We may not always have the sheer physical support for mental calm. We can get very tired, and overwrought. We can lose the wholesomeness that goes with health. When this takes place, we will be likely to be anxious. Just as we say that we have mastered the air and yet we have aeronautical accidents, or that we have mastered certain diseases, and yet people die because we cannot always cure them.
All human mastery is incomplete.
Let me try to give the answer, so far as I know it, in terms that are utterly practical. There are four principles or rules, which, taken together, will give us mastery over anxiety, at least a great deal of the time. Nothing is so terrible when you face it as when you run away from it. Now, the first reality, and one which superficial sorts of religion try to mask or hide, is that of limited security. We live our physical lives within our own precarious bodies, subject to all the perils of disease, all the dangers of accident.