human body is only a small individual fraction of the great organised physical world. The great general principles of theoretical physics and chemistry are now in a position to afford to this unifying conception of nature an exact, to a certain extent, indeed, a mathematical confirmation. In establishing the law of the "conservation of energy," Robert Mayer and Helmholtz showed that the energy of the universe is a constant unchangeable magnitude; if any energy whatever seems to vanish or to come anew into play, this is only due to the transformation of one form of energy into another. In the same way Lavoisier's law of the "conservation of matter" shows us that the material of the cosmos is a constant unchangeable magnitude; if any body seems to vanish (as, for example, by burning), or to come anew into being (as, for example, by crystallisation), this also is simply due to change of form or of combination. Both these great laws--in physics, the fundamental law of the conservation of energy, and in chemistry, of the conservation of matter--may be brought under one philosophical conception as the law of the conservation of substance; for, according to our monistic conception, energy and matter are inseparable, being only different inalienable manifestations of one single universal being-substance.[7] In a certain sense we can regard the conception of "animated atoms" as essentially partaking of the nature of this pure monism--a very ancient idea which more than two thousand years ago Empedocles enunciated in his doctrine of "hate and love of the elements." Modern physics and chemistry have indeed in the main accepted the atomic hypothesis first enunciated by Democritus, in so far as they regard all bodies as built up of atoms, and reduce all changes to movements of these minutest-discrete particles. All these changes, however, in organic as well as in inorganic nature, become truly intelligible to us only if we conceive these atoms not as dead masses, but as living elementary particles endowed with the power of attraction and repulsion. "Pleasure" and "pain," and "love" and "hate," as predicates of atoms are only other expressions for this power of attraction and repulsion.