'while consciousness lasts'.[14] But if all existence, distinguishable as subjective, is resolvable into units of consciousness, which in their obverse or objective aspect are oscillations of molecules,[15] what more is required to round off the explanation of every thing, save the Unknowable--save the Ultimate Reality in which subject and object are united? In the end we are baffled by mystery; let us, therefore, make the best of it and rejoice. 'We can think of Matter only in terms of Mind. We can think of Mind only in terms of Matter. When we have pushed our explorations of the first to its uttermost limit we are referred to the second for a final answer; and when we have got the final answer to the second we are referred back to the first for an interpretation of it.'[14] And so neither answer is final. Finality is only reached when both are swallowed up, not in victory, but in defeat. Shall we not then glory in defeat and sing its praises often? I must leave to some future Herbert Spencer lecturer the discussion of his doctrine of the Unknowable and the critical consideration of its place and value in philosophy. I would fain leave it altogether on one side; but that is impossible. Although the _First Principles_ is divided into two Parts, dealing respectively with the Unknowable and the Knowable, we have not by any means done with the former when we turn from the First Part to the Second. With Spencer we have never done with the Unknowable, the Unconditioned Reality and the other aliases by which it goes. His persistence of force is the persistence of Unknowable Force. In a leading passage, at any rate, it is avowedly 'the persistence of some Cause which transcends our knowledge and conception. In asserting it we assert an Unconditioned Reality without beginning or end'.[16] There must, he holds, be something at the back of the evolutionary drama which we study--something that is both a principle of activity and a permanent _nexus_.[17] The pity of it is that we know not, and can never know, what on earth (or in heaven!) it is. We only know that it exists, and somehow produces the whole show. Now it would much conduce to clearness of thought and of statement if we could agree to eliminate those terribly ambiguous words 'force' and 'cause' when we are dealing with the fundamental postulate (if such it be) that there must be something at the back of evolution to make it what it is; and the word Source seems ready to our hand and might well be given this special significance. But Spencer uses Agency, Power, Cause, Force, in this connexion. In how many senses he uses the word 'force' I am not prepared to say. It is often a synonym for cause; it stands alike for matter and energy;[18] it is the objective correlate of our subjective sense of effort.[19] There is a 'correlation and equivalence between external forces and the mental forces generated by them under the form