Objection 1. It seems that the virtue of truth does not incline to that which is less. For as one incurs falsehood by saying more, so does one by saying less: thus it is no more false that four are five, than that four are three. But "every falsehood is in itself evil, and to be avoided," as the Philosopher declares (Ethic. iv, 7). Therefore the virtue of truth does not incline to that which is less rather than to that which is greater.
Objection 2. Further, that a virtue inclines to the one extreme rather than to the other, is owing to the fact that the virtue's mean is nearer to the one extreme than to the other: thus fortitude is nearer to daring than to timidity. But the mean of truth is not nearer to one extreme than to the other; because truth, since it is a kind of equality, holds to the exact mean. Therefore truth does not more incline to that which is less.
Objection 3. Further, to forsake the truth for that which is less seems to amount to a denial of the truth, since this is to subtract therefrom; and to forsake the truth for that which is greater seems to amount to an addition thereto. Now to deny the truth is more repugnant to truth than to add something to it, because truth is incompatible with the denial of truth, whereas it is compatible with addition. Therefore it seems that truth should incline to that which is greater rather than to that which is less.
On the contrary, The Philosopher says (Ethic. iv, 7) that "by this virtue a man declines rather from the truth towards that which is less."
I answer that, There are two ways of declining from the truth to that which is less. First, by affirming, as when a man does not show the whole good that is in him, for instance science, holiness and so forth. This is done without prejudice to truth, since the lesser is contained in the greater: and in this way this virtue inclines to what is less. For, as the Philosopher says (Ethic. iv, 7), "this seems to be more prudent because exaggerations give annoyance." For those who represent themselves as being greater than they are, are a source of annoyance to others, since they seem to wish to surpass others: whereas those who make less account of themselves are a source of pleasure, since they seem to defer to others by their moderation. Hence the Apostle says (2 Corinthians 12:6): "Though I should have a mind to glory, I shall not be foolish: for I will say the truth. But I forbear, lest any man should think of me above that which he seeth in me or anything he heareth from me."
Secondly, one may incline to what is less by denying, so as to say that what is in us is not. On this way it does not belong to this virtue to incline to what is less, because this would imply falsehood. And yet this would be less repugnant to the truth, not indeed as regards the proper aspect of truth, but as regards the aspect of prudence, which should be safeguarded in all the virtues. For since it is fraught with greater danger and is more annoying to others, it is more repugnant to prudence to think or boast that one has what one has not, than to think or say that one has not what one has.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)