Saints Barsanuphius the Great and John the Prophet. "Letters".
Note: English translation and letter numbering is from the translation by John Chryssavgis. The numbering in the English and Russian translations are different.
(There are 15 letters in this article; 6 more letters on discernment are in False Visions, and also there is Letter 421)
Question from a monk named Theodore to the Great Old Man: 'How can I know which thoughts are from God, which are natural and which are from the demons?" Response by Barsanuphius.
My child, Theodore, when you ask a question, you should understand what you are asking and prepare yourself for work. For it is written: "Do not be haughty, but give yourselves to humble tasks."(Rom 12.16) Your questions, brother, belong to someone with high measures. Therefore, unless the inner eye is purified by means of much sweat, you cannot be detached from thorns and prickles in order to seize the grape that strengthens and gladdens the heart. If one does not reach this measure, then one is unable to discern whether one is ridiculed and deceived by the demons in trusting them. For they transform matters as they desire, especially for those who are not familiar with their tricks. Therefore, beloved one, hope in the Lord, "and he shall give you the desires of your heart."(Ps 36.4) Cut off your own will, and in all things say to the Lord: "Not as I want, but as you want,"(Mt 26.39) and then he shall treat you in accordance with his mercy.
Child, listen to the way of discerning the three thoughts, about which you have inquired. When your thought suggests that you are doing something according to the will of God, and you find joy in the matter but also find affliction resisting you, learn that this is from God and struggle to endure, according to the words of Paul: "I punish my body and enslave it, so that after proclaiming to others I myself should not be disqualified";(1 Cor 9.27) so just fulfill the will of God.