The extreme importance of the subject leads us to oppose to these untenable "proofs of immortality" a brief exposition of the sound scientific arguments against it. The _physiological_ argument shows that the human soul is not an independent, immaterial substance, but, like the soul of all the higher animals, merely a collective title for the sum-total of man's cerebral functions; and these are just as much determined by physical and chemical processes as any of the other vital functions, and just as amenable to the law of substance. The _histological_ argument is based on the extremely complicated microscopic structure of the brain; it shows us the true "elementary organs of the soul" in the ganglionic cells. The _experimental_ argument proves that the various functions of the soul are bound up with certain special parts of the brain, and cannot be exercised unless these are in a normal condition; if the areas are destroyed, their function is extinguished; and this is especially applicable to the "organs of thought," the four central instruments of mental activity. The _pathological_ argument is the complement of the physiological; when certain parts of the brain (the centres of speech, sight, hearing, etc.) are destroyed by sickness, their activity (speech, vision, hearing, etc.) disappears; in this way nature herself makes the decisive physiological experiment. The _ontogenetic_ argument puts before us the facts of the development of the soul in the individual; we see how the child-soul gradually unfolds its various powers; the youth presents them in full bloom, the mature man shows their ripe fruit; in old age we see the gradual decay of the psychic powers, corresponding to the senile degeneration of the brain. The _phylogenetic_ argument derives its strength from palæontology, and the comparative anatomy and physiology of the brain; co-operating with and completing each other, these sciences prove to the hilt that the human brain (and, consequently, its function--the soul) has been evolved step by step from that of the mammal, and, still further back, from that of the lower vertebrate.