But the more frankly the student of natural science acknowledges these appointed limits, and the more humbly he is reconciled to this ignorance, the more profoundly conscious is he of his right inductively to fashion his own views as to the relations between mind and matter, with perfect freedom, and untrammeled by myths, dogmas, or time-honored philosophies.
He sees material conditions in a thousand ways influencing mental life. To his unprejudiced mind there seems no reason to doubt that sense-impressions are really communicated to the so-called Soul. He sees the human mind grow with the brain as it were, and, according to the empiricists, he finds that the actual forms of his thought are constituted by means of external perceptions. In sleep, and in dreams, in fainting, in intoxication and narcosis, in the delirium of fever and in inanition, in mania, epilepsy, idiocy, microcephaly—in a thousand morbid states he sees the soul to be dependent on the constant or transient condition of the brain. No theological prejudice prevents him, as it did Descartes, from recognizing in the souls of animals the relatives of the human soul, and less perfect members of the same series of development. On the contrary, he sees that in the vertebrates those parts of the brain which physiological research and pathological experience prove to be the seat of the higher mental activities keep pace, in their comparative development, with the growth of these activities. Where mental capacity makes the immense leap from the anthropoid apes to man which is indicated by the power of speech, we find a corresponding leap in cerebral mass. The varied arrangement of similar elementary particles in the invertebrates instructs the investigator of Nature that here, as in other organs, there is question less of the general architecture than of the structural elements.
With awe and wonder he regards the microscopic molecule of nervous substance which is the seat of the laborious, constructive, orderly, loyal, dauntless soul of the ant. Finally, the development theory, coupled with the doctrine of natural selection, forces upon him the theory that the soul came into being as the result, gradually attained, of certain material combinations, and that probably, like other heritable endowments that are of use to the individual in the struggle for life, it has risen and perfected itself up to its present state through a countless series of generations.
Now, if the ancient thinkers found every interaction between body and soul unintelligible and impossible on their theories, and if their undoubted simultaneous cooperation is to be explained only by a Preestablished Harmony of the two substances, then the notion they formed of the soul, in conformity with their scholastic conceptions, must have been erroneous. The necessity of a scholastic conclusion so plainly in conflict with the reality, is, as it were, an apogogical demonstration of the falsity of their premises. In his simile of the two watches, Leibnitz, as has been well observed by Fechner, over-