ABOVE all we ought at least to know that there are three origins of our thoughts, i.e., from God, from the devil, and from ourselves. They come from God when He vouchsafes to visit us with the illumination of the Holy Ghost, lifting us up to a higher state of progress, and where we have made but little progress, or through acting slothfully have been overcome, He chastens us with most salutary compunction, or when He discloses to us heavenly mysteries, or turns our purpose and will to better actions, as in the case where the king Ahasuerus, being chastened by the Lord, was prompted to ask for the books of the annals, by which he was reminded of the good deeds of Mordecai, and promoted him to a position of the highest honour and at once recalled his most cruel sentence concerning the slaughter of the Jews. Or when the prophet says: "I will hearken what the Lord God will say in me." Another too tells us "And an angel spoke, and said in me," or when the Son of God promised that He would come with His Father, and make His abode in us, and "It is not ye that speak, but the Spirit of your Father which speaketh in you." And the chosen vessel: "Ye seek a proof of Christ that speaketh in me." But a whole range of thoughts springs from the devil, when he endeavours to destroy us either by the pleasures of sin or by secret attacks, in his crafty wiles deceitfully showing us evil as good, and transforming himself into an angel of light to us: as when the evangelist tells us: "And when supper was ended, when the devil had already put it into the heart of Judas Iscariot, Simon's son, to betray" the Lord: and again also "after the sop," he says, "Satan entered into him." Peter also says to Ananias: "Why hath Satan tempted thine heart, to lie to the Holy Ghost?" And that which we read in the gospel much earlier as predicted by Ecclesiastes: "If the spirit of the ruler rise up against thee, leave not thy place." That too which is said to God against Ahab in the third book of Kings, in the character of an unclean spirit: "I will go forth and will be a lying spirit in the mouth of all his prophets." But they arise from ourselves, when in the course of nature we recollect what we are doing or have done or have heard. Of which the blessed David speaks: "I thought upon the ancient days, and had in mind the years from of old, and I meditated, by night I exercised myself with my heart, and searched out my spirit." And again: "the Lord knoweth the thoughts of man, that they are vain:" and "the thoughts of the righteous are judgments." In the gospel too the Lord says to the Pharisees: "why do ye think evil in your hearts?"
From the Conferences of John Cassian