Many a man, finding himself with certain muscles flabby and powerless takes that as his natural condition, and regards their weakness as a kind of destiny imposed upon him; but anyone who understands a little of the human body is aware that by continued exercise those muscles can be brought into a state of health and the whole body eventually put in order.
In exactly the same way, many a man finds himself possessed of a bad temper or a tendency to avarice or suspicion or self-indulgence, and when in consequence of any of these vices he commits some great mistake or does some great harm he offers it as an excuse that he is a hasty-tempered man, or that he possesses this or that quality by nature--implying that therefore he cannot help it.
In this case just as in the other the remedy is in his own hands.

Regular exercise of the right kind will develop a certain muscle, and regular mental exercise of the right kind will develop a missing quality in a man's character.

The ordinary man does not realize that he can do this, and even if he sees that he can do it, he does not see why he should, for it means much effort and much self-repression.

He knows of no adequate motive for undertaking a task so laborious and painful.
The motive is supplied by the knowledge of the truth.

One who gains an intelligent comprehension of the direction of evolution feels it not only his interest but his privilege and his delight to co-operate with it.