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By Root 3515 0

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