The balance of reasonable presumption, however, is not even. The most inclusive movements known to us in nature, the astronomical, are calculable, and so are the most minute and pervasive processes, the chemical. These are also, if evolution is to be accepted, the earliest processes upon which all others have supervened and out of which, as it were, they have grown. Apart from miraculous intervention, therefore, the assumption seems to be inevitable that the intermediate processes are calculable too, and compounded out of the others. The appearance to the contrary presented in animal and social life is easily explicable on psychological grounds. We read inevitably in terms of our passions those things which affect them or are analogous to what involves passion in ourselves; and when the mechanism of them is hidden from us, as is that of our bodies, we suppose that these passions which we find on the surface in ourselves, or read into other creatures, are the substantial and only forces that carry on our part of the world. Penetrating this illusion, dispassionate observers in all ages have received the general impression that nature is one and mechanical. This was, and still remains, a general impression only; but I suspect no one who walks the earth with his eyes open would be concerned to resist it, were it not for certain fond human conceits which such a view would rebuke and, if accepted, would tend to obliterate. The psychological illusion that our ideas and purposes are original facts and forces (instead of expressions in consciousness of facts and forces which are material) and the practical and optical illusion that everything wheels about us in this world--these are the primitive persuasions which the enemies of naturalism have always been concerned to protect.