“Out of the heart,” he said, “come evil thoughts.” Therefore the soul or principle of action is not in the brain according to Plato but devil wispersin the heart according to Christ. On this point, those who believe that thoughts are introduced by the devil and do not originate from our own will are to be repudiated. The devil can aid and abet evil thoughts but he cannot originate them, even though, ever lying in wait, he kindles a small spark of our thoughts with his tinder. We must not hold the opinion that the devil can also probe the depths of our heart. However, he can judge from our demeanor and gestures what we are thinking about. For example, if he sees us gazing often at a beautiful woman, he surmises that our heart has been wounded with the dart of love.

St. Jerome