It is worth noticing that with the characteristic readiness of the French materialist school to turn metaphysical and psychological discussion to practical uses, Holbach discerned the immense new field which the materialist account of mind opened to the physician. "If people consulted experience instead of prejudice, medicine would furnish morality with the key of the human heart; and in curing the body, it would be often assured of curing the mind too.... The dogma of the spirituality of the soul has turned morality into a conjectural science, which does not in the least help us to understand the true way of acting on men's motives.... Man will always be a mystery for those who insist on regarding him with the prejudiced eyes of theology, and on attributing his actions to a principle of which they can never have any clear ideas" (ch. ix.). It is certainly true as a historical fact that the rational treatment of insane persons, and the rational view of certain kinds of crime, were due to men like Pinel, trained in the materialistic school of the eighteenth century. And it was clearly impossible that the great and humane reforms in this field could have taken place before the decisive decay of theology. Theology assumes perversity as the natural condition of the human heart, and could only regard insanity as an intolerable exaggeration of this perversity. Secondly, the absolute independence of mind and body which theology brought into such overwhelming relief naturally excluded the notion that, by dealing with the body, you might be doing something to heal the mind. Perhaps we are now in some danger of overlooking the potency of the converse illustration of what Holbach says: namely, the efficacy of mental remedies or preventives in the case of bodily disease. If you complain--to resume our exposition--that the mechanism is not sufficient to explain the principle of the movements and faculties of the soul, the answer is, that it is in the same case with all the bodies in nature. In them the simplest movements, the most ordinary phenomena, the commonest actions, are inexplicable mysteries, whose first principles are for ever sealed to us. How shall we flatter ourselves that we know the first principle of gravity, by virtue of which a stone falls? What do we know of the mechanism that produces the attraction of some substances, and the repulsion of others? But surely the incomprehensibility of natural effects is no reason for assigning to them a cause that is still more incomprehensible than any of those within our cognisance. It is not given to man to know everything; it is not given to him to know his own origin, nor to penetrate into the essence of things, nor to mount up to the first principle of things. What is given to him is to have reason, to have good faith, to concede frankly that he is ignorant of what he cannot know, and not to supplement his lack of certainty by words that are unintelligible, and suppositions that are absurd.