Later philosophers, accepting this notion of the soul, were naturally much perplexed to understand how, if matter and spirit had nothing in common, they could act and react on one another. All the changes of matter being modes of motion, the difficulty of understanding how a moving extended material body was to affect a thinking thing which had no dimension, was as great as that involved in solving the problem of how to hit a nominative case with a stick. Hence, the successors of Descartes either found themselves obliged, with the Occasionalists, to call in the aid of the Deity, who was supposed to be a sort of go-between betwixt matter and spirit; or they had recourse, with Leibnitz, to the doctrine of pre-established harmony, which denied any influence of the body on the soul, or _vice versa_, and compared matter and spirit to two clocks so accurately regulated to keep time with one another, that the one struck when ever the other pointed to the hour; or, with Berkeley, they abolished the "substance" of matter altogether, as a superfluity, though they failed to see that the same arguments equally justified the abolition of soul as another superfluity, and the reduction of the universe to a series of events or phenomena; or, finally, with Spinoza, to whom Berkeley makes a perilously close approach, they asserted the existence of only one substance, with two chief attributes, the one, thought, and the other, extension. There remained only one possible position, which, had it been taken up earlier, might have saved an immensity of trouble; and that was to affirm that we do not, and cannot, know anything about the "substance" either of the thinking thing, or of the extended thing. And Hume's sound common sense led him to defend this thesis, which Locke had already foreshadowed, with respect to the question of the substance of the soul. Hume enunciates two opinions. The first is that the question itself is unintelligible, and therefore cannot receive any answer; the second is that the popular doctrine respecting the immateriality, simplicity, and indivisibility of a thinking substance is a "true atheism, and will serve to justify all those sentiments for which Spinoza is so universally infamous." In support of the first opinion, Hume points out that it is impossible to attach any definite meaning to the word "substance" when employed for the hypothetical substratum of soul and matter. For if we define substance as that which may exist by itself, the definition does not distinguish the soul from perceptions. It is perfectly easy to conceive that states of consciousness are self-subsistent. And, if the substance of the soul is defined as that in which perceptions inhere, what is meant by the inherence? Is such inherence conceivable? If conceivable, what evidence is there of it? And what is the use of a substratum to things which, for anything we know to the contrary, are capable of existing by themselves?