Desire is the mood we have to do something, to ask for something is a movement of the heart, while thought moves in the area of the intellect.
First we desire and after this desire it is expressed internally through thoughts. Desire is the beginning, the root. By cutting off sinful desires we are essentially freeing ourselves from sinful thoughts.
This is why our Lord emphasized: "But I tell you that anyone who looks at a woman with desire has already committed adultery with her in his heart" (Matt. 5:28). He wanted to cut off the root of the passions.
Saint Gregory Palamas says that the intellect of the believer who struggles in prayer is easily purified of thoughts, just like the heart of a person (which is the power that produces thoughts), cannot be purified unless all the other powers of the soul, the appetitive and spirited, are purified.
Elder Ephraim of Vatopaidi
