"Because of the workings of the memory whatever has preoccupied our mind before the time for prayer must of necessity intrude on our actual prayers. Therefore in advance of prayer we must strive to dispose ourselves as we would wish to be during prayer. The praying spirit is shaped by its earlier condition. As we prostrate ourselves for prayer our deeds, words, fastings rise up in our imagination. They are as they were before our prayer, and they move us to anger or gloom. We turn back toward desire or worldly affairs. Stupidly - and I am ashamed to say it - we laugh as we recall some clownish act or word, and the mind flits back to the earlier concerns of our talk. So therefore before we pray we must hasten to drive from our heart’s sanctuary anything we would not wish to intrude on our prayers, and all this so that we might do as the apostle bids us: ‘Pray ceaselessly’ (I Thes. 5:17). ‘In every place lift up pure hands, with no anger and no rivalry’ (I Tim. 2:8). But we will not be able to fulfill this injunction unless the mind within us is cleansed of the contagion of sin, is devoted to virtue as its natural good, and feeds continuously on the contemplation of the all-powerful God."
— St. John Cassian
