My brother in profession, Paul Byrd, O.P., who blogs over at Dominican Cooperator Brother, has raised the vexed question about the ethics of lying, especially in liminal situations in which the lives of others are at stake. Rather than clog up his comment box, I decided to put my thoughts here.
Paul raised a rather classic question in ethics, both in general and in the history of moral theology itself. I will say from the outset that I believe his resolution (viz. that it is at least on some occasions morally acceptable, perhaps even necessary or praiseworthy to lie) to be mistaken.
To begin with, we want to admit that the overwhelming tradition of moral inquiry in this matter has held lying to be, in and of itself and not merely circumstantially, an evil, one precisely forbidden by the eighth commandment. To help see why this is so, we can look to St Thomas Aquinas. For Thomas, truthfulness (veritas) is a virtue by which one is disposed to tell the truth, i.e. to speak and act in ways that are consistent with one's inner thoughts, to be "one in mind and heart" not merely with others but with oneself. While this is the principal object of the virtue of truthfulness, it also includes the inclination readily and easily to speak the truth at the appropriate time and in the appropriate manner.
For Thomas, the vice opposed to truthfulness is lying, by which one represents externally (by speech or action) what is contrary to what one knows or thinks within. All of the kinds of lying (e.g. simulation, hypocrisy, iactantia [exaggeration of one's merits], ironia [untrue minimizing of one's merits]) share this feature. What is important to note here is that, for Thomas, there is not a "right to the truth" which makes lying to be bad. Simply speaking, others do not have a right that we disclose to them what is true, nor even that we be truthful. Furthermore, lying is not bad principally because of the intention to deceive. For Thomas, even if one had no reasonable expectation he would be believed, he could still be guilty of lying through a seriously intended misrepresentation of his mind/thoughts through contrary external speech/acts. It is the deformation of the one who lies that is at stake, and with that deformation, the whole project of both personal and interpersonal flourishing that is at stake here.
It is worth noting that the definition of the Catechism published in 1992, i.e. "To lie is to speak or act against the truth in order to lead into error someone who has the right to know the truth," was revised in the official edition of 1997 to remove the final phrase, viz. "someone who has the right to know the truth" precisely because of the problematic idea of the "right to the truth." The intention to deceive remains there, and a Thomist could readily accept that generally one lies in order to deceive, but this seems more a near universal circumstance, rather than getting at the essence of lying. Overall, the revision also helps ward off ultimately unhelpful approaches to the "difficult cases" often presented by, e.g. distinguishing, say, lies from "false speech" on the basis of the presence or absence, respectively, of either the intent to deceive or else of the recipient's relative "right" to the truth.
So, how have the hard cases been classically resolved? Thomas' solution, ultimately derived from Augustine, and developed later as the theory of "mental reservation", held that one could, legitimately, speak or act in such a way that, while not misrepresenting what was in one's own mind, nonetheless was possible, even likely, to mislead or misdirect the undesired, even wickedly motivated, inquiry (e.g. the Nazi looking for Jews in one's basement). While some versions of mental reservation have been rejected as hopelessly lax (i.e. saying what is clearly a material falsehood while adding a "corrective" phrase in one's head), the general theory understands that one can justifiably speak in a potentially equivocal but nonetheless truthful way. Moreover, one need not always say anything at all, nor is one generally obliged to tell the whole truth. ("I do not know if he is in," for example, is an acceptable response even when you just saw the person 20, or even 5 minutes ago. The fact is, you *don't* know that he is in. That the listener might conclude you have not seen him in some time is not your moral responsibility to correct. The circumstances when one is giving evidence in court, of course, alter one's moral obligations in the latter regard.)
While some may find this theory a bit too fine-grained, it is not actually a way to allow for lying. Indeed, it is quite the opposite of the proposal that one can morally lie. As tempting as such proposal seems, it tries to get at some ultimately passing good (even if one of great value) through the dissolution of one's own integrity. Even when done in small ways, this erosion of one's own authenticity is destructive of one's own self and, through that, of one's capacity to be personally related to others. This is the good truthfulness protects, and it is the good every lie, even "in a good cause", wounds or destroys.
As to Paul's other claims: (a) The case of lying is distinct from justifiable homicide for the reasons seen above. The claim that some people are authorized to kill (or justified to do so) is not rooted in the claim that right intention makes something otherwise evil nonetheless justified. (b) The cases from Scripture have a long, rich, detailed exegetical heritage. The *simplest* thing to note here is that nothing in the Scriptures requires (in fact much of the narrative disallows) the presumption that the acts of the patriarchs and matriarchs, of the kings and prophets, are all praiseworthy. That good that flows from their misdeeds comes from God's providence, not from the virtue of their deceptions (if, that is, these were not cases of mental reservation). (c) One *cannot* justify the deliberate choice of evil on the basis that the sin is not mortal. Knowingly doing what is evil is knowingly to turn away from the love of God, and in doing so from love of neighbor and of self as well. This is why it is obligatory to avoid any sin at any cost. The world is not made in any way better by the lessening of divine charity within us, much less its being extinguished in our hearts. It is *never* furthering God's Kingdom to draw away from God. (d) See the CCC for what the Church universal holds, and which I note above [bearing in mind to use the revised version of 1997], as well as the Thomistic approach, which represents the longer-standing and majority approach to the question.