Lastly, in the third place, we saw that even if Mr. Spencer's argument were fully subscribed to, and Mind in its substantial essence were conceded to be causeless, the central position of Materialism would still remain unaffected. For Mr. Spencer does not suppose that his "units of Force" are themselves endowed with consciousness, any more than Professor Clifford supposes his "moving molecules of inorganic matter" to be thus endowed. So that the only change which these possibilities, even if conceded to be actualities, produce in the speculative standing of Materialism, is to show that the raw material of consciousness, instead of requiring to be _caused_ by other substances--Matter and Force, _x_ and _y_,--occurs ready made as those substances. But the essential feature of Materialism remains untouched--namely, that what we know as Mind is dependent (whether by way of causality or not is immaterial) on highly complex forms of _what we know_ as Matter, in association with highly peculiar distributions of _what we know_ as Force. * * * * * IV. THE FINAL MYSTERY OF THINGS. Some physicists are inclined to dispute the fundamental proposition in which the whole of Mr. Spencer's system of philosophy may be said to rest--the proposition, namely, that the fact of the "persistence of force" constitutes the ultimate basis of science. For my own part, I cannot but believe that any disagreement on this matter only arises from some want of mutual understanding; and, therefore, in order to anticipate any criticisms to which the present work may be open on this score, I append this explanatory note. I readily grant that the term "persistence of force" is not a happy one, seeing that the word "force," as used by physicists, does not at the present time convey the full meaning which Mr. Spencer desires it to convey. But I think that any impartial physicist will be prepared to admit that, in the present state of his science, we are entitled to conclude that energy of position is merely the result of energy of motion; or, in other words, that potential energy is merely an expression of the fact that the universe, as a whole, is replete with actual energy, whose essential characteristic is that it is indestructible. And this may be concluded without committing ourselves to any particular theory as to the physical explanation of gravity; all we need assert is, that in some way or other gravity is the result of ubiquitous energy. And this, it seems to me, we must assert, or else conclude that gravity can never admit of a physical explanation. For all that we mean by a physical explanation is the proved establishment of an equation between two quantities of energy; so that if energy of position does not admit of being interpreted in terms of energy of motion, we must conclude that it does not admit of being interpreted at all--at least not in any physical sense.