Now, the materialist allows these two classes to run together; so does the idealist. The one says that everything is matter; the other, that everything is mind. It would be foolish to maintain that nothing can be said for either doctrine, for men of ability have embraced each. But one may at least say that both seem to be refuted by our common experience of the world, an experience which, so far as it is permitted to testify at all, lifts up its voice in favor of _Dualism_. Dualism is sometimes defined as the doctrine that there are in the world two kinds of substances, matter and mind, which are different in kind and should be kept distinct. There are dualists who prefer to avoid the use of the word substance, and to say that the world of our experiences consists of physical phenomena and of mental phenomena, and that these two classes of facts should be kept separate. The dualist may maintain that we have a direct knowledge of matter and of mind, and he may content himself with such a statement, doing little to make clear what we mean by matter and by mind. In this case, his position is little different from that of the plain man who does not attempt to philosophize. Thomas Reid (section 50) belongs to this class. On the other hand, the dualist may attempt to make clear, through philosophical reflection, what we mean by the matter and mind which experience seems to give us. He may conclude:-- (1) That he must hold, as did Sir William Hamilton, that we perceive directly only physical and mental phenomena, but are justified in inferring that, since the phenomena are different, there must be two kinds of underlying substances to which the phenomena are referred. Thus, he may distinguish between the two substances and their manifestations, as some monists distinguish between the one substance and its manifestations. (2) Or he may conclude that it is futile to search for substances or realities of any sort _behind_ phenomena, arguing that such realities are never revealed in experience, and that no sound reason for their assumption can be adduced. In this case, he may try to make plain what mind and matter are, by simply analyzing our experiences of mind and matter and coming to a clearer comprehension of their nature. As the reader has probably remarked, the philosophy presented in the earlier chapters of this book (Chapters III to XI) is _dualistic_ as well as _realistic_. That is to say, it refuses to rub out the distinction between physical phenomena and mental phenomena, either by dissolving the material world into ideas; by calling ideas secretions or functions of the brain; or by declaring them one in a fictitious entity behind the veil and not supposed to be exactly identical with either. And as it teaches that the only reality that it means anything to talk about must be found in experience, it is a dualism of the type described in the paragraph which immediately precedes.