An objection, as to how we ought to believe that devils see into the thoughts of men.
Germanus: In this way, which you describe, those spirits cannot possibly see into our thoughts. But we think it utterly absurd to hold such an opinion, when Scripture says: “If the spirit of him that has power ascend upon you;” and again: “When the devil had put it into the heart of Simon Iscariot to betray the Lord.” How then can we believe that our thoughts are not open to them, when we feel that for the most part they spring up and are nursed by their suggestions and instigation?
Source: Conferences (New Advent)