Objection 1. It would seem that original sin is not concupiscence. For every sin is contrary to nature, according to Damascene (De Fide Orth. ii, 4,30). But concupiscence is in accordance with nature, since it is the proper act of the concupiscible faculty which is a natural power. Therefore concupiscence is not original sin.
Objection 2. Further, through original sin "the passions of sins" are in us, according to the Apostle (Romans 7:5). Now there are several other passions besides concupiscence, as stated above (Question 23, Article 4). Therefore original sin is not concupiscence any more than another passion.
Objection 3. Further, by original sin, all the parts of the soul are disordered, as stated above (2, Objection 3). But the intellect is the highest of the soul's parts, as the Philosopher states (Ethic. x, 7). Therefore original sin is ignorance rather than concupiscence.
On the contrary, Augustine says (Retract. i, 15): "Concupiscence is the guilt of original sin."
I answer that, 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.
Reply to Objection 1. 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.
Reply to Objection 2. As stated above (Question 25, Article 1), all the irascible passions are reducible to concupiscible passions, as holding the principle place: and of these, concupiscence is the most impetuous in moving, and is felt most, as stated above (25, 2, ad 1). Therefore original sin is ascribed to concupiscence, as being the chief passion, and as including all the others, in a fashion.
Reply to Objection 3. As, in good things, the intellect and reason stand first, so conversely in evil things, the lower part of the soul is found to take precedence, for it clouds and draws the reason, as stated above (77, A1,2; 80, 2). Hence original sin is called concupiscence rather than ignorance, although ignorance is comprised among the material defects of original sin.
Source: Summa Theologica (New Advent)