Of course in saying this I am aware that some of the more clear-sighted of the materialists have plainly perceived this difficulty in all its magnitude, and so have felt that unless it can be met, any theory of Materialism must necessarily contain a radical contradiction of principles. Some few materialists have therefore sought to meet the difficulty in the only way it can be met, viz. by boldly asserting the possibility of thought and energy being transmutable. On this view thought becomes a mode of motion, and takes its rank among the forces as identical in nature with heat, light, electricity, and the rest. But this view is also inherently impossible. For suppose, as a matter of argument, that physiologists should discover a mechanical equivalent of thought, so that we might estimate the value of a calculation in thermal units, or the 'labour of love' in foot-pounds: still we should not be out of our difficulties; we should only have to cut a twist of flax to find a lock of iron. For by thus assimilating thought with energy, we should in no wise have explained the fundamental antithesis between subject and object. The fact would remain, if possible, more unaccountable than ever, that mind should present absolutely no point of real analogy with motion. Involved with the essential idea of motion is the idea of extension; suppress the latter and the former must necessarily vanish, for motion only means transition in space of something itself extended. But thought, as far as we can possibly know it, is known and distinguished by the very peculiarity of not having extension. Therefore, even if we were to find a mechanical equivalent of thought, thought would still not be proved a mode of motion. On the contrary, what would be proved would be that, in becoming transformed into thought, energy had ceased to be energy; in passing out of its relation to space it would cease to exist as energy, and if it again passed into that relation it would only be by starting _de novo_ on a new course of history. Therefore the proof that thought has a mechanical equivalent would simply amount to the proof, not that thought _is_ energy, but that thought _destroys_ energy. And if Materialism were to prove this, Materialism would commit suicide. For if once it were proved that the relation of energy to thought is such that thought is able to absorb or temporarily to annihilate energy, the whole argument of Materialism would be inverted, and whatever evidence there is of causation as between mind and matter would become available in all its force on the side of Spiritualism. This seems plain, for if it even were conceivable--which most distinctly it is not--that a motor could ever become a motive, and so pass from the sphere of dynamics into the sphere of consciousness, the fact would go to prove, not that the motor was the cause of the motive, but rather that the motive was the cause of destroying the motor; so that at that point the otherwise unbroken chain