To the second one proceeds thus. It seems that in one human being there are many original sins. For it is said in Psalm I, For behold I was conceived in iniquity, and in sins my mother conceived me. But sin in which a human being is conceived is original. Therefore there are many original sins in one human being.
Further, one and the same habitude does not incline to contraries, for habitude inclines by way of nature, which tends to one. But original sin, even in one human being, inclines to diverse and contrary sins. Therefore original sin is not one habitude, but several.
Further, original sin infects all the parts of the soul. But the diverse parts of the soul are diverse subjects of sin, as is obvious from the foregoing. Since therefore one sin is not able to be diverse subjects, it seems that original sin is not one but many.
But contrariwise is what is said in Ioan. I, Behold the lamb of God, behold he who takes away the sin of the world [peccatum mundi]. Which is said singularly because the sin of the world, which is original sin, is one; as the Gloss explains there.
I reply that it must be said that in one human being is one original sin. The reason for this is able to be taken in two ways. In one way, on the part of the cause of original sin. For it was said above that only the first sin of the first parent is handed down to posterity. Whence original sin in one human being is one in number; and in all human beings it is proportioned to one, to wit, in reference to the first source [primum principium]. In another way the reason for this is able to be taken from the essence itself of original sin. For in every disordered [inordinata] disposition, specific unity is considered on the part of the cause, but numerical unity on the part of the subject, as is obvious in corporeal illness, for there are diverse specific illnesses which proceed from diverse causes, such as superabundance of heat or cold, or from lesion of lung or liver, but one illness according to species in one human being is only one numerically. But the cause of this corrupt disposition that is called original sin is just one, namely, privation of original justice, through which subjection of the human mind to God is suppressed. And in one human being it cannot but be one, but in diverse human being it is specifically and proporationately one, but numerically diverse.
To the first, therefore, it must be said that sins are said plurally according to that custom of divine Scripture in which the plural is frequently put for the singular, as in Matth. II, They died who sought the soul of the child. Or else because in original sin all actual sins pre-exist, as in a kind of source, when it is multiple in force. Or else because in the sin of the first parent, which is handed down by origin, there were several deformities, to wit, pride, disobedience, gluttony, and suchlike other things. Or else because many parts of the soul are infected by original sin.
To the second it must be said that one habitude is not able through itself [per se] and directly, that is, through its own form, to contraries. But indirectly and incidentally [per accidens], to wit, through removing the impeding [per remotione prohibentis] nothing impedes, just as, the harmony of mixed body being dissolved, elements tend to contrary places. And likewise, the harmony of original justice being dissolved, diverse powers of soul are impelled diversely.
To the third it must be said that original sin infects diverse parts of the soul, according as they are parts of one whole, just as original justice is contained in all parts of the soul in one. And thus there is but one original sin, just as there is one fever in one human being although diverse parts of the body are affected.
[St. Thomas Aquinas, ST 2-1.82.2, my translation. The Latin is here, the Dominican Fathers translation is here.]