Thomas Aquinas

“ Through the bond of original justice being broken, which held together all the powers of the soul in a certain order, each power of the soul tends to its own proper movement, and the more impetuously, as it is stronger. Now it happens that some of the soul's powers are stronger in one man than in another, on account of the different bodily temperaments. Consequently if one man is more prone than another to acts of concupiscence, this is not due to original sin, because the bond of original justice is equally broken in all, and the lower parts of the soul are, in all, left to themselves equally ”

“ Since, in man, the concupiscible power is naturally governed by reason, the act of concupiscence is so far natural to man, as it is in accord with the order of reason; while, in so far as it trespasses beyond the bounds of reason, it is, for a man, contrary to reason. Such is the concupiscence of original sin. ”

“ In this way original sin is not a habit. The second kind of habit is the disposition of a complex nature, whereby that nature is well or ill disposed to something, chiefly when such a disposition has become like a second nature, as in the case of sickness or health. In this sense original sin is a habit. For it is an inordinate disposition, arising from the destruction of the harmony which was essential to original justice, even as bodily sickness is an inordinate disposition of the body, by reason of the destruction of that equilibrium which is essential to health. ”