Aquinas starts here when he discusses the virtue of truth, and, in fact, gives Aristotle's definition for truthfulness; distinguishing it from other senses of truth, he says (in ST 2-2.109) that it is "that truth whereby a man, both in life and in speech, shows himself to be such as he is, and the things that concern him, not other, and neither greater nor less, than they are". But he uses this as a leverage-point to fit more into the concept of truthfulness than Aristotle ever explicitly does. The way he does this is by recognizing that what we know and believe is part of who we are. So if we are going to be truthful about ourselves, truthfulness about our beliefs and knowledge. For the same reason Aristotle suggested, "to say what is true is a good act," but truthfulness about oneself necessarily requires that we be truthful about the world as we know it to be. This truthfulness gives us a simplicity, in a positive sense, because it excludes duplicity, "whereby a man pretends one thing and intends another." Aquinas's virtue of truthfulness is annexed to, or a potential part of justice, "through having something in common with justice, while falling short from the complete virtue thereof." Like justice, truthfulness is directed to others and involves a kind of equal exchange (in this case, sign for thing). Also, like justice, it involves rendering someone what is due to them:
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 a manner, regard something as being due.
It differs from justice, however, in the sense that the debt, i.e., what is due, is purely moral. In this sense it is more like equity (which, roughly, concerns the spirit of the law) than like justice in the strictest sense.
William Whewell, in his The Elements of Morality, includes Truth as one of the five principal virtues: Benevolence, Justice, Truth, Purity, and Order. Justifying Truth's placement in this august company, he says (Vol. I, Book III, Chapter II),
[A]mong the necessary conditions of a Rule of human action, is the existence of a Common Understanding among men, such that they can depend on each other's actions. Lying and Deceit tend to separate and disunite men; and to make all actions implying mutual dependence, that is, all social action and social life, impossible.
Like Aquinas, Whewell holds that truthfulness involves a moral right rather than a jural or legal one; and because Truth in this sense involves a consistency between internal and external actions, he also calls it Integrity.
Each of the principal virtues in Whewell's scheme involves duties that are characterized by principles or precepts. The principle that belongs to Truth as such (discussed in Volume I, Book III, Chapter IX) is "that we must conform to the universal understanding among men which the use of language implies," or, to put it more colloquially and briefly, that we must not lie. As one would expect a Victorian to do, he takes a very strong stance on the duties associated with truth: the duties of Truth extend beyond lying in the proper sense to rule out any form of deception whatsoever. Because of this, Whewell thinks, we ought to cultivate what he calls the Spirit of Truth: we ought to cultivate virtues such as Openness, Frankness, Simplicity of Character, and Singleness of Heart, by repeatedly engaging in truthful actions, so that we are far removed from any kind of deceit.
Whewell's conception of virtue, however, is slightly different from Aristotle's and Aquinas's; for Whewell, morality consists in conforming our lives as best we can to Moral Ideas. Thus Whewell posits Truth and the rest as ideals of moral life, to be continually approached and approximated.
