Tim points to Brian Cox's claim that "engineering is the foundation of our economy" as corroboration of Richard Feynman's quip that "a scientist looking at nonscientific problems is just as dumb as the next guy."

This, I suspect, reflects a general psychological fact - that many of us have a "lump of rationality", such that if we use a lot of it in one activity, there's little left in another. People who are rational in some contexts can be irrational or downright barmy in others. Vicky Pryce is perhaps an extreme example of this, but many would add Richard Dawkins, and I guess you can all think of other less famous examples.

There are good reasons why this might be:

- Ego depletion. Our willpower is limited, so if we use it in one context, we lack it in another. This is why men often get drunk after a hard day's work, why people put on weight when they give up smoking, or why women on diets go on spending sprees. A similar thing applies to thinking. Rationality requires effort, so if we use it in our day job we don't have much left elsewhere.

- Incentives. In Professor Cox's day job, irrationality will cost him heavily; his papers won't be published and he'll not get funding or promotion. But his political opinions are a free hit; stupidity is not penalized in politics. Simple incentives, then, mean that Cox - and indeed many professionals - will be more rational in their day job than in their political opinions.

- Motivated reasoning. We often think most clearly when we have no skin in the game, no interest at stake. But this is almost never true for all domains. There's always some interest to promote (science teaching in Cox's case or hating Huhne in Pryce's) or some opinion to bolster (atheism for Dawkins), and this leads to poor judgment.

- Other objectives. Human history surely teaches us that decisions made at two in the morning are rarely fully rational. However, political agreement is often reached then - look at many EU summits or this week's agreement on press regulation. Taking decisions at stupid o'clock might be irrational in the sense of not fitting the facts or not leading to the most efficient policy. But these are not the objectives. Sometimes, rough compromise and collegiality matter more than cold-headed rationality.

This last point is under-appreciated. A while ago, I was in a long queue in a greengrocers in Belsize Park, behind a man who was taking an eternity to choose some plums. When he turned round, I gave a cry: "fuckin' 'ell, it's Mike Brearley." Mr Brearley was committing the error of not seeing that, sometimes, any decision is better than the rational one. Jon Elster quotes Boswell on Samuel Johnson:

He did not approve of late marriages, observing that more was lost in point of time than compensated for by any possible advantages. Even ill-assorted marriages were preferable to cheerless celibacy.

Perhaps, then, it is not only impossible for us to be fully rational, but undesirable too.