Nor, as we have seen, did he think that the adaptation in living organisms of each part to every other required an external explanation. Far less did it occur to him that the production of impressions on our senses was due to the agency of a supernatural power. It is absolutely certain that he would have rejected the Cartesian argument, according to which a perfect being must exist if it be only conceivable—existence being necessarily involved in the idea of perfection.[252] Finally, not recognising such a faculty as conscience, he would not have admitted it to be the voice of God speaking in the soul. On the other hand, Aristotle’s own theistic arguments cannot stand for a moment in the face of modern science. We know by the law of inertia that it is not the continuance, but the arrest or the beginning of motion which requires to be accounted for. We know by the Copernican system that there is no solid sidereal sphere governing the revolutions of all Nature. And we know by the Newtonian physics that gravitation is not dependent on fixed points in space for its operation. _The_ Philosophy of _the_ Philosopher Aristotle is as inconsistent with the demonstrations of modern astronomy as it is with the faith of mediaeval Catholicism. It remains to be seen whether the system which we are examining is consistent with itself. It is not. The Prime Mover, being unextended, cannot be located outside the sidereal sphere; nor can he be brought into immediate contact with it more than with any other part of the cosmos. If the aether has a motion proper to itself, then no spiritual agency is required to keep it in perpetual rotation. If the crystalline spheres fit accurately together, as they must, to avoid leaving a vacuum anywhere, there can be no friction, no production of heat, and consequently no effect produced on the sublunary sphere. Finally, no rotatory or other movement can, taken alone, have any conceivable connexion with the realisation of a possibility, in the sense of progress from a lower to a higher state of being. It is merely the perpetual exchange of one indifferent position for another.