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What's the difference between lying and misleading? And does it matter if our politicians don't always manage to stay on the right side of the line?

"I want you to listen to me. I'm going to say this again - I did not have sexual relations with that woman, Miss Lewinsky."

Was this the most blatant lie in modern politics? Or had Bill Clinton convinced himself, when he uttered those words in January 1998, that his physical relationship with Monica Lewinsky did not constitute "sexual relations"?

But even if we grant that generous interpretation, his statement was, according to any normal understanding of "sexual relations", certainly misleading.

These days, if a politician is caught in an outright lie - claiming something to be true which he or she knows to be false - it can be political suicide. At the very least it can prove highly damaging.

Image copyright Getty Images Image caption Bill Clinton denied "sexual relations" with Monica Lewinsky

Take an example from another Clinton, Hillary. Her bid to become the Democratic presidential candidate in 2008 never fully recovered from her untrue claim that she had landed in Bosnia under sniper fire during the Yugoslav civil war. She never admitted to lying - she only conceded that she "did misspeak".

The problem is that politicians are routinely put in a position where telling the truth would be problematic. They might hold market-sensitive information, they might have a negotiation stance that can't be revealed, there might be internal party dissent on an issue which it would be embarrassing or self-destructive to admit, they might hold information that could be useful to foreign rivals or enemies.

Sometimes the bald truth is unpalatable. Politicians want to get first elected and then re-elected, and telling people what they want to hear is usually more effective than confronting them with miserable realities.

Since politicians can't lie, and since it is often inexpedient for them to tell the truth, they have become adept at positioning their remarks in the squidgy marshland between lies and truth. It's a phenomenon of modern democratic politics, and now completely routine, but it's not entirely new.

The Italian diplomat and philosopher Niccolo Machiavelli wrote in The Prince, a founding bible of modern political science, that for the statesman, "occasionally words must serve to veil the facts".

Image copyright Alamy Image caption Machiavelli: "Occasionally words must serve to veil the facts"

The former adviser to British Prime Minister Gordon Brown, Damian McBride, calls political misleading "lying without lying", and confesses to having a "talent for avoiding the truth without lying".

He recounts one episode before the 2005 budget when a journalist sent McBride a photograph of an official document which appeared to have the new borrowing figures. "Were these figures correct?" he was asked.

In fact, they were very nearly right, but not quite. McBride was able to tell the journalist, without exactly lying, that these were not the budget figures - and the story didn't run.

Image caption Damian McBride with Gordon Brown in 2007

Gordon Brown, then Chancellor of the Exchequer, had fretted that if the figures leaked he would have to consider his position. As it happens, says McBride, he "didn't have to resign, went on to become prime minister and the rest is history".

The practice of misleading works through what philosophers call "conversational implicature". When we communicate with one another, there are a set of background rules which govern the meaning of our sentences. Take one example.

Suppose there's a referendum and a politician says that if the people vote "yes", she will resign. The politician doesn't explicitly state that if the vote goes "no" she will remain in her job - but it's implied.

In July this year, the Greek finance minister said he would resign if Greece voted to accept the creditors' demands for more austerity. The country voted decisively to reject them. Yet within hours, Yanis Varoufakis quit anyway.

Image copyright Alamy

The idea that deception is wrong can probably be dated to the Bible, and the ninth commandment: "Thou shalt not bear false witness against thy neighbour".

But the question of whether outright lying is morally worse than other kinds of deception seems to have been first addressed by Thomas Aquinas in the 13th Century.

Modern philosophers disagree about whether lying and misleading are on a moral par. On the one hand, the intention behind telling a porky and uttering a misleading sentence is the same - to con the hearer of the sentence into believing an untruth.

But Jonathan Webber of Cardiff University argues that, although all forms of deception are generally wrong, lying is its worst form. This, he says, has to do with reputation. If a politician lies and is discovered, trust in this politician will be undermined.

"Economical with the truth"

One of the most famous euphemisms for political lying, thought to have been coined by Edmund Burke, who wrote in 1796: "Falsehood and delusion are allowed in no case whatsoever: But, as in the exercise of all the virtues, there is an economy of truth"

Phrase entered modern political lexicon UK cabinet secretary Sir Robert Armstrong used the phrase during the 1986 Spycatcher trial (when the government tried to prevent the publication of a spy's memoirs); asked to define the difference between a "misleading impression" and a lie he replied: "As one person said, it is perhaps being 'economical with the truth'

Former UK defence minister Alan Clark (pictured) hastened the collapse of the Matrix Churchill trial in 1992, after admitting being "economical with the actualite" regarding what he knew about arms exports to Iraq

But if he simply misleads, you become "unable to trust the implicatures" of what he says, but not necessarily unable to trust the politician himself. You still have no reason to believe that he would actually lie. "Lying", Webber argues, "damages your reputation more thoroughly than misleading."

Damian McBride is unconvinced. Having spent his career as an inveterate misleader, he now believes that lying-without-lying is no better than outright lying. What's more, that modern politicians mislead every day of their lives is directly "connected to the fact that trust in politicians has been corroded over the last 40 years".

There's also a cost to the character of the political misleader. Being economical with the truth "starts to seep into the rest of their lives", he says.

"I don't think it's any coincidence it leads to the higher proportion that we see in politics of people living secret lives. Having affairs, being secret alcoholics, secret gamblers, secret drug addicts, having that cloak of secrecy around them at all times, because that's become so much a part of who they are."

In Britain. MPs are not permitted to call each other liars - this is regarded as un-parliamentary language, the ultimate insult. To call MPs misleaders however, is acceptable. And it has the additional virtue of being true.

More from the Magazine

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Everybody knows lying is usually a bad thing, but is there a positive side, asks novelist Clare Allan.

The good side of lying (January 2012)

The Philosopher's Arms about politicians and lying is on BBC Radio 4 on 7 December at 20:00 GMT, or you can listen to it online now as part of BBC Radio 4's Online First collection.