Democritus of Abdera taught that nothing exists except atoms and empty space. The atoms, he maintained, differ from one another in size, shape, and position. In other respects they are alike. They have always been in motion. Perhaps he conceived of that motion as originally a fall through space, but there seems to be uncertainty upon this point. However, the atoms in motion collide with one another, and these collisions result in mechanical combinations from which spring into being world-systems. According to this doctrine, nothing comes from nothing, and nothing can become nonexistent. All the changes which have ever taken place in the world are only changes in the position of material particles--they are regroupings of atoms. We cannot directly perceive them to be such, for our senses are too dull to make such fine observations, but our reason tells us that such is the case. Where, in such a world as this, is there room for mind, and what can we mean by mind? Democritus finds a place for mind by conceiving it to consist of fine, smooth, round atoms, which are the same as the atoms which constitute fire. These are distributed through the whole body, and lie among the other atoms which compose it. They are inhaled with and exhaled into the outer air. While they are in the body their functions are different according as they are located in this organ or in that. In the brain they give rise to thought, in the heart to anger, and in the liver to desire. I suppose no one would care, at the present time, to become a Democritean. The "Reason," which tells us that the mind consists of fine, round atoms, appears to have nothing but its bare word to offer us. But, apart from this, a peculiar difficulty seems to face us; even supposing there are atoms of fire in the brain, the heart, and the liver, what are the _thought_, _anger_, and _desire_, of which mention is made? Shall we conceive of these last as atoms, as void space, or as the motion of atoms? There really seems to be no place in the world for them, and _these are the mind so far as the mind appears to be revealed_--they are _mental phenomena_. It does not seem that they are to be identified with anything that the Atomistic doctrine admits as existing. They are simply overlooked. Is the modern materialism more satisfactory? About half a century ago there was in the scientific world something like a revival of materialistic thinking. It did not occur to any one to maintain that the mind consists of fine atoms disseminated through the body, but statements almost as crude were made. It was said, for example, that the brain secretes thought as the liver secretes bile.