Lust is the resting in the sense of enjoyment.

This has been explained again and again. Sensation, as, for example, the sense of taste, is meant to be the guide to action; in this case, the choice of wholesome food, and the avoidance of poisonous and hurtful things. But if we rest in the sense of taste, as a pleasure in itself; rest, that is, in the psychical side of taste, we fall into gluttony, and live to eat, instead of eating to live. So with the other great organic power, the power of reproduction. This lust comes into being, through resting in the sensation, and looking for pleasure from that.