The motives for this development were various; perhaps the most important was the development of physics immediately after Descartes’ time. A law was discovered called the “conservation of momentum”. This states that, if a system of bodies is in any sort of motion, and is free from outside influences, the amount of motion _in any direction_ is constant. This showed that the kind of action of the will on the “animal spirits” which Descartes had assumed was contrary to the principles of dynamics. It seemed to follow that mind cannot influence matter, and it was inferred that matter cannot influence mind, since the two were regarded as co-equal substances. It was held that each goes it own way, according to its own laws. The fact that our arm moves when we will it to move was regarded as analogous to the fact that two perfectly accurate clocks strike at the same moment, though neither has any effect upon the other. The series of mental events and the series of physical events were parallel, each going at the same rate as the other, therefore they continued to synchronise, in spite of their independence of each other. Spinoza sought to make this parallelism less mysterious by denying that there are two separate substances, mind and matter. He maintained that there is only one substance, of which thought and extension are attributes. But there seemed still no good reason why the events belonging to the two attributes should develop along parallel lines. Spinoza is in many ways one of the greatest philosophers, but his greatness is rather ethical than metaphysical. Accordingly he was regarded by contemporaries as a profound metaphysician but a very wicked man. The notion of the impossibility of interaction between mind and body has persisted down to our own day. One still hears of “psychophysical parallelism”, according to which to every state of the brain a state of mind _corresponds_ and vice versa, without either acting on the other. This whole point of view, though not exactly that of Descartes, derives from him. It has a number of sources, religious, metaphysical, and scientific; but there seems no ground whatever for regarding it as true.