A little thought renders it evident that a feeble-minded person, an idiot, or an imbecile, is always one with a defective memory. He is unable to profit like the normal individual from experience. The truth that the higher faculties are more often absent in the feeble-minded than the lower is due entirely to the fact that they can be acquired only by people whose receptive powers are well developed. In effect and in fact the feeble-minded person is an instance of reversion to a prehuman mental state. Judged by the human standard, every monkey is an idiot. But the reversion is not complete, for, though the imbecile loses some part of his power of profiting by experience, he regains no part of the lost power of being guided by instinct. Therefore he is correspondingly helpless as compared with a lower animal. Owing to the constitution of the human mind, some decay of the faculty of profiting by experience accompanies advancing age. But it need seldom be so great as it usually is, and never so great as it often is. Certain mental attitudes, certain systems of education, certain environments, leave the mind of the man almost as open as that of a little child; others inflict on it premature senility. An Aristotle or a Darwin learns to the last year of his long life; a Mohammedan or a Tibetan ecclesiastic is old before he has ceased to be young. Convinced that pestilence is due directly to the wrath of God, he scorns the notion that sanitation can be right or useful; believing that the earth is flat, no evidence will convince him that it is round; holding his sacred religion with a steadfast faith, he will murder the heretic rather than think out his propositions. [Sidenote: How the Minds of Men Differ]