But although Hobbes took psychology out of the hands of the theologian and made it a mechanical science, he did not identify it with physics. It is still psychology. The mental states are the physical motion of bodies, but they are not external motions, nor are they the copies of the external motions of bodies. Mental states are brain movements; they are the _result_ of external motions. They come about in this way. A moving body in the outer world makes an impression on the sense organ, and this motion is transmitted by the nerves to the heart and brain. A reaction is effected in the brain, and this is a mental state. The brain transformations, and not the movement of the external object, is that of which we are conscious. The mental state is an “apparition” of the actual fact in the external world; it is an effect in a causal series. Our perception of light is, for example, a modification of the cerebral substance, and not of the external body itself. We deceive ourselves when we think that the sensations of light, sound, heat are outside us. These qualities of things are modifications of ourselves. There is nothing external to us, except the motions of bodies which are the causes of these modifications. The external world is no doubt real, but we have no knowledge of it――no knowledge of aught save the motions of bodies within ourselves. _This is the point of view of all subsequent English philosophy: the substance of things is quite different from our knowledge of them. The substance of things is real; but is not the object of our knowledge. The object of our knowledge is a modification of ourselves._ The independence of knowledge with reference to theology on the one side, and to physical reality on the other, is well illustrated in Hobbes’s discussion of language. Speech consists of words, which are only the counters of things. Words are markers by which men may know a thing as “seamen mark a rock.” Science consists in their manipulation. Science combines them by addition and subtraction into judgments and syllogisms, and thereby constructs a body of demonstrated principles. Words are only counters, and he is a fool who mistakes the counter for the coin of reality. Words only represent reality, and the law of their use is mathematics. Truth and falsity are terms that are concerned with the correct or incorrect manipulation of these verbal counters and not with real things.