Health is the balance between physical life and external nature, and it is maintained only by sufficient vitality on the inside against things on the outside. Everything outside my physical life is designed to put me to death. Things which keep me going when I am alive, disintegrate me when I am dead. If I have enough fighting power, I produce the balance of health. The same is true of the mental life. If I want to maintain a vigorous mental life, I have to fight, and in that way the mental balance called thought is produced.
Oswald Chambers