Truthfulness, or veracity, is a virtue, allied to justice, by which its possessor is inclined to manifest himself not otherwise than he is. Plato (Rep. 381) recognized it as a divine characteristic, and he taunted polytheists with the question: "… can you imagine that God will be willing to lie, whether in word or deed, to put forth a phantom of himself?" Aristotle, after briefly portraying the habitually truthful person, is unequivocal in his evaluation of him. He says: "The man who loves truth, and is truthful where nothing is at stake will still more be truthful where something is at stake; he will avoid falsehood as something base, seeing that he avoided it even for its own sake; and such a man is worthy of praise." (Eth. Nic. 1127.) Praise for the truthful person has been unstinting and constant in Christian tradition, both by condemnations of its opposite, lying, and by its association, in a theological context, with the most exalted of moral virtues, justice.
In other than a moral context, truth is a relationship, a formal identity or conformity, between what is in the mind and reality that exists apart from the mind. If the relationship is considered as emanating from an intellect, primarily divine, to the thing, then the truth is called ontological or metaphysical. If it is viewed from the thing to the intellect, so that the intellect is formed according to reality, then it is called logical. Truth, in a moral sense, exists where there is conformity between one's thought and one's speech. This is subject to voluntary control. Because there are various ways in which that conformity may be distorted, there is need for habituation of the will to a proper standard of conformity. That habituation is a virtue, the special virtue truthfulness or veracity.
The good action, which distinguishes the virtue of truthfulness from all others, is one "whereby a man, both in life and in speech, shows himself to be such as he is, and other things not differently than they are in his regard, and neither greater nor less, than they are." (St. Thomas Aquinas, Summa theologiae, 2a2ae, 109.3 ad 3.)
The basic reason that a person is made good by being truthful is that he finds his own fulfillment in fulfilling his social responsibility. "Since man is a social animal, one man naturally owes another whatever is necessary for the preservation of human society. Now it would be impossible for men to live together, unless they believed one another, as declaring the truth one to another. Hence the virtue of truth does in some sense regard the truth as something owed." (Summa theologiae, 2a2ae, 109.3 ad1.)
Bibliography: thomas aquinas, Summa theologiae, 2a2ae, 109. j. a. mchugh and c. j. callan, Moral Theology, 2 v. (New York 1958) 2:436–438. r. middleton, "The Obligation of Veracity," American Ecclesiastical Review 19 (1898) 163–173.
"Truthfulness (Veracity)." New Catholic Encyclopedia. . Encyclopedia.com. (August 18, 2018). http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/truthfulness-veracity
"Truthfulness (Veracity)." New Catholic Encyclopedia. . Retrieved August 18, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/truthfulness-veracity