Continuing the “debate”:https://www.crookedtimber.org/archives/002956.html about preventive war begun by “Judge Richard Posner”:https://www.crookedtimber.org/archives/002956.html (the discussion was begun by him, I mean, not the war) the Medium Lobster “presents a competing analysis”:http://fafblog.blogspot.com/2004_12_05_fafblog_archive.html#110238611093456642:

bq. [T]the probability of an attack from the moon is less than one – indeed, it is miniscule. However, the potential offensive capabilities of a possible moon man invasion could be theoretically staggering. … The Medium Lobster has calculated this probability to be 5×10^-9. … the resulting costs would include the end of civilization, the extinction of the human race, the eradication of all terrestrial life, the physical obliteration of the planet, and the widespread pollution of the solar system with a mass of potentially radioactive space debris. The Medium Lobster conservatively values these costs at 3×10¹², bringing the expected cost of the moon man attack on earth to 1500 (5×10^-9 x 3×10¹²), a truly massive sum. Even after factoring in the cost of exhausting earth’s nuclear stockpile and the ensuing rain of moon wreckage upon the earth (200 and 800, respectively), the numbers simply don’t lie: our one rational course of action is to preventively annihilate the moon.

I’m a bit sorry to break it to the Medium Lobster, but Judge Posner considers scenarios of precisely this kind, and uses pretty much this methodology, in his new book, “Catastrophe: Risk and Response”:http://www.amazon.com/exec/obidos/ASIN/0691070148/kieranhealysw-20/ref=nosim/. Cases treated include the nanotechnology gray-goo apocalypse, the rise of superintelligent robots, and a strangelet disaster at Brookhaven Labs that would annihilate a substantial chunk of spacetime in the vicinity of our solar system. A “recent review”:http://www.slate.com/id/2109600 of the book raises most of the relevant critical points about the approach Posner takes. In essence, it’s all good geeky fun to apply the methods to cases like these but it’s a stretch to pretend we’re learning anything decisive about what we should do, as opposed to gaining insights on the scope and limits of some techniques for assessing alternatives.

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Now that, thanks to Kieran and the Medium Lobster, we’ve all had our fun with Richard Posner’s case for pre-emptive war, complete with toy numerical example, it’s time for me to play straight man.

Posner’s starting assumption is consequentialism: that we should evaluate an action based on whether its probable consequences are, on balance, good or bad. I broadly agree with this, so I’ll try to explain why it shouldn’t lead to conclusions like those derived by Posner.

I’ll ignore a range of more complex objections and come straight to the first distinction learned by beginning students of the subject. Should we evaluate the consequences of general rules such as “don’t engage in pre-emptive wars” (rule-consequentialism) or should we evaluate each action on a case by case basis (act-consequentialism)

For perfectly rational decision makers, following the rules of Bayesian decision theory, the answer is easy and, in fact, trivial. It’s best to make the optimal decision on a case by case basis, and an optimal set of rules would be so detailed and precise as to yield the optimal decision in every case. Posner routinely assumes this kind of perfect rationality, which is why he doesn’t see any big problems with toy examples, or with claiming that this kind of reasoning can usefully be applied to improbable catastrophes with incalculable consequences.

There are two objections that can be made here

* Human beings are not perfectly rational and do not follow the rules of Bayesian decision theory

* Since war is a negative sum game, rational decision makers do not fight wars[1]

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There’s been a lot of hubbub, both here and elsewhere in the blogworld, about the Becker-Posner blog. But if it’s intellectual firepower in a group blog you’re after, you should be reading “Left2Right”:http://left2right.typepad.com/. Here’s its “mission statement”:http://left2right.typepad.com/main/2004/11/why_left2right.html, which should be good for setting off a round of debates.

bq. In the aftermath of the 2004 Presidential election, many of us have come to believe that the Left must learn how to speak more effectively to ears attuned to the Right. How can we better express our values? Can we learn from conservative critiques of those values? Are there conservative values that we should be more forthright about sharing? “Left2Right” will be a discussion of these and related questions.

bq. Although we have chosen the subtitle “How can the Left get through to the Right?”, our view is that the way to get through to people is to listen to them and be willing to learn from them. Many of us identify ourselves with the Left, but others are moderates or independents. What we share is an interest in exploring how American political discourse can get beyond the usual talking points.

The contributors so far include “Elizabeth Anderson”:http://left2right.typepad.com/main/2004/11/what_hume_can_t.html, “Kwame Appiah”:http://left2right.typepad.com/main/2004/11/less_contempt.html, “Josh Cohen”:http://left2right.typepad.com/main/2004/11/the_moral_value.html, “Stephen Darwall”:http://left2right.typepad.com/main/2004/11/school_resegreg.html, “Gerald Dworkin”:http://left2right.typepad.com/main/2004/11/less_contempt_m.html, “David Estlund”:http://left2right.typepad.com/main/2004/12/the_first_data_.html, “Don Herzog”:http://left2right.typepad.com/main/2004/12/public_private_.html, “Jeff McMahan”:http://left2right.typepad.com/main/2004/12/supporting_our_.html, “Seana Shiffrin”:http://left2right.typepad.com/main/2004/11/being_forthrigh.html, and “David Velleman”:http://left2right.typepad.com/main/2004/12/debunking_a_dea.html. Wowsa. And many other names you may have heard of, from Peter Railton to Richard Rorty, are listed as being part of the team. This should be worth following.