"A brother asked one of the Fathers, "What shall I do? My thoughts are always turned to lust without allowing me an hour's respite, and my soul is tormented by it." He said to him, "Every time the demons suggest these thoughts to you, do not argue with them. For the activity of demons always is to suggest, and suggestions are not sins, for they cannot compel; but it rests with you to welcome them, or not to welcome them. Do you know what the Midianites did? They adorned their daughters and presented them to the Israelites. They did not compel anyone, but those who consented, sinned with them, while the others were enraged and put them to death. It is the same with thoughts."
The brother answered the old man, "What shall I do, then, for I am weak and passion overcomes me?" He said to him, "Watch your thoughts, and every time they begin to say something to you, do not answer them but rise and pray; kneel down, saying, 'Son of God, have mercy on me.'" - Anonymous Desert Father