Everything takes its species from its form: and it has been stated (2) that the species of original sin is taken from its cause. Consequently the formal element of original sin must be considered in respect of the cause of original sin. But contraries have contrary causes. Therefore the cause of original sin must be considered with respect to the cause of original justice, which is opposed to it. Now the whole order of original justice consists in man's will being subject to God: which subjection, first and chiefly, was in the will, whose function it is to move all the other parts to the end, as stated above (Question 9, Article 1), so that the will being turned away from God, all the other powers of the soul become inordinate. Accordingly the privation of original justice, whereby the will was made subject to God, is the formal element in original sin; while every other disorder of the soul's powers, is a kind of material element in respect of original sin. Now the inordinateness of the other powers of the soul consists chiefly in their turning inordinately to mutable good; which inordinateness may be called by the general name of concupiscence. Hence original sin is concupiscence, materially, but privation of original justice, formally.
St. Thomas Aquinas, ST 2-1.82.3
