The simile of an instrument and the master who plays upon it, which is often used of the relation between body and mind, is in many respects a very imperfect one; for the master does not develop with and in his instrument. But in regard to the most oppressive arguments of naturalism, the influence of disease, of old age, of mental disturbances due to brain changes, the comparison serves our turn well enough, for undoubtedly the master is dependent upon his instrument; upon an organ which is going more and more out of tune, rusting, losing its pipes, his harmonies will become poorer, more imperfect. And if we think of the association between the two as further obstructed, the master becoming deaf, the stops confused, the relation between the notes and pipes altered, then what may still live within him in perfect and unclouded purity, and in undiminished richness, may present itself outwardly as confused and unintelligible, may even find only disconnected expression, and finally cease altogether; so that no conclusion would be possible except that the master himself had become different or poorer. The melancholy field of mental diseases perhaps yields proofs against naturalism to an even greater degree than for it. It is by no means the case that all mental diseases are invariably diseases of the brain, for even more frequently they are real sicknesses of the mind, which yield not to physical but to psychical remedies. And the fact that the mind can be ill, is a sad but emphatic proof that it goes its own way. Immortality.