Those who maintain that concupiscence is not sin, are wont to found on the passage of James, “Then, when lust has conceived, it bringeth forth sin,” (James 1:15). But this is easily refuted: for unless we understand him as speaking only of wicked works or actual sins, even a wicked inclination will not be accounted sin. But from his calling crimes and wicked deeds the fruits of lust, and also giving them the name of sins, it does not follow that the lust itself is not an evil, and in the sight of God deserving of condemnation.
