It were comparatively an easy task to explain psychological phenomena by asserting, as did the metaphysicians of the past, and as some do even at the present, that the human brain—the physical sanctuary of thought—is merely an instrument through which various spiritual beings operate, producing at one time the prophetic utterances of the seer, at another time the gifted words of genius, and yet again the extravagant and discordant expressions of madness...

We will, therefore, examine the question from the standpoint of more modern times, when the supernatural agency in insanity gives place to the deteriorating influences which unite it to other forms of nervous disease; and genius becomes a product of an age, in the expansive growth of the human mind.