BOB ABERNETHY, anchor: We want to expand now on that debate underway about what religious traditions and international law have to say about military retaliation. Michael Young is an international lawyer and Dean of the George Washington University School of Law in Washington D.C. Farid Esack is a professor at Auburn Theological Seminary in New York and teaches Islam at Union Theological Seminary. Lisa Sowle Cahill teaches moral theology at Boston College. Professor Cahill, let me begin with you and with the Christian idea of a "just war." What guidance can that tradition offer about responding to something as awful as the terrorist attack?

DR. LISA CAHILL (Professor of Ethics, Department of Theology at Boston College): Well, one of the first things that it can remind us is that resorting to violence should be a last resort, that Christian just war tradition especially has emphasized a presumption against war. As Thomas Aquinas famously asked in the thirteenth century: "Is it always a sin to wage a war?" The just war tradition has developed because an exception can be made to that presumption, particularly in the case of self-defense.

ABERNETHY: And what about this particular situation? What would be the guidelines?

DR. CAHILL: Well, some of the most important guidelines are, as I mentioned, last resort. Another one of them is proportionality. Will more harm be caused by the means used than by good achieved? Another would be a reasonable hope of success. Are the aims clear and can they be accomplished? And perhaps the most important criterion is non-combatant immunity. Civilians should not be the subjects of direct attack.

ABERNETHY: Professor Esack in New York, is there anything in Islamic tradition that would be a guide to anybody considering how to respond to a terrorist attack?

DR. FARID ESACK (Visiting Professor and Islam Specialist, Union Theological Seminary): Yes, certainly, I think that the Islamic tradition does allow for notions of retaliation and I think that in this particular case, one can also, I mean we can also have a theory of what is comparable to Christianity in the just war theory. And by and large, our guidelines very much parallel or correspond to what we just heard. In this particular case, I think that the Islamic position would be far more looking at a long term of what has induced this, so that one doesn't see this necessarily as an immediate retaliation for an immediate act. But how does a response to this lead to the creation of a more just world, of a more peaceful world, in the long term rather than "how do you get at a particular victim in a particular space or a particular network?"

ABERNETHY: Let me get back to that. Michael Young, let me switch over to international law now. What are the guidances there?

DEAN MICHAEL YOUNG (George Washington University Law School): Well, international law recognizes the inherent right of self-defense and really bounds it largely by notions of proportionality and necessity. That it has to be necessary, and the actions have to be proportional to eliminate the risk or to stop the action.

ABERNETHY: And retaliation, vengeance?

DEAN YOUNG: Well, retaliation can certainly be considered appropriate under international law if it's thought necessary to forestall any future attacks, which is often the justification that states use.

ABERNETHY: And does it matter what international law says in this situation?

DEAN YOUNG: Well, I think it matters importantly in two senses. One is that we want to continue to work to create a society in which -- an international society in which other countries do things that we consider appropriate and not do things simply because they're powerful and strong. And secondly, this is a war in particular that is going to require an enormous amount of cooperation among allies. And the legitimacy of it, the way in which we prosecute it, will matter to them as well.

ABERNETHY: Professor Cahill, in Boston, another issue here is one of holding responsible for terrorism, a government or nation that harbors terrorists, knowingly, I suppose, or unknowingly. What does the Christian tradition have to say about that?

DR. CAHILL: Well, I think that, I'm not sure that the Christian tradition replies specifically to terrorism as a traditional issue but I think that something that I've heard mentioned by many religious thinkers is the need for clearly defining who the enemy is, separating that enemy from others who are much more loosely related, and having some kind of persuasive and clear evidence that the adversary that we've identified is really guilty of the crimes of which he is accused.