Objection 1. It would seem that virtue is in us by nature. For Damascene says (De Fide Orth. iii, 14): "Virtues are natural to us and are equally in all of us." And Antony says in his sermon to the monks: "If the will contradicts nature it is perverse, if it follow nature it is virtuous." Moreover, a gloss on Matthew 4:23, "Jesus went about," etc., says: "He taught them natural virtues, i.e. chastity, justice, humility, which man possesses naturally."
Objection 2. Further, the virtuous good consists in accord with reason, as was clearly shown above (55, 4, ad 2). But that which accords with reason is natural to man; since reason is part of man's nature. Therefore virtue is in man by nature.
Objection 3. Further, that which is in us from birth is said to be natural to us. Now virtues are in some from birth: for it is written (Job 31:18): "From my infancy mercy grew up with me; and it came out with me from my mother's womb." Therefore virtue is in man by nature.
On the contrary, Whatever is in man by nature is common to all men, and is not taken away by sin, since even in the demons natural gifts remain, as Dionysius states (Div. Nom. iv). But virtue is not in all men; and is cast out by sin. Therefore it is not in man by nature.
I answer that, With regard to corporeal forms, it has been maintained by some that they are wholly from within, by those, for instance, who upheld the theory of "latent forms" [Anaxagoras; Cf. I, 45, 8; 65, 4]. Others held that forms are entirely from without, those, for instance, who thought that corporeal forms originated from some separate cause. Others, however, esteemed that they are partly from within, in so far as they pre-exist potentially in matter; and partly from without, in so far as they are brought into act by the agent.
In like manner with regard to sciences and virtues, some held that they are wholly from within, so that all virtues and sciences would pre-exist in the soul naturally, but that the hindrances to science and virtue, which are due to the soul being weighed down by the body, are removed by study and practice, even as iron is made bright by being polished. This was the opinion of the Platonists. Others said that they are wholly from without, being due to the inflow of the active intellect, as Avicenna maintained. Others said that sciences and virtues are within us by nature, so far as we are adapted to them, but not in their perfection: this is the teaching of the Philosopher (Ethic. ii, 1), and is nearer the truth.
To make this clear, it must be observed that there are two ways in which something is said to be natural to a man; one is according to his specific nature, the other according to his individual nature. And, since each thing derives its species from its form, and its individuation from matter, and, again, since man's form is his rational soul, while his matter is his body, whatever belongs to him in respect of his rational soul, is natural to him in respect of his specific nature; while whatever belongs to him in respect of the particular temperament of his body, is natural to him in respect of his individual nature. For whatever is natural to man in respect of his body, considered as part of his species, is to be referred, in a way, to the soul, in so far as this particular body is adapted to this particular soul.
In both these ways virtue is natural to man inchoatively. This is so in respect of the specific nature, in so far as in man's reason are to be found instilled by nature certain naturally known principles of both knowledge and action, which are the nurseries of intellectual and moral virtues, and in so far as there is in the will a natural appetite for good in accordance with reason. Again, this is so in respect of the individual nature, in so far as by reason of a disposition in the body, some are disposed either well or ill to certain virtues: because, to wit, certain sensitive powers are acts of certain parts of the body, according to the disposition of which these powers are helped or hindered in the exercise of their acts, and, in consequence, the rational powers also, which the aforesaid sensitive powers assist. In this way one man has a natural aptitude for science, another for fortitude, another for temperance: and in these ways, both intellectual and moral virtues are in us by way of a natural aptitude, inchoatively, but not perfectly, since nature is determined to one, while the perfection of these virtues does not depend on one particular mode of action, but on various modes, in respect of the various matters, which constitute the sphere of virtue's action, and according to various circumstances.
It is therefore evident that all virtues are in us by nature, according to aptitude and inchoation, but not according to perfection, except the theological virtues, which are entirely from without.
This suffices for the Replies to the Objections. For the first two argue about the nurseries of virtue which are in us by nature, inasmuch as we are rational beings. The third objection must be taken in the sense that, owing to the natural disposition which the body has from birth, one has an aptitude for pity, another for living temperately, another for some other virtue.
Source: Summa Theologica (New Advent)