Sarah Sands: Liars are thrilling on the stage, but there's no room for them in real life

There are many kinds of lies – and Kant rejected them all, even if done to protect others or to bring about a worthy end. A background of deception creates a world without trust

The Prime Minister and son of the manse has been pushed around a bit lately, but The Apprentice has reaffirmed his moral compass. Asked about the embroidered CV submitted by the show's winner, Lee McQueen, Gordon Brown answered unequivocally: "I think people should tell the truth."

This appears to be a statement of Kantian absolutism. People should tell the truth because a lie has a corrupting effect on the body politic. If everyone followed Lee's example, there would be a total breakdown in trust. We would not believe the pilot or the lollipop lady or the doctor or the teacher or the policeman.

Perhaps this is what Brown meant when he yelled at Tony Blair: "I cannot believe a word you say." It was not merely a question of Blair reneging on a promise to hand over his job to his Chancellor. The timber of humanity rested on the truth of the promise.

McQueen's false claim to have completed two years of a hospitality course at Thames Valley University, when he only managed a few months, was a lie without resonance. His career owed nothing to his student accomplishments. Jeff Randall points out that business leaders such as Sir Alan Sugar and Sir Philip Green are not university men. McQueen sold himself on his determination, energy and footballer's appearance, rather than scholarly reflection. It is not as if we discovered that Simon Schama was secretly a bit of a thicko.

McQueen could have made a worthy political point about the new universities and their failure to inspire students. He might have become the spokesman for the solid and growing constituency of university dropouts.

After everybody else weighed in with their theories of Lee's lie, he offered his own opinion. He said, with a curious self detachment, that the lie exposed "frailties and insecurities about my education".

This places him at some distance from Iago on the spectrum of liars, yet it does not satisfy Brown/Kant. There are consequential and utilitarian considerations. What does Lee's lie mean for all those other students at Thames Valley who completed their courses? Why should they have bothered? (This question is aimed at moralists rather than educationalists.)

McQueen has also misled his future employer Sir Alan Sugar. To echo Brown on Blair, why should Sir Alan believe a word McQueen says? What if McQueen one day, feeling insecure about his profit figures, embellishes them a little? What was the Enron scandal if not a serious spasm of performance frailty and insecurity?

Presumably this is why a quarter of bosses have withdrawn job offers after spotting discrepancies, according to the Chartered Institute of Personnel and Development.

I was slightly surprised, having been headhunted for my present job, when my boss asked for evidence of my university degree. It was not a particularly good degree and it was not from a famous university, so I could not see why he cared.

Then I realised that, because my boss was American and did not know me, he needed to establish that I was not a psychopathic fantasist. By producing a sepia fragment of paper, I gave him veracity (though not, of course, a guarantee of psychological stability). Globalisation demands a greater degree of trust than ever before.

A lie, even if it leads nowhere and harms no one, can devastate character and reputation. Hillary Clinton must wonder if her "misspoken" moment, in which she claimed to have landed in Bosnia under sniper fire, lost her the presidency. Hillary wanted to prove that a woman was tough enough to be Commander in Chief. She had already voted for the Iraq War. So what harm was there in a bit of boy's talk about dashing from a helicopter to the safety of the base?

Yet a politician as practiced and a wife as worldly as Hillary Clinton did not make an elementary calculation; could the facts be checked? Her visit to Bosnia was a public engagement. Television news recorded her beaming as an eight-year-old child read out a poem. This was about as life-threatening as a paddling pool.

Kant might say that Hillary was true to her personality – insanely ambitious and reckless about objectivity – but also true to her deeper human self. She lied very badly because she knew that all lying is wrong.

Most of us are more relative about lying. It is distasteful to lie for professional advancement but it is not as despicable as lying to a loved one. Wasn't Bill Clinton's lawyerly distinction between oral and penetrative sex meaner than Hillary's fantastic display of her balls of steel?

The journalist Jeff Randall is particularly unforgiving about professional lies. David Cameron's achievements are wiped out because he was not straight with Randall about a company merger when he was working for Carlton. When Cameron entered the Tory leadership race, Randall wrote of him: "I wouldn't trust him with my daughter's pocket money."

Whom you lie to is morally significant. Many public figures believe that there is no moral imperative to tell journalists the truth. Cherie Blair was taken aback when Alastair Campbell told her that she could not lie to the public about her miscarriage.

Boris Johnson was wounded at being sacked as a Conservative frontbench spokesman by Michael Howard for telling a newspaper that allegations of his affair with Petronella Wyatt were a " pyramid of piffle". The late Auberon Waugh, an eminent journalist, always protested that it was perfectly respectable to lie your head off to tabloid journalists poking their noses in your private business.

Lying to Parliament is in a completely different category, even if it is about private matters. John Profumo suffered an extraordinary punishment and did life-long penance for his denial of "impropriety" with the call girl Christine Keeler.

I do not know where Gordon Brown stands on Kant's logical assertion that it is also wrong to lie to murderers. Under this ruling, you would have to answer the Gestapo honestly when they asked if you were hiding Jews. Kant's critics accuse him of putting truth before life.

There is also the question of kindness, which we might also call humanity. The example usually given is the person at the bedside of a dying woman. The woman asks if her son is on his way and the person assents, despite knowing that the son has been killed in a car accident on the way to the hospital. There are milder variations of the kindness of lying, which usually apply to doctors.

The relativists among us prefer Augustine's degrees of lying to Kant's zero tolerance. There are lies that hurt somebody and help nobody, which is at the heart of Othello's tragedy. There are lies that hurt somebody, but benefit someone else. Cameron might argue that Randall had to suffer so that Cameron's boss, Michael Green, could benefit from a cleaner deal.

Then there are lies told for the pleasure of deception, which amount to mischief. Boris Johnson's biographer Andrew Gimson describes the day journalists from The New York Times came to The Spectator to do a profile of Boris. Their photographer turned up after the writer had left. Puckishly, Boris ordered Gimson to pose as him. He had a joyful notion of the august and po-faced American paper publishing a photograph of an impostor. The former publisher of The Spectator, Kimberly Fortier, an American, was horrified and put a stop to the prank.

Would the mortification and fury of The New York Times have outweighed the childish pleasure of the Spectator crowd?

A crueller deception was perpetrated on Malvolio in Twelfth Night, but the Augustine bringing about of repentance or redemption could be offered in mitigation.

The noblest form of lying is the lie that hurts nobody and physically protects someone. This is the lie to the murderer at the door.

Most of us do not have to trouble our consciences about the emergency lie. Our lies are a question of manners and convenience. Does it matter if we say we are ill or busy when we are not, or if we pretend to admire a dress or hairstyle that we do not?

It is easy to lie and it is habit-forming. Lies slip out so easily that, like Hillary Clinton, sometimes you are not even aware that you are lying. Yet society cannot function without trust and we cannot trust in a climate of lying. That is why Sir Alan should have fired Lee McQueen.