The story of Zhuangzi and the butterfly must be one of the best-known anecdotes in the philosophical literature. It is also one of the most annoying: a tease that irritates rather than illuminates. But, as is so often the case, it is when we are walking away from philosophical problems that we realize they point, however unsteadily, to something we cannot entirely dismiss.

According to legend, the great Taoist philosopher fell asleep one day and dreamed that he was a butterfly. When he woke up he did not know whether he really was a man who had dreamed he was a butterfly or whether he was now a butterfly dreaming that he was a man. The story is intended as more than a charming episode in the life of a sage: it is meant to make a general philosophical point about what we take to be real. Our dreams are utterly compelling and, so long as we are dreaming, we think they are real: there are, as Descartes said in his Meditations, “no certain indications by which we may clearly distinguish wakefulness from sleep”. If last night I dreamed that I was giving a lecture wearing no trousers and was so convinced of this that I woke up sweating, how do I now know that I was dreaming then?