processes--is not proof against doubt. Hence, at the very beginning of all philosophy we must explicitly set down all knowledge which transcends ideas as open to doubt." These are the opening sentences of Volkelt's book on Kant's Theory of Knowledge. What is here put forward as an immediate and self-evident truth is, in reality, the conclusion of a piece of argument which runs as follows. Naïve common sense believes that things, just as we perceive them, exist also outside our minds. Physics, Physiology, and Psychology, however, teach us that our percepts are dependent on our organisation, and that therefore we cannot know anything about external objects except what our organisation transmits to us. The objects which we perceive are thus modifications of our organisation, not things-in-themselves. This line of thought has, in fact, been characterised by Ed. von Hartmann as the one which leads necessarily to the conviction that we can have direct knowledge only of our own ideas (cp. his Grundproblem der Erkenntnistheorie, pp. 16-40). Because outside our organisms we find vibrations of particles and of air, which are perceived by us as sounds, it is concluded that what we call sound is nothing more than a subjective reaction of our organisms to these motions in the external world. Similarly, colour and heat are inferred to be merely modifications of our organisms. And, further, these two kinds of percepts are held to be the effects of processes in the external world which are utterly different from what we experience as heat or as colour. When these processes stimulate the nerves in the skin of my body, I perceive heat; when they stimulate the optical nerve I perceive light and colour. Light, colour, and heat, then, are the reactions of my sensory nerves to external stimuli. Similarly, the sense of touch reveals to me, not the objects of the outer world, but only states of my own body. The physicist holds that bodies are composed of infinitely small particles called molecules, and that these molecules are not in direct contact with one another, but have definite intervals between them. Between them, therefore, is empty space. Across this space they act on one another by attraction and repulsion. If I put my hand on a body, the molecules of my hand by no means touch those of the body directly, but there remains a certain distance between body and hand, and what I experience as the body's resistance is nothing but the effect of the force of repulsion which its molecules exert on my hand. I am absolutely external to the body and experience only its effects on my organism.