Objection 1. It seems that truth is not a virtue. For the first of virtues is faith, whose object is truth. Since then the object precedes the habit and the act, it seems that truth is not a virtue, but something prior to virtue.
Objection 2. Further, according to the Philosopher (Ethic. iv, 7), it belongs to truth that a man should state things concerning himself to be neither more nor less than they are. But this is not always praiseworthy--neither in good things, since according to Proverbs 27:2, "Let another praise thee, and not thy own mouth"--nor even in evil things, because it is written in condemnation of certain people (Isaiah 3:9): "They have proclaimed abroad their sin as Sodom, and they have not hid it." Therefore truth is not a virtue.
Objection 3. Further, every virtue is either theological, or intellectual, or moral. Now truth is not a theological virtue, because its object is not God but temporal things. For Tully says (De Invent. Rhet. ii) that by "truth we faithfully represent things as they are were, or will be." Likewise it is not one of the intellectual virtues, but their end. Nor again is it a moral virtue, since it is not a mean between excess and deficiency, for the more one tells the truth, the better it is. Therefore truth is not a virtue.
On the contrary, The Philosopher both in the Second and in the Fourth Book of Ethics places truth among the other virtues.
I answer that, Truth can be taken in two ways. First, for that by reason of which a thing is said to be true, and thus truth is not a virtue, but the object or end of a virtue: because, taken in this way, truth is not a habit, which is the genus containing virtue, but a certain equality between the understanding or sign and the thing understood or signified, or again between a thing and its rule, as stated in I, 16, 1; I, 21, 2. Secondly, truth may stand for that by which a person says what is true, in which sense one is said to be truthful. This truth or truthfulness must needs be a virtue, because to say what is true is a good act: and virtue is "that which makes its possessor good, and renders his action good."
Reply to Objection 1. This argument takes truth in the first sense.
Reply to Objection 2. To state that which concerns oneself, in so far as it is a statement of what is true, is good generically. Yet this does not suffice for it to be an act of virtue, since it is requisite for that purpose that it should also be clothed with the due circumstances, and if these be not observed, the act will be sinful. Accordingly it is sinful to praise oneself without due cause even for that which is true: and it is also sinful to publish one's sin, by praising oneself on that account, or in any way proclaiming it uselessly.
Reply to Objection 3. A person who says what is true, utters certain signs which are in conformity with things; and such signs are either words, or external actions, or any external thing. Now such kinds of things are the subject-matter of the moral virtues alone, for the latter are concerned with the use of the external members, in so far as this use is put into effect at the command of the will. Wherefore truth is neither a theological, nor an intellectual, but a moral virtue. And it is a mean between excess and deficiency in two ways. First, on the part of the object, secondly, on the part of the act. On the part of the object, because the true essentially denotes a kind of equality, and equal is a mean between more and less. Hence for the very reason that a man says what is true about himself, he observes the mean between one that says more than the truth about himself, and one that says less than the truth. On the part of the act, to observe the mean is to tell the truth, when one ought, and as one ought. Excess consists in making known one's own affairs out of season, and deficiency in hiding them when one ought to make them known.
Source: Summa Theologica (New Advent)