Those who are struggling in battle ought always to keep their souls free of the tumultuous waves of distraction. If they do this, the mind will be able to distinguish among the thoughts that come to it. The good thoughts, sent by God, they can store in the treasure-house of their memory. The evil thoughts, sent by the devil, they can throw out.
–Saint Diadochus of Photike (c. 400 – c. 486)