According to Wundt, all changes of nature are mere changes of place. All causes are motional causes (page 26). Any discussion of the philosophical grounds on which Wundt supports his theory would lead us deep into the speculations of the Eleatics and the Herbartians. Change of place, Wundt holds, is the _only_ change of a thing in which a thing remains identical with itself. If a thing changed _qualitatively_, we should be obliged to imagine that something was annihilated and something else created in its place, which is not to be reconciled with our idea of the identity of the object observed and of the indestructibility of matter. But we have only to remember that the Eleatics encountered difficulties of exactly the same sort in motion. Can we not also imagine that a thing is destroyed in _one_ place and in _another_ an exactly similar thing created? After all, do we really know _more_ why a body leaves one place and appears in another, than why a _cold_ body grows _warm_? Granted that we had a perfect knowledge of the mechanical processes of nature, could we and should we, for that reason, _put out of the world_ all other processes that we do not understand? On this principle it would really be the simplest course to deny the existence of the whole world. This is the point at which the Eleatics ultimately arrived, and the school of Herbart stopped little short of the same goal. Physics treated in this sense supplies us simply with a diagram of the world, in which we do not know reality again. It happens, in fact, to men who give themselves up to this view for many years, that the world of sense from which they start as a province of the greatest familiarity, suddenly becomes, in their eyes, the supreme "world-riddle." Intelligible as it is, therefore, that the efforts of thinkers have always been bent upon the "reduction of all physical processes to the motions of atoms," it must yet be affirmed that this is a chimerical ideal. This ideal has often played an effective part in popular lectures, but in the workshop of the serious inquirer it has discharged scarcely the least function. What has really been achieved in mechanical physics is either the _elucidation_ of physical processes by more familiar _mechanical analogies_, (for example, the theories of light and of electricity,) or the exact _quantitative_ ascertainment of the connexion of mechanical processes with other physical processes, for example, the results of thermodynamics. III. THE PRINCIPLE OF ENERGY IN PHYSICS.