The great advantage of this theory is, that it supposes only one stream of causation, in which both mind and motion are simultaneously concerned. The theory, therefore, escapes all the difficulties and contradictions with which both spiritualism and materialism are beset. Thus, motion is supposed to be producing nothing but motion; mind-changes nothing but mind-changes--both producing both simultaneously: neither could be what it is without the other, because without the other neither could be the cause which in fact it is. Impossible, therefore, is the supposition of the materialist that consciousness is adventitious, or that in the absence of mind the changes of the brain could be what they are; for it belongs to the very causation of these movements that they should have a mental side. And equally impossible is the supposition of the spiritualist that the cerebral processes are adventitious, or that in the absence of brain the changes of the mind could be what they are; for it belongs to the very causation of these changes that they should have a material side. Furthermore, the use of mind to animals and to men is thus rendered apparent; for intelligent volition is thus shown to be a true cause of adjustive movement, in that the cerebration which it involves could not otherwise be possible: the causation would not otherwise be complete.
[Footnote 8: _Lectures and Essays_, vol. ii. pp. 56-7.]
In the Introduction to this essay I have sought to show that there are, for the purposes of practical discussion, but three theories of the World of Being. There is, first, the theory of Materialism, which supposes matter in motion to be the ultimate or self-existing Reality, and, therefore, the cause of mind. Next, there is the theory of Spiritualism, which supposes mind to be the ultimate Reality, and, therefore, the cause of matter in motion. Lastly, there is the theory of Monism which supposes matter in motion to be substantially identical with mind, and, therefore, that as between mind and matter in motion there is no causal relation either way. In the foregoing chapters I have considered these three theories, and argued that of them the last-mentioned is the only one which satisfies all the facts of feeling on the one hand, and of observation on the other. The theory of Monism alone is able to explain, without inherent contradiction, the phenomena both of the subjective and objective spheres.