Q: Can the devil know your thoughts, and tempt you when he sees weakness?
A: Satan cannot act directly on our higher faculties of the soul– the intellect and the will. Even with diabolical possession, Satan can control the body, but the soul remains in suspended animation. Nevertheless, he can act indirectly, meaning he tempts a person to consent to commit sin. A person remains free to give or refuse consent, to reject or fall to temptation. A person must also keep in mind that the Lord will not allow us to be tempted beyond our strength; with God’s grace, a person can always defeat the Devil.
However, Satan can act directly on the body, on our senses, and on the imagination and the memory. So, Satan can observe our behavior and prey upon the weaknesses due to Original Sin (concupiscence)– the eyes (avarice and curiosity), the flesh (passions), and pride. He can afflict our imagination and even dreams; while a person has no control over dreams and would not be culpable for anything dreamed, a person must recognize and reject promptly anything in the imagination that, if accepted and entertained, would be sinful.