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. It is my present purpose to extend the considerations already presented. Assuming the theory of Monism, I desire to ascertain the result to which it will lead when applied to the question whether we ought to regard the external world as of a character mental or non-mental. As observed in my Rede Lecture (_supra_, p. 33), this question has already been considered by the late Professor Clifford, who decided that on the monistic theory the probability pointed towards the external world being of a character non-mental; that, although the whole universe is composed of 'mind-stuff,' the universe as a whole is mindless. This decision I then briefly criticized; it is now my object to contemplate the matter somewhat more in detail. I will assume, on account of reasons previously given, that when we speak of matter in motion we do not at all know what it is that moves, nor do we know at all what it is that we mean by motion. Therefore if, as unknown quantities, we call matter _a_ and motion _b_, all we are entitled to affirm is that _a + b = z_, where _z_ is a known quantity, or mind. Obversely stated, we may say that the known quantity _z_ is capable of being resolved into the unknown _a + b_. But, inasmuch as both _a_ and _b_ are unknown, we may simplify matters by regarding their sum as a single unknown quantity _x_, which we take to be substantially identical with its obverse aspect known as _z_.