원죄
Involuntary sexual arousal is explored in the "Confessions" of Augustine, wherein he used the term "concupiscence" to refer to sinful lust. He taught that Adam's sin is transmitted by concupiscence, or "hurtful desire", resulting in humanity becoming a "massa damnata" (mass of perdition, condemned crowd), with much enfeebled, though not destroyed, freedom of will. When Adam sinned, human nature was thenceforth transformed. Adam and Eve, via sexual reproduction, recreated human nature. Their descendants now live in sin, in the form of concupiscence, a term Augustine used in a metaphysical, not a psychological sense. Augustine insisted that concupiscence was not "a being" but a "bad quality", the privation of good or a wound. He admitted that sexual concupiscence ("libido") might have been present in the perfect human nature in paradise, and that only later it became disobedient to human will as a result of the first couple's disobedience to God's will in the original sin. In Augustine's view (termed "Realism"), all of humanity was really present in Adam when he sinned, and therefore all have sinned. Original sin, according to Augustine, consists of the guilt of Adam which all humans inherit.