Objection 1. It seems that truth is not a special virtue. For the true and the good are convertible. Now goodness is not a special virtue, in fact every virtue is goodness, because "it makes its possessor good." Therefore truth is not a special virtue.
Objection 2. Further, to make known what belongs to oneself is an act of truth as we understand it here. But this belongs to every virtue, since every virtuous habit is made known by its own act. Therefore truth is not a special virtue.
Objection 3. Further, the truth of life is the truth whereby one lives aright, and of which it is written (Isaiah 38:3): "I beseech Thee... remember how I have walked before Thee in truth, and with a perfect heart." Now one lives aright by any virtue, as follows from the definition of virtue given above (I-II, 55, 4). Therefore truth is not a special virtue.
Objection 4. Further, truth seems to be the same as simplicity, since hypocrisy is opposed to both. But simplicity is not a special virtue, since it rectifies the intention, and that is required in every virtue. Therefore neither is truth a special virtue.
On the contrary, It is numbered together with other virtues (Ethic. ii, 7).
I answer that, The nature of human virtue consists in making a man's deed good. Consequently whenever we find a special aspect of goodness in human acts, it is necessary that man be disposed thereto by a special virtue. And since according to Augustine (De Nat. Boni iii) good consists in order, it follows that a special aspect of good will be found where there is a special order. Now there is a special order whereby our externals, whether words or deeds, are duly ordered in relation to some thing, as sign to thing signified: and thereto man is perfected by the virtue of truth. Wherefore it is evident that truth is a special virtue.
Reply to Objection 1. The true and the good are convertible as to subject, since every true thing is good, and every good thing is true. But considered logically, they exceed one another, even as the intellect and will exceed one another. For the intellect understands the will and many things besides, and the will desires things pertaining to the intellect, and many others. Wherefore the "true" considered in its proper aspect as a perfection of the intellect is a particular good, since it is something appetible: and in like manner the "good" considered in its proper aspect as the end of the appetite is something true, since it is something intelligible. Therefore since virtue includes the aspect of goodness, it is possible for truth to be a special virtue, just as the "true" is a special good; yet it is not possible for goodness to be a special virtue, since rather, considered logically, it is the genus of virtue.
Reply to Objection 2. The habits of virtue and vice take their species from what is directly intended, and not from that which is accidental and beside the intention. Now that a man states that which concerns himself, belongs to the virtue of truth, as something directly intended: although it may belong to other virtues consequently and beside his principal intention. For the brave man intends to act bravely: and that he shows his fortitude by acting bravely is a consequence beside his principal intention.
Reply to Objection 3. The truth of life is the truth whereby a thing is true, not whereby a person says what is true. Life like anything else is said to be true, from the fact that it attains its rule and measure, namely, the divine law; since rectitude of life depends on conformity to that law. This truth or rectitude is common to every virtue.
Reply to Objection 4. Simplicity is so called from its opposition to duplicity, whereby, to wit, a man shows one thing outwardly while having another in his heart: so that simplicity pertains to this virtue. And it rectifies the intention, not indeed directly (since this belongs to every virtue), but by excluding duplicity, whereby a man pretends one thing and intends another.
Source: Summa Theologica (New Advent)