City of God

Although, therefore, lust may have many objects, yet when no object is specified, the word lust usually suggests to the mind the lustful excitement of the organs of generation. And this lust not only takes possession of the whole body and outward members, but also makes itself felt within, and moves the whole man with a passion in which mental emotion is mingled with bodily appetite, so that the pleasure which results is the greatest of all bodily pleasures. So possessing indeed is this pleasure, that at the moment of time in which it is consummated, all mental activity is suspended.

What friend of wisdom and holy joys, who, being married, but knowing, as the apostle says, “how to possess his vessel in santification and honor, not in the disease of desire, as the Gentiles who know not God,” would not prefer, if this were possible, to beget children without this lust, so that in this function of begetting offspring the members created for this purpose should not be stimulated by the heat of lust, but should be actuated by his volition, in the same way as his other members serve him for their respective ends?

But even those who delight in this pleasure are not moved to it at their own will, whether they confine themselves to lawful or transgress to unlawful pleasures; but sometimes this lust importunes them in spite of themselves, and sometimes fails them when they desire to feel it, so that though lust rages in the mind, it stirs not in the body. Thus, strangely enough, this emotion not only fails to obey the legitimate desire to beget offspring, but also refuses to serve lascivious lust; and though it often opposes its whole combined energy to the soul that resists it, sometimes also it is divided against itself, and while it moves the soul, leaves the body unmoved.