Weakness of the brain produces heat. In such a condition, however nice, however good a person may be, he is at one moment very hot-tempered, then cold as snow, then hot again. To worry very much, to repent very much, to sorrow very much, to think that life is a great burden upon us, to make a great thing out of every small event, of every small responsibility - it all weakens the brain. Normal deeds and actions and normal rest strengthen the brain.