The tiresome kerfuffle about whether Jeremy Corbyn lied about being unable to get a seat on a train raises a question: so what if he did lie? We all do. Often, a lie can be a way of expressing a bigger truth. Telling your partner that she looks good in that new dress is a lie that expresses the more important truth that you love her. Likewise, Corbyn's video gets at the truth that trains are often overcrowded and expensive.

The question is: how wide is the domain of truthful lies?

I suspect it's very wide indeed. One example of course is art. Literature, film and TV are fictions that aim at telling truths. As Stephen King said: "Fiction is a lie, and good fiction is the truth inside the lie."

Another example is that lies can embolden us to succeed. The team that believes "we can do this" has more chance of winning than one with more realistic beliefs. As Arsene Wenger said, "If you do not believe you can do it then you have no chance at all." The lie of overconfidence might lead to the truth of success.

Another class of examples arises from a variant of the theory of the second-best. We know from welfare economics that, in a sub-optimal world, policies that would otherwise be undesirable can actually increase well-being. For example, where there is a monopoly, price controls might be justifiable. Likewise, in a world where there are barriers to truth, lying might be a justifiable way of shifting us towards the truth.

This might apply in Corbyn's case. If he had made an honest speech about the shortcomings of our railways it would have been soon forgotten if it had been noticed at al. As it is, his stunt has focused more attention on the issue.

Another case where lying might be acceptable is that it might be easier to pander to people's beliefs than to confront them head-on. Nick Robinson once told Jonathan Portes that the man who was honest about immigration "would not have a chance of getting elected in a single constituency in the country". It might be better for pro-immigration candidates to lie and then enact reasonable policies and show that these have worked than to try to change voters' beliefs beforehand. As the old saying goes, it's easier to ask forgiveness than permission. Whether this strategy works is a matter of tactics, not morality.

Lying is, therefore, often acceptable and, as Sir Malcolm Bruce said, it is common in politics. So, when is it wrong? There are several occasions.

One is the simple prudential one - that you might not get away with it. Corbyn's critics claim that "traingate" has been deflected onto the question of his honesty. The objection here is not that he was morally wrong to lie, but that he was an incompetent liar. Here, the left is fighting an uphill battle; its lies will be more quickly exposed by the media than those of the right.

Also, of course, what matter is the cause the lie serves. Lying to a murderer is acceptable; lying to the policeman chasing him is not. What was wrong about Brexiteers' lies is that they served a bad cause. It's for this reason that children can get confused: we teach them to tell the truth, but to lie when great-aunt Jemima asks if they like her new hat.

A third problem is that you can easily get high on your supply. I suspect many decent Labour candidates lied about immigration and fiscal policy at the last election. My fear was that they would come to believe the lies. A similar thing might have happened to Brexiteers: their lie that negotiating Brexit would be a doddle led them to being woefully under-prepared for the slog of doing so.

You might add that there's a further objection to lying - that it debases the quality of political debate. A bit of me sympathizes. But another bit makes me think this ship sailed long ago. Politics is about power, not truth. Expecting politicians to tell the truth all the time is a childishly naïve Kantianism, rather like expecting to win a war without firing a shot.