In the last chapter (section 52) I have called him a Naive Realist; and here I shall call him a _Naive Dualist_, for a man may regard mind and matter as quite distinct kinds of things, without trying to elevate his opinion, through reflection, into a philosophical doctrine. The reflective man may stand by the opinion of the plain man, merely trying to make less vague and indefinite the notions of matter and of mind. He then becomes a _Philosophical Dualist_. There are several varieties of this doctrine, and I shall consider them a little later (section 58). But it is possible for one to be less profoundly impressed by the differences which characterize matter and mind. One may feel inclined to refer mental phenomena to matter, and to deny them the prominence accorded them by the dualist. On the other hand, one may be led by one's reflections to resolve material objects into mere ideas, and to claim that they can have no existence except in a mind. Finally, it is possible to hold that both minds and material things, as we know them, are only manifestations, phenomena, and that they must be referred to an ulterior "reality" or "substance." One may claim that they are "aspects" of the one reality, which is neither matter nor mind. These doctrines are different forms of _Monism_. In whatever else they differ from one another, they agree in maintaining that the universe does not contain two kinds of things fundamentally different. Out of the duality of things as it seems to be revealed to the plain man they try to make some kind of a unity. 35. MATERIALISM.--The first of the forms of monism above mentioned is _Materialism_. It is not a doctrine to which the first impulse of the plain man leads him at the present time. Even those who have done no reading in philosophy have inherited many of their ways of looking at things from the thinkers who lived in the ages past, and whose opinions have become the common property of civilized men. For more than two thousand years the world and the mind have been discussed, and it is impossible for any of us to escape from the influence of those discussions and to look at things with the primitive simplicity of the wholly untutored. But it was not always so. There was a time when men who were not savages, but possessed great intellectual vigor and much cultivation, found it easy and natural to be materialists. This I have spoken of before (section 30), but it will repay us to take up again a little more at length the clearest of the ancient forms of materialism, that of the Atomists, and to see what may be said for and against it.