The systems of Descartes, Spinoza and Leibniz have one very important characteristic in common, namely, that they all depend upon the category of “substance”. This is a concept which has developed out of the common-sense notion of “thing”. A “substance” is that which has qualities, and is in general supposed to be indestructible, though it is difficult to see why. It acquired its hold over metaphysicians partly because both matter and the soul were held to be immortal, and partly through a hasty transference to reality of ideas derived from grammar. We say “Peter is running”, “Peter is talking”, “Peter is eating”, and so on. We think that there is one entity, Peter, who does all these things, and that none of them could be done unless there were someone to do them, but that Peter might quite well do none of them. Similarly we assign qualities to Peter: we say he is wise and tall and blond and so on. All these qualities, we feel, cannot subsist by themselves in the void, but only when there is a subject to which they belong; but Peter would remain Peter even if he became foolish and short and dyed his hair. Thus Peter, who is regarded as a “substance”, is self-subsistent as compared with his qualities and states, and he preserves his substantial identity throughout all sorts of changes. Similarly in the material world an atom is supposed (or rather was supposed until recently) to preserve its identity throughout all time, however it might move and whatever combinations it might form with other atoms. The concept of “motion”, upon which all physics seemed to depend, was only strictly applicable to a substance which preserves its identity while changing its spatial relations to other substances; thus “substance” acquired an even firmer hold upon physics than upon metaphysics.