But to the very last the idea of causation retains the traces of its origin.  For in the first place every step in this building up of science assumes a permanence underlying all phenomena.  We cannot believe that the future will be like the past except because we believe that there is something permanent which was in the past and will be in the future.  And this assumption of something permanent in things around us comes from the consciousness of something permanent within us.  We know our own permanence.  Whatever else we know or do not know about ourselves, we are sure of our own personal identity through successive periods of life.  And as our explanation of things outside begins by classing them with things inside we still continue to ascribe permanence to whatever underlies phenomena even when we have long ceased to ascribe individual wills to any except beings like ourselves.  And without this assumption of permanence our whole science would come to the ground.

And in the second place let it be remembered that we began with the will causing the motions of the limbs.  Now there is, as far as we know, no other power in us to affect external nature than by setting something in motion.  We can move our limbs, and by so doing move other things, and by so doing avail ourselves of the laws of Nature to produce remoter effects.  But, except by originating motion, we cannot act at all.  And, accordingly, throughout all science the attempt is made to reduce all phenomena to motions.  Sounds, colours, heat, chemical action, electricity, we are perpetually endeavouring to reduce to vibrations or undulations, that is, to motion of some sort or other.  The mind seems to find a satisfaction when a change of whatever kind is shown to be, or possibly to be, the result of movement.  And so too all laws of Nature are then felt to be satisfactorily explained when they can be traced to some force exhibited in the movement of material particles.  The law of Gravitation has an enormous evidence in support of it considered simply as a fact.  And yet how many attempts have been made to represent it as the result of vortices or of particles streaming in all directions and pressing any two bodies together that lie in their path!  The facts which establish it are enough.  Why then these attempts?  What is felt to be yet wanting?  What is felt to be wanting is something to show that it is the result of some sort of general or universal motion, and that it thus falls under the same head as other motions, either those which originate in ourselves and are propagated from our bodies to external objects, or those which, springing from an unknown beginning, are for ever continuing as before.