their systems. I shall therefore begin by setting forth a few typical philosophical constructions of earlier centuries. Modern philosophy is generally taken as beginning with Descartes, who flourished in the first half of the seventeenth century. We have already had occasion, in Chapter XVI, to consider his argument “I think, therefore, I am”, but now we will deal with him somewhat more generally. He inaugurated two movements, one in metaphysics, one in theory of knowledge. In metaphysics, he emphasised the gulf between mind and matter, or between soul and body; in theory of knowledge he advocated a critical scrutiny of premises. These two movements had different histories, each of them interesting. The science of dynamics was rapidly developing in Descartes’ time, and seemed to show that the motions of matter could be calculated mathematically, given sufficient data. As the motions of matter include our bodily acts, even speaking and writing, it seemed as if the consequence must be a materialistic theory of human behaviour. This consequence, however, was distasteful to most philosophers, and they therefore invented various ways of escaping from it. Descartes himself thought that the will could have certain direct physical effects. He thought that the brain contains a fluid called the “animal spirits”, and that the will could influence the direction of its motion, though not the velocity. In this way he was still able to hold that the will is effective in the manner in which common sense supposes it to be. But this view did not fit in at all well with the rest of his philosophy. He held that, apart from the Supreme Substance, namely God, there are two created substances, mind and matter; that the essence of mind is thought, and that the essence of matter is extension. He made these two substances so different that interaction between them became difficult to understand, and his followers decided that there is never any effect either of mind on matter or of matter on mind.