This commandment forbids not only acts of impurity, but sensual thoughts and desires, or any practice that tends to excite them. Purity is demanded not only in the outward life but in the secret intents and emotions of the heart. Christ, who taught the far-reaching obligation of the law of God, declared the evil thought or look to be as truly sin as the unlawful deed. (Patriarchs and Prophets, 308)
When the thought of evil is loved and cherished, however, secretly, said Jesus, it shows that sin still reigns in the heart. The soul is still in the gall of bitterness and in the bond in iniquity. He who finds pleasure in dwelling upon scenes of impurity, who indulges the evil thought, the lustful look, may behold in the open sin, with its burden of shame and heart-breaking grief, the true nature of the evil which he has hidden in the chambers of the soul. The season of temptation, under which, it may be, one falls into grievous sin, does not create the evil that is revealed, but only develops or makes manifest that which was hidden and latent in the heart. As a man "thinketh in his heart, so is he;" for out of the heart "are the issues of life" (Proverbs 23:7; 4:23). (Thoughts from the Mount of Blessing, 60)
Reflection: The devil will put thoughts into our minds but we don't have to dwell on them. We need to be like Jesus and say, "Get behind me Satan." When these sinful thoughts come into the mind, think of something else. Singing a Christian song, even if sung in the mind and not out loud, will change your thoughts and feelings.