I am endeavouring in a general way to explain the laws of double refraction on this hypothesis, but have not yet arrived at any results sufficiently … - William Kingdon Clifford

In the cases of heat and electricity the form of the persisting energy is pretty well ascertained. But there are cases in which we do not know if it is energy of motion or energy position, such as that of . In the burning of coal there is a falling together of carbon and oxygen [and heat is produced] but we do not know in which of the two forms, if either, the energy which comes out as heat existed in the chemical process. For such a case the is only a probable statement (though of great probability) to the effect that in all cases where a physical quality is convertible with energy, that quality is itself either energy of motion or energy position.

Is mind a force? It is held by some that the will acts as the match to gunpowder, by setting loose a store of energy, the matter of the brain being in unstable equilibrium. But you cannot have in nature an absolutely unstable equilibrium [i.e., an equilibrium capable of being upset by an infinitesimal force], because the universe is not at rest [and every motion in the universe produces a finite change, however small, in the resultant force at every point of space]. Therefore if mind is force, operating in the way suggested, it must be able to create a determinate quantity of energy. This is a supposition which, if true, would destroy its own evidence; for it would destroy the uniformity of nature, on which all of inference ultimately rests.

I hold in fact
(1) That small portions of space are in fact of a nature analogous to little hills on a surface which is on the average flat; namely, that the ordinary laws of geometry are not valid in them.
(2) That this property of being curved or distorted is continually being passed on from one portion of space to another after the manner of a wave.
(3) That this variation of the curvature of space is what really happens in that phenomenon which we call the motion of matter, whether ponderable or etherial.
(4) That in the physical world nothing else takes place but this variation, subject possibly to the law of continuity.