To reach the right meaning of this we must begin with physiology. All impressions of the nervous system, that is to say, all feelings, may be compared or studied with reference to three criteria, their Quality, their Intensity, and their Persistence. Feelings of the same quality, as a rule, heighten each other’s intensity, but persistence is usually inversely to intensity. The keener the sensation, the shorter its duration. The story is told of a French scholar who, for suspected heresy, was subjected to judicial torture on the rack. When the instrument was extended the first time, dislocating several of his joints, he uttered a cry of agony; but at the second extension he burst into laughter. “At my own ignorance,” he explained, “to suppose that I could feel such suffering twice.” It is essential to anything like a constant flow of pleasurable feelings that we maintain a high state of vigor in the organs of sensibility; and this can only be accomplished by a careful limitation of intensity in favor of persistence of feeling. Occasional nervous impressions of a very high degree of intensity are not only consistent with health, but increase it; but their frequent repetition, and especially the determined effort to maintain them for long periods, inevitably result in a deadening of the sensibility and a lack of response to ordinary and healthful stimuli. The ignorance or disregard of these physiological laws explains some of the most disastrous and conspicuous failures to attain happiness where every circumstance seems propitious. The neglect of them is the origin of that morbid condition of the mind which has been called “the disease of the century,” _la maladie de la siècle_,――Ennui. The bitter pessimist, Schopenhauer, delighted to show the worthlessness of life, whose only variety is from the toil of pursuit to the ennui of possession; while the sweet mystic, Pascal, discovering in the same feeling the greatest misery of man, saw in it that which would prove his salvation, for it would lead him to renounce the vanities of the world and give himself unto God. The one opinion is worth as much as the other. If we make an anatomy of Ennui, as Burton made an anatomy of Melancholy, we shall find that two different, though allied, mental conditions have been grouped under the name. The one is that sense of immeasurable boredom which we feel when placed in uncongenial conditions, especially such as ought to be welcome to us, as listening to good advice, or hearing instructive lectures, or reading useful books,――like this one. We are driven to any revolt by such inflictions. The scholar will turn gypsy and the virtuous youth a vagrant to escape them. As a boy, at stiff company dinners, I used to suffer from a keen desire to throw a plate through the window, or commit some other outrageous breach of decorum.