, that is no argument, for dreams are involuntary although they come from within. The reasons for supposing that ideas of sense come from without are therefore inconclusive.

Moreover there are sometimes two different ideas of the same external object, e.g. the sun as it appears to the senses and the sun in which the astronomers believe. These cannot both be like the sun, and reason shows that the one which comes directly from experience must be the less like it of the two.

But these considerations have not disposed of the sceptical arguments which threw doubt on the existence of the external world. This can only be done by first proving the existence of God.

Descartes's proofs of the existence of God are not very original; in the main they come from scholastic philosophy. They were better stated by Leibniz, and I will omit consideration of them until we come to him.

When God's existence has been proved, the rest proceeds easily. Since God is good, He will not act like the deceitful demon whom Descartes has imagined as a ground for doubt. Now God has given me such a strong inclination to believe in bodies that He would be deceitful if there were none; therefore bodies exist. He must, moreover, have given me the faculty of correcting errors. I use this faculty when I employ the principle that what is clear and distinct is true. This enables me to know mathematics, and physics also, if I remember that I must know the truth about bodies by the mind alone, not by mind and body jointly.

The constructive part of Descartes's theory of knowledge is much less interesting than the earlier destructive part. It uses all sorts of scholastic maxims, such as that an effect can never have more perfection than its cause, which have somehow escaped the initial critical scrutiny. No reason is given for accepting these maxims, although they are certainly less self-evident than one's own existence, which is proved with a flourish of trumpets. Plato, St Augustine, and St Thomas contain most of what is affirmative in the Meditations.

The method of critical doubt, though Descartes himself applied it only half-heartedly, was of great philosophic importance. It is clear, as a matter of logic, that it can only yield positive results if scepticism is to stop somewhere. If there is to be both logical and empirical knowledge, there must be two kinds of stopping points: indubitable facts, and indubitable principles of inference. Descartes's indubitable facts are his own thoughts—using 'thought' in the widest possible sense. 'I think' is his ultimate premiss. Here the word 'I' is really illegitimate; he ought to state his ultimate premiss in the form 'there are thoughts'. The word 'I' is grammatically convenient, but does not describe a datum. When he goes on to say 'I am a thing which thinks', he is already using uncritically the apparatus of categories handed down by scholasticism. He nowhere proves that thoughts need a thinker, nor is there reason to believe this except in a grammatical sense. The decision, however, to regard thoughts rather than external objects as the prime empirical certainties was very important, and had a profound effect on all subsequent philosophy.

In two other respects the philosophy of Descartes was important. First: it brought to completion, or very nearly to completion, the dualism of mind and matter which began with Plato and was developed, largely for religious reasons, by Christian philosophy. Ignoring the curious transactions in the pineal gland, which were dropped by the followers of Descartes, the Cartesian system presents two parallel but independent worlds, that of mind and that of matter, each of which can be studied without reference to the other. That the mind does not move the body was a new idea, due explicitly to Geulincx but implicitly to Descartes. It had the advantage of making it possible to say that the body does not move the mind. There is a considerable discussion in the Meditations as to why the mind feels 'sorrow' when the body is thirsty. The correct Cartesian answer was that the body and the mind were like two clocks, and that when one indicated 'thirst' the other indicated 'sorrow'. From the religious point of view, however, there was a grave drawback to this theory; and this brings me to the second characteristic of Cartesianism that I alluded to above.

In the whole theory of the material world, Cartesianism was rigidly deterministic. Living organisms, just as much as dead matter, were governed by the laws of physics; there was no longer need, as in the Aristotelian philosophy, of an entelechy or soul to explain the growth of organisms and the movements of animals. Descartes himself allowed one small exception: a human soul could, by volition, alter the direction though not the quantity of the motion of the vital spirits. This, however, was contrary to the spirit of the system, and turned out to be contrary to the laws of mechanics; it was therefore dropped. The consequence was that all the movements of matter were determined by physical laws, and, owing to parallelism, mental events must be equally determinate. Consequently Cartesians had difficulty about free will. And for those who paid more attention to Descartes's science than to his theory of knowledge, it was not difficult to extend the theory that animals are automata: why not say the same of men, and simplify the system by making it a consistent materialism? This step was actually taken in the eighteenth century.

There is in Descartes an unresolved dualism between what he learnt from contemporary science and the scholasticism that he had been taught at La Flèche. This led him into inconsistencies, but it also made him more rich in fruitful ideas than any completely logical philosopher could have been. Consistency might have made him merely the founder of a new scholasticism, whereas inconsistency made him the source of two important but divergent schools of philosophy.

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