A family of cannibals move next door and want to eat your child. Do you respect their beliefs and allow them to go ahead?
Christopher Ross, a part-time ticket clerk with London Underground at Oxford Circus station, asked the question seriously. He is a writer working on "Tunnel visions. Journeys of an underground philosopher". This consists of his reflections on urban madness, based on watching people for four hours every week for a year in an underground station.
"Are we running on rails? Or are we free?" he asked. "My obsession is the freedom of the human mind."
Iain Williamson, a former social anthropologist who is about to withdraw to a Buddhist monastery, picked up the challenge: "I would be happy to accommodate your rights to be a cannibal but if you started to impose them on me, then I would have to think. I do not want you to eat my children. I like children but I would not want to eat a whole one."
This exchange took place during the opening session of Britain's first Cafe Philosophique at the Institut Francais in London's South Kensington last Saturday. More than 100 men and women met in the institute's cafe to ponder the meaning of life for a couple of hours over coffee and croissants.
There are now about 100 cafes philosophiques in France. The first was founded by Marc Sautet, a French philosopher, in a cafe in the Bastille area of Paris.
Originally, he and a group of friends and colleagues started meeting regularly in the cafe to discuss philosophical questions. The gatherings became more popular and gradually led to other cafes springing up elsewhere in France.
Today there are cafes philosophiques in North America, South America, Austria, Germany and Japan and even an international association.
Gale Prawda, who is organising the British cafe philosophiques (there will be more in December, January and February), studied under the eminent French philosopher Paul Ricoeur, known for his work on phenomenology and hermeneutics.
She regularly officiates at English language cafes philosophiques in Paris's Cafe de Flore where Jean-Paul Sartre, Simone de Beauvoir and Albert Camus used to chew the epistemological fat.
Saturday's first meeting in the Institut Francais was slightly inhibited by the overwhelming presence of reporters, photographers and cameramen.
The room bristled with microphones, tape recorders, cameras and notebooks vying for soundbites and philosophical bon mots while earnest amateur and professional philosophers strove to make serious and sometimes abstruse moral and epistemological points.
There were middle-aged French and English housewives, students, full and part-time philosophy lecturers, ageing and earnest freewheeling Italian and Indian intellectuals. It was a cosmopolitan and eclectic gathering.
American-born Ms Prawda solicited philosophical questions from the floor. Does evolution exist for man or is man an animal? Does life have meaning or do we have to find our own meaning? Was philosophy a matter of emotion or logic? Is man a political animal? Are technology and philosophy compatible? Would it be better not to have been born? Is virtue the road to happiness?
She invited the hirsute man who asked whether it would have been better never to have been born to kick off the discussion. Someone else recalled that Socrates once voiced the same opinion.
Another contributor added that perhaps Socrates said that knowing he was about to swallow hemlock having been accused of corrupting the minds of Athenian youth.
A sceptic suggested the question was self-contradictory because if you had not been born you could not say one way or another. Perhaps, he added, it would be better if this question had never been born and the meeting got on with discussing more interesting issues.
The discussion sputtered on. Would it be better if badly-deformed people had not been born? Better for whom? What about Hitler? Should children be brought into a war-torn world? Are individual lives intrinsically good? What is the point of living? How should we live?
"The meaning of life is your life," someone said not altogether seriously. Another pointed out that most people just have to get on with life and don't have the luxury of speculating about its meaning over cafe au lait and croissants.
Is this life all we have? Or is it a rehearsal? Is there an afterlife? Speaking as a Buddhist, Mr Williamson thought reincarnation was something of a gamble. "But if we prepare for our deaths, then maybe we can get a life, " he said.
Which came first: the brain or the hand? How do we tolerate intolerance in a tolerant society? Would it be good to live in harmony or not? Harmony to a cannibal, someone helpfully reminds the meeting, is eating someone else. "What about the cannibalisation of ideas?", someone else chipped in.
One man asked: "Why is everybody being so friendly? This is meant to be about arguments."
"What we have here is a lot of clever people in one room disagreeing," another volunteered.
Afterwards Donatella Bernstein, a social anthropologist and theatrical agent from Islington, said that she thought such meetings were an excellent idea.
"The way society is structured is too vertical," she said. "University lecturers and newspapers hand information down to people but what we need are more horizontal, sideways discussions like this. I'd like to see a cafe philosophique on social, political and economic questions."
Peter Cave, a part-time university philosophy lecturer from Hampstead who had come along to see what the level of debate would be, thought that too many false philosophical claims had been left hanging in the air.
Mr Ross, from Herne Hill, had wanted to see whether a room full of strangers could reach a consensus. The answer was no, sadly.
The discussion, he said, had been "too sclerotic". There was too much "grandstanding" by more confident speakers and he would have been keen to hear what the quieter, more reticent folk had to say.