Objection 1. It would seem that the above four virtues are not diverse and distinct from one another. For Gregory says (Moral. xxii, 1): "There is no true prudence, unless it be just, temperate and brave; no perfect temperance, that is not brave, just and prudent; no sound fortitude, that is not prudent, temperate and just; no real justice, without prudence, fortitude and temperance." But this would not be so, if the above virtues were distinct from one another: since the different species of one genus do not qualify one another. Therefore the aforesaid virtues are not distinct from one another.
Objection 2. Further, among things distinct from one another the function of one is not attributed to another. But the function of temperance is attributed to fortitude: for Ambrose says (De Offic. xxxvi): "Rightly do we call it fortitude, when a man conquers himself, and is not weakened and bent by any enticement." And of temperance he says (De Offic. xliii, xlv) that it "safeguards the manner and order in all things that we decide to do and say." Therefore it seems that these virtues are not distinct from one another.
Objection 3. Further, the Philosopher says (Ethic. ii, 4) that the necessary conditions of virtue are first of all "that a man should have knowledge; secondly, that he should exercise choice for a particular end; thirdly, that he should possess the habit and act with firmness and steadfastness." But the first of these seems to belong to prudence which is rectitude of reason in things to be done; the second, i.e. choice, belongs to temperance, whereby a man, holding his passions on the curb, acts, not from passion but from choice; the third, that a man should act for the sake of a due end, implies a certain rectitude, which seemingly belongs to justice; while the last, viz. firmness and steadfastness, belongs to fortitude. Therefore each of these virtues is general in comparison to other virtues. Therefore they are not distinct from one another.
On the contrary, Augustine says (De Moribus Eccl. xi) that "there are four virtues, corresponding to the various emotions of love," and he applies this to the four virtues mentioned above. Therefore the same four virtues are distinct from one another.
I answer that, As stated above (Article 3), these four virtues are understood differently by various writers. For some take them as signifying certain general conditions of the human mind, to be found in all the virtues: so that, to wit, prudence is merely a certain rectitude of discretion in any actions or matters whatever; justice, a certain rectitude of the mind, whereby a man does what he ought in any matters; temperance, a disposition of the mind, moderating any passions or operations, so as to keep them within bounds; and fortitude, a disposition whereby the soul is strengthened for that which is in accord with reason, against any assaults of the passions, or the toil involved by any operations. To distinguish these four virtues in this way does not imply that justice, temperance and fortitude are distinct virtuous habits: because it is fitting that every moral virtue, from the fact that it is a "habit," should be accompanied by a certain firmness so as not to be moved by its contrary: and this, we have said, belongs to fortitude. Moreover, inasmuch as it is a "virtue," it is directed to good which involves the notion of right and due; and this, we have said, belongs to justice. Again, owing to the fact that it is a "moral virtue" partaking of reason, it observes the mode of reason in all things, and does not exceed its bounds, which has been stated to belong to temperance. It is only in the point of having discretion, which we ascribed to prudence, that there seems to be a distinction from the other three, inasmuch as discretion belongs essentially to reason; whereas the other three imply a certain share of reason by way of a kind of application (of reason) to passions or operations. According to the above explanation, then, prudence would be distinct from the other three virtues: but these would not be distinct from one another; for it is evident that one and the same virtue is both habit, and virtue, and moral virtue.
Others, however, with better reason, take these four virtues, according as they have their special determinate matter; each of its own matter, in which special commendation is given to that general condition from which the virtue's name is taken as stated above (Article 3). In this way it is clear that the aforesaid virtues are distinct habits, differentiated in respect of their diverse objects.
Reply to Objection 1. Gregory is speaking of these four virtues in the first sense given above. It may also be said that these four virtues qualify one another by a kind of overflow. For the qualities of prudence overflow on to the other virtues in so far as they are directed by prudence. And each of the others overflows on to the rest, for the reason that whoever can do what is harder, can do what is less difficult. Wherefore whoever can curb his desires for the pleasures of touch, so that they keep within bounds, which is a very hard thing to do, for this very reason is more able to check his daring in dangers of death, so as not to go too far, which is much easier; and in this sense fortitude is said to be temperate. Again, temperance is said to be brave, by reason of fortitude overflowing into temperance: in so far, to wit, as he whose mind is strengthened by fortitude against dangers of death, which is a matter of very great difficulty, is more able to remain firm against the onslaught of pleasures; for as Cicero says (De Offic. i), "it would be inconsistent for a man to be unbroken by fear, and yet vanquished by cupidity; or that he should be conquered by lust, after showing himself to be unconquered by toil."
From this the Reply to the Second Objection is clear. For temperance observes the mean in all things, and fortitude keeps the mind unbent by the enticements of pleasures, either in so far as these virtues are taken to denote certain general conditions of virtue, or in the sense that they overflow on to one another, as explained above.
Reply to Objection 3. These four general conditions of virtue set down by the Philosopher, are not proper to the aforesaid virtues. They may, however, be appropriated to them, in the way above stated.
Source: Summa Theologica (New Advent)