The inquest into the death of Princess Diana is drawing to a close, and the coroner Lord Justice Scott Baker has been summing up to help the jury to reach a verdict. Among other things, he has advised them on the credibility of one of the witnesses at the inquest, Diana’s butler Paul Burrell.

The BBC summarised his comments about Burrell like this:

Lord Baker suggested to the jury that Mr Burrell may have given evidence while thinking that “whatever he said might have an impact on his future enterprises”. Mr Burrell worked for Diana from 1992 and described himself as “Diana’s rock” - but Lord Baker said he was “quite a porous rock” given that many of the princess’ secrets were made public. The coroner said: “I advise you to proceed with caution especially if you are left with the impression that he only told you what he wanted you to hear. “On the other hand he was close to Diana and was particularly well-placed to hear information that others were not. The fact that he has not told you the truth on some occasions does not mean you cannot accept anything he he has told you. But you should proceed with caution.”

[Source: BBC News: Diana's butler 'obviously' lying]

In other words, although Burrell, being close to Diana, has a good ability to see, his vested interest to advance his own career means that he can’t be relied on.

How far would you stretch the truth to impress people?

A chef from Swindon faces losing his US cooking show after it was discovered that he had lied about his achievements. His desire to promote himself to open up career opportunities seems to have got the better of him.

Robert Irvine claimed to have made part of Charles and Diana’s wedding cake, and to have prepared meals for Presidents, despite having only picked fruit for the cake and worked in the White House mess. He also claimed to have a Knighthood, and that the Queen had given him a Scottish castle as a present, both of which Buckingham Palace dispute.

Irvine had not only worked on Dinner: Impossible, but had also written a book to accompany the show and lent his name to a “Royal Titanium” cookware range (which some vendors are now withdrawing from sale).

Irvine is far from alone in getting a career boost from half-truths, however; reportedly, one in four people lie on their CVs. Vested interest can affect us all.

John Lennon once explained that he had known he was a genius since about the age of twelve: “I thought, ‘I’m a genius or I’m mad. Which is it? I can’t be mad because nobody’s put me away. Therefore I must be a genius.’”

On the face of it, this is a simple example of the restricting the options (or “false dilemma”) fallacy. The starting point of the argument, “I’m a genius or I’m mad”, discounts for no good reason the possibility of sane normality; plenty of people are neither mad nor a genius, so why couldn’t John Lennon have been one of those?

However, the argument isn’t about people in general, but about John Lennon specifically. He didn’t reason “Everyone is either a genius or mad, and I’m not mad so I must be a genius”; he reasoned “I’m either a genius or I’m mad, and I’m not mad so I must be a genius”. If there were some feature of John Lennon that justified this narrowing of the options then the argument might just go through.

So why think that John Lennon must either have been mad or a genius?

Well, how about the fact that he was already convinced he was a genius? He described his sense that he was smarter than everyone else, including his teachers, and his constant mystification that no one else had noticed this. Given that he thought like that, either he was right (in which case he really was a genius) or he was wrong (in which case he was mad in at least some mild sense of suffering from persistent delusions of grandeur).

So perhaps, at a push, we can grant Lennon the first part of the argument; either he was a genius, or he was mad (in some sense).

However, the argument still presents us with a false dilemma, it still commits the restricting the options fallacy.

The problem is with the penultimate step in the argument: “I can’t be mad because nobody’s put me away”. This short sentence contains a whole sub-argument of its own, with the same structure as the main argument. Filling in the implicit reasoning in the sub-argument, we get this: “Either I’m not mad or they would have put me away, but they haven’t put me away so I must not be mad.”

Again, Lennon begins by narrowing the options to two: “Either I’m not mad or they would have put me away.” This time, however, there aren’t good grounds for doing so. Suffering from persistent delusions of grandeur need not be particularly dangerous, so it would be perfectly possible for Lennon to have been mad in that sense without getting locked up.

The argument fails, therefore, not because Lennon can’t prove that he isn’t sane but normal, but because he can’t prove that he isn’t mad.