In the Times, Janice Turner asks: "Why did Vicky Pryce, a pre-eminent economist lavishly paid to predict the future of nations, not foresee her own doom?" This raises an awkward point in the philosophy of economics - that rationality is more ambiguous than often realized.

The standard conception in economics comes from Hume:

Reason is, and ought only to be, the slave of the passions, and can never pretend to any other office than to serve and obey them.

In this conception, rationality consists in maximizing "utility", in choosing that action which best satisfies whatever passion we have. This allows economists to speak (pdf) of rational addiction. From this point of view, Ms Pryce was rational. Her passion was negative altruism, a desire to hurt Huhne even at a cost to herself, and she achieved this. She was rational in the way that Eli Berman and David Laitin claim (pdf) suicide bombers to be.

But surely, there's a distinction between Ms Pryce and suicide bombers? There is. It lies in our second conception of rationality. This requires that beliefs are proportionate to evidence. Most of us would claim that suicide bombers are irrational in that their beliefs in paradise and in the evil of the west are not proportioned to the evidence. But Ms Pryce's beliefs were not so irrational; Huhne had treated her terribly.

The passions are not always beyond reason. We can judge Ms Pryce's negative altruism as rationally motivated, but suicide bombers' as not.

You might wonder why I'm talking of negative altruism rather than hatred or anger. I do so for two reasons.

First, to note a distinction between hatred and anger. Anger often clouds rational thought whereas hatred can be consistent with it - as, for example, when a military commander pursues the best strategy.

Secondly, it's because that word "altruism" brings me to my third conception of rationality.

Altruism - positive or negative - sits oddly with economists' presumption of rationality.They might think of it as just a passion beyond reason, or try to embed within a system of reciprocity, or look for an evolutionary explanation for it.

But there's another possibility, stressed by Alasdair Macintyre in Whose Justice? Which Rationality? This says that rationality is not a feature of desiccated calculating machines who exist outside of society and history, but is instead embodied withni practices and tradition. Take, for example, the doctor who treats patients outside his contractual hours or the soldier risking his life for his comrades. These might seem irrationally altruistic to economists. To Macintyre, however, such behaviour is rational - it's part of what it means to be a good doctor or good soldier. In this conception, the question "how can I serve God?" makes sense - as a striving to continue a tradition - even though many of us would say it doesn't according to our second conception of rationality.

But - Macintyre would say - just as we can ask "what is a good doctor or good soldier?" so we can ask: "what is a good life?" And by this standard, Ms Pryce has been irrational because most of us would say that a good life consists partly in staying out of the Big House.

Ms Pryce has therefore inadvertently done us a service. She's shown that rationality has many, sometimes incompatible, meanings. (And this is before I mention Nozick!) Faced with these, the challenge is to avoid the twin dangers of either a Dawkinsite pigheadedness ("That's not what I think, so it's stupid") or a sloppy "anything goes" relativism. I fear that nobody tries hard to steer between these.