The rule of honest nature is that enjoyment should steadily increase up to the full maturity of the powers, mental and physical. This, under favorable circumstances, is between forty-five and fifty years of age. After that, physical decadence sets in, and only by exceptional strength or by increased effort can its fatal progress be for a while stayed. From inquiry of many persons, I am persuaded that the rule of the increase of enjoyment up to this turning-point is on the average correct. That old age is synonymous with wisdom is a comical deception which the graybeards have palmed off on the world, because by laws and customs they hold most of the property, and want most of the power as well. In fact, diminution of the physical powers means decay all round. “As we grow old,” observes Thoreau, “we cease to obey our finer instincts.” It is an error to talk of the accumulated wisdom of years. The experience of youth serves but to lead old age astray; and this is seen nowhere so plainly as when an old man pretends a zest for the pleasures of the young. “No fool like an old fool” is the proverb. Such men are “out of their class,” as a trainer of athletes would say. Every age has pleasures sufficient, which are appropriate to it, and these alone should be sought for. To those who know and respect these laws of nature, old age is very tolerable. It brings many compensations for its inevitable losses; and though not likely to be so happy as the best of middle life, it should be and often is superior in this respect to youth. Probably it would generally be so were it more willing to learn the lessons appropriate to it. Bonstetten goes so far as to say, “No one can be happy till he is past sixty;” but the proverb of the ancient Rabbis holds forever good,――“He who teacheth the old, is like one who writes on blotted paper.” V. _Principles of a Self-Education for the Promotion of One’s Own Happiness._ The purpose of this chapter is not to enter into practical details of an education for happiness――that is the mission of the rest of the book,――but to establish certain general theoretical principles on which such an education must be carried out in order to be successful. And first, I must repeat what I have already intimated, that any permanent and even moderate stock of happiness does not fall into the open mouth, like the roasted quails in the fairy story, but can be obtained only by methodical pursuit and constant watchfulness. Eternal vigilance is said to be the price of liberty; and liberty is only one of the necessary elements of happiness. Meditation, forethought, and the formation of a clearly defined Plan of Life are all required, and even then success is far from certain.