We learn to know man's body through bodily senses, and the manner of observing it cannot differ from the way in which we learn to know other objects perceived by the senses. As we observe minerals, plants and animals, so can we also observe man. He is related to these three forms of existence. Like the minerals, he builds his body out of natural substances; like the plants, he grows and propagates his species; like the animals, he perceives the objects around him and builds up his inner experiences on the basis of the impressions they make on him. Thus, a mineral, a plant and an animal existence may be ascribed to man.
The differences in structure of minerals, plants and animals correspond with the three forms of their existence. It is this structure the shape that is perceived through the senses, and that alone can be called body. Now the human body is different from that of the animal. This difference must be recognized, whatever may otherwise be thought of the relationship of man to animals. Even the most extreme materialist who denies all soul cannot but admit the truth of this passage uttered by Carus in his Oragnon der Natur und des Geistes. The finer, inner construction of the nervous system and especially of the brain remains still an unsolved problem for the physiologist and the anatomist. That this concentration of structures ever increases in the animal kingdom and reaches in man a stage unequalled in any other being is a fully established fact a fact that is of the deepest significance in regard to the mental evolution of man. Indeed, we may go so far as to say it is really a sufficient explanation of that evolution. Where, therefore, the structure of the brain has not developed properly, where its smallness and poverty are in evidence as in the case of microcephali and idiots, it goes without saying that we can no more expect the appearance of original ideas and of knowledge than we can expect the propagation of the species from persons with completely stunted reproductive organs. On the other hand, a strong and beautifully developed build of the whole man, and especially of the brain, will certainly not in itself take the place of genius but it will at any rate supply the first and indispensable condition for higher knowledge.
Just as one ascribes to the human body the three forms of existence, mineral, plant and animal, so one must ascribe to it a fourth the distinctively human form. Through his mineral existence man is related to everything visible; through his plantlike existence to all beings that grow and propagate their species; through his animal existence to all those that perceive their surroundings and by means of external impressions have inner experiences; through his human form of existence he constitutes, even in regard to his body alone, a kingdom by himself.