By a tranquil mind I mean nothing else than a mind well ordered.

Calm is not an absence of feeling; it is an arrangement. A mind that knows what it is doing, what it has decided and what it has set aside has nothing rattling loose. Most of what passes for anxiety is unsorted material rather than genuine danger.

When you feel scattered, treat it as a filing problem before you treat it as a mood. Write down everything currently unresolved, decide for each one the next action and when, and close the list. The feeling usually changes within the hour, because the disorder was the thing you were feeling.