result of his premature plunge into ontology, before the way was adequately prepared by a theory of knowledge. Enumerating perception as one species of thinking, he begins by treating it, not as perception, but simply as the presence of certain states of mind or mental modes. As he says in the opening of the Third Meditation : "The things which I perceive or imagine are perhaps nothing at all apart from me; but, in any case, I am assured that those modes of consciousness which I call perceptions and imaginations, in so far only as they are modes of consciousness, exist in me." But beyond their purely subjective or factual character as states of consciousness, our perceptions possess, according X to Descartes, a representative character, as referred to objects beyond themselves—as images and effects, indeed, of things existing outside the thinking substance. It is only in this latter aspect as symbolic of something beyond themselves that they are ideas or knowledge, and that truth or falsity belongs to them. Otherwise they are merely internal facts, that come and go and have no meaning.1 But if we start with mental modes unreferred, this subsequent reference of ideas to objects is evidently an inference which

1 Cf. Bradley's Principles of Logic, Book I. chap. i.

may or may not be false. Nor does Descartes deny its problematical character. But he defends its truth, in the case of extension and its derivative notions, by reference to the truthfulness or trustworthiness of God. God cannot be supposed to deceive us in the case of ideas which are clearly and distinctly realised. Reid points out, however, the weakness of this argument; for, according to Descartes' principles, "our senses testify no more but that we have certain ideas, and if we draw conclusions from this testimony, which the premisses will not support, we deceive onrselves." If we had a clear consciousness of extended substance as a permanent and relatively independent existence, the whole position would be changed; but, as it is, our fallacy lies at our own door. If we start with a selfcontained subject, the time can never arrive when such a being would have any justification for referring its states beyond itself. Descartes, however, is here under the shadow of his own presuppositions. The abstraction of the thinking substance has its necessary counterpart in the abstraction of the extended substance. These are the two dead entities into which, as we may say, Descartes broke up the living whole of know1 Works, p. 286.

ledge. Modern philosophy thus starts with two self-contained substances, each with its proper quality. The knowledge which the one acquires of the other is the result of the mechanical action of the other upon it. The ideas which represent material things are produced by the action of extended substance upon the thinking substance at the single point of location in the brain.

It would be needless to emphasise the difficulties which such a theory has to contend with. It is sufficient to point to the history of the Cartesian school for its immediate consequences. Occasionalism, which is simply logical Cartesianism, denies the possibility of any such interaction between the two substances as Descartes had admitted. Between mind and matter-thought and extension-an impassable gulf is fixed; the miraculously exerted will of God forms the only intermediary between the two worlds. Even in Spinoza, where the two finite substances pass into two sides of the divine nature, the existence of the two sides is empirically assumed, and their parallelism is also matter of dogmatic assertion. Malebranche, taking up the question from the point of view of knowledge, goes even further than Occasionalism. So far as the material world is concerned, the sole object of knowledge for

Malebranche is the idea of extension with its implications, or, as he calls it, intelligible extension. This, which is an ideal world, we know through our union with God, who illumines our minds. The existence of a real extended world, on the other hand-that is to say, Descartes' second substance-is not known at all, but is believed by Malebranche on grounds of supernatural revelation. In other words, it is maintained that our clear and distinct ideas do not, as Descartes had said, ground any inference to a non-ideal archetype or cause.

In English philosophy we can trace on a larger scale the evolution and self-refutation of the twosubstance doctrine and the complementary theory of Representative Perception. To it, then, we now turn.

It was Locke who made the terms and distinctions of modern philosophy current coin in England. Locke's philosophy is also peculiarly interesting, because in it "the theory of ideas" is seen just detaching itself, as it were, from the groundwork of common-sense and ordinary belief. Analysis has only begun to do its work, and as yet we are but a single remove from the consciousness of the ordinary man. The account given of the human understanding commends itself as

eminently credible, and not even very new. Only on looking closer do we see how far the first step in analysis has in reality carried us, and to what strange conclusions it has occasionally conducted Locke. In the course of his theorising, as Reid truly remarks, "the author is led into some paradoxes, although in general he is not fond of paradoxes." Let us look, then, with some care at the main features of the system elaborated in the Essay.

First, then, it may be noted that Locke took for granted the independent existence, on the one hand, of a system of material substances, which we may call the material world; and, on the other hand, of a number of separate minds or substances with the power of thinking. He also took for granted the interaction of these substances, supposing that, in perception, the material object perceived communicates a knowledge of itself to the perceiving mind by a species of impact, or mechanical impression. "Bodies," he says, "produce ideas in us . . . manifestly by impulse, the only way which we can conceive bodies operate in." So much he found warrant for alike in the common consciousness of mankind, and in the philosophy with which he was Essay, ii. 8, 11.

1 Works, p. 294.

2