The prior probability of justification for war?

The difficulty of figuring out whether or not a given war is just doesn't reside in "probabilistic" uncertainty (is there a good term for this), but in uncertainty about what exactly is meant by a "just war".

Those are two very different kinds of uncertainty. Probability theory only helps us for the first.

This is a poorly defined and inflammatory question.

It also seems to be a question with the obvious answer of "yes" from almost any perspective.

Once you decide on a moral system and a capacity for judgment--i.e., "vengeful general" or "concerned liberal citizen"--you could use standard brands of decision and probability theory to determine optimal actions--i.e., "go to war" or "vote against war"--but basically the only claim this entails is that "it is theoretically possible to think clearly when it comes to politics and wars" which is (hopefully) not a controversial (or, really, interesting) claim.

You could start by looking at recent wars and deciding whether each one was "just" according to whatever standard you're using. The percentage that were just would be a rough estimate of the prior probability that a war is just.

Thanks for the comments everyone! Here are some of my initials thoughts that prompted my question.

I was thinking that I could come to an estimate of the prior probability in this way:

For every given war there is a side that is justified in violence and a side that is not justified in violence.

OR

Both side are unjustified in violence

It can't be the case that both sides are justified. Violent conflicts can't happen between two justified parties.

Given the above realization, I should expect to randomly find myself in the justified country no more than half the time a violent conflict arises. Actually, the prior probability that I live in justified country is most likely less than .5 since many wars have probably involved both parties sharing guilt in the conflict.

That's as far as I got and then I posed the question here.

Thoughts?

I think you really want decision theory rather than probability theory for a question like that.

Definition of Just War

There is at least one part which might be interestingly subject to Bayesian analysis: "Arms may not be used in a futile cause or in a case where disproportionate measures are required to achieve success".

More generally, a utilitarian analysis of how to tell whether a war is worth fighting could be very valuable.

Could you use Bayes Theorem to figure out whether or not a given war is just?

If so, I was wondering how one would go about estimating the prior probability that a war is just.

From my understanding of Bayesian reasoning, you'd just put a prior of 0.5; you're not supposed to do any reasoning or look at any evidence in order to determine your prior. You then use all the other evidence (have other wars been just? has the invading country allowed free access to evidence or does it act as if it's been hiding things? does it profit from the war? etc.) to update your probability.

So your use of "prior" seems technically incorrect here (though I'd be glad to be corrected if I'm wrong!)

Could you use Bayes Theorem to figure out whether or not a given war is just? If so, I was wondering how one would go about estimating the prior probability that a war is just.

I think it is a fascinating question. After all, if we claim to be Bayesians, we ought to be interested in applying our art to useful public issues. But coming up with a prior: P(H), is only part of the problem. We also need to specify our evidence: E, the probability of encountering that evidence in any old war: P(E), the probability of encountering that evidence if the hypothesis is true that the war is just: P(E|H), and most importantly, the specification of the hypothesis: H.

Specifying the hypothesis is the easy part, if you are Roman Catholic. Here is what the Catholic catechism says about just war:

In this regard Just War doctrine gives certain conditions for the legitimate exercise of force, all of which must be met:

H1. the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain;

H2. all other means of putting an end to it must have been shown to be impractical or ineffective;

H3. there must be serious prospects of success;

H4. the use of arms must not produce evils and disorders graver than the evil to be eliminated. The power of modern means of destruction weighs very heavily in evaluating this condition.

Well, as compared to a typical law passed by the US Congress, that seems pretty clearcut. So I guess we can just define H ::= H1 & H2 & H3 & H4. Note that it is assumed that for a war to be just from the standpoint of one side, the other side must be the aggressor. It is not easy to be just - you need to clear some legal hurdles to even defend yourself from aggression justly.

So, to see whether Bayesianism can help us here, let us try to estimate P(H3|E) where E is taken to be "We lost the first three battles in this war". Well, that certainly seems to be evidence that our prospects of success are not good, and hence that our war is not just. But just how strong is this evidence, and how severely does it depress our estimate of P(H3)?

Well, I suppose we could look at statistics from past wars - how often did a side that lost the first three battles eventually "succeed"? Of course H3 doesn't require that we actually succeed, only that we have "serious prospects of success". If we arbitrarily define "serious prospects" as a probability > 20%, and our historical statistics tell us that the defender succeeds against the aggressor 40% of the time, but only 25% after losing the first three battles, do we have the information we need to compute P(H3|E)?

To be honest, I don't know. Perhaps better Bayesians than me can help us out. But I'm certainly beginning to see that applying Bayes to real-world questions can get difficult very quickly.

Emile, yeah, what ""justification" is is certainly a question, but what if we just assume a definition of just war and work from there? Could you not then look at the evidence of the other countries wrongs and so forth and come up with a probability?