If we always tell the truth we are unlikely to contradict ourselves; but if we resort to lies we are liable, when asked a question for a second time, to forget what our answer was on the first occasion.
‘I tell you I couldn’t have done the murder,’ protested the man suspected of the crime, ‘because I was up in London when it happened.’
‘Which part of London?’
‘Last time we asked you you said it was Bermondsey. Come on, let’s have the truth for a change.’