I answer that, Truth can be taken in two ways. Secondly, truth may stand for that by which a person says what is true, in which sense one is said to be truthful. 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. 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.