Germanos then asked: 'How does it happen that even against our will many wicked ideas and thoughts trouble us, entering by stealth and undetected to steal our attention? Not only are we unable to prevent them from entering, but it is extremely difficult even to recognize them. Is it possible for the mind to be completely free of them and not be troubled by them at all?'
Abba Moses replied: 'It is impossible for the mind not to be troubled by these thoughts. But if we exert ourselves it is within our power either to accept them and give them our attention, or to expel them. Their coming is not within our power to contro, but their expulsion is. The amending of our mind is also within the power of our choice and effort. When we meditate wisely and continually on the law of God, study psalms and canticles, engage in fasting and vigils, and always bear in mind what is to come - the kingdom of heaven, the Gehenna of fire and all God's works - our wicked thoughts diminish and find no place. But when we devote our time to worldly concerns and to matters of the flesh, to pointless and useless conversation, then these base thoughts multiply in us. Just as it is impossible to stop a watermill from turning, although the miller has power to choose between grinding either wheat or tares, so it is impossible to stop our mind, which is ever-moving, from having thoughts, although it is within our power to feed it either with spiritual meditation or with worldly concerns.'
-St. John Cassian, On the Holy Fathers of Sketis (4th century)