Elder Fr. Leonid answering questions from Alexey Polikarpovich Bochkov, later Antony, Hegumen of the Cheremenetsky monastery.
6. How do we know which thoughts are actually ours and which are from the devil?
I was told by a hermit of Konevets that while he practiced the noetic prayer for a long time, he could not distinguish his own thoughts from ones of the enemy. The enemy when drawing your thought to something, does not say to you, ‘Go, do this and that,’ but as if thinks for you, and tells you with your thought, ‘I want to do this and that; I believe that this is useful, and this is harmful; I have decided to do this and that.’ And all these things are often not yours, but the thoughts of the enemy, covered by your or his self. You think they are your thoughts. No, you are only listening to the enemy's suggestions [Those who heed their salvation should remember this carefully].