A great German thinker began his work on the human soul with a discussion of the law of gravitation. All study of man must begin with the study of the atom. Man's life we have seen to be the aggregate of the work of all the cells of his body. But the protoplasm which composes his cells is a chemical compound, and hence subject to all the laws of all the atoms of which it is composed. And its molecules, or the smallest mechanically separable compounds of these atoms, are arranged and related according to the laws of physics, so as to permit or produce the play of certain forces which are always the result of atomic or molecular combination. Every motive or thought demands the combustion of a certain amount of material which has been already assimilated in the microscopic cellular laboratories of our body. Every vital activity is manifested at least through chemical and physical forces. And the elements of the fuel for our engines we receive through plants from the inorganic world. For the plant, as we have seen, stores up as potential energy in its compounds the actual energy of the sun's rays. And thus man lives and thinks by energy, obtained originally from the sun. But man not only consumes food and fuel. The complicated protoplasm is continually wearing out and being replaced. Every cell in our bodies is a centre toward which particles of material stream to be assimilated and form for a time a part of the living substance, and then to be cast out again as dead matter. Our very existence depends upon this continual change. There is synthesis of simple substances into more complex compounds, and then analysis of these complex compounds into simpler, and from this latter process results the energy manifested in every vital action. We are all whirlpools on the surface of nature; when the whirling ceases we disappear. Man, like every other living being, exists in a condition of constant interchange with surrounding nature; he is rooted in innumerable ways in the inorganic world.