What does a free society require of believers and non-believers alike?

A great debate of our time concerns how people with different religions, ethnicities and values can live together as full citizens of free societies. Here's the common thread that runs through half-a-dozen news stories every day. Yesterday, for example: a schoolteacher arrested and charged in Sudan for allowing children to call a teddy bear Muhammad; the poor, ethnically mixed housing estates around Paris going up in smoke again; Israel-Palestine peace talks, with their implications for relations between Muslims and non-Muslims everywhere; a Jewish school in London criticised for insisting that for a child to qualify for admission the applicant's mother had to be born Jewish; angry scenes in Oxford as a student debating society offers a platform to a Holocaust denier.

A large part of this debate is about the position of Muslims in Europe, but it's important to remember that the issues are much wider. Recently, discussion of Muslims in Europe has crystallised around a few personalities, including some views attributed to me. Such a personalisation of the issues helps to dramatise them, but it also risks disappearing down obscure polemical back alleys of the "who did or did not say what about whom" variety. It's probably more useful to put personalities aside for the moment and restate some of the basics of the secular liberal position that I propose. Obviously I can't spell this out in a single column - that needs a book - but here are just a few of the bare bones.

Muslims start from Islam. Liberals start from liberalism. I'm a liberal, so I start from liberalism - not in the parody version propagated by the American right, but liberalism properly understood as a quest for the greatest possible measure of individual human freedom, compatible with the freedom of others. I believe that, faced with the challenges of growing diversity, we, the citizens, need to agree and spell out more clearly the essentials of a free society. A charter of citizens' rights and duties, as proposed by Gordon Brown, would be one way to take this forward.

Among the essentials is freedom of expression, which has been eroded to an alarming degree, both by death threats from extremists and by misconceived pre-emptive appeasement on the part of the state and private bodies. Freedom of expression necessarily includes the right to offend; not the duty, but the right. We must, in particular, be free to say what we like about historical figures, be they Moses, Jesus, Muhammad, Churchill, Hitler or Gandhi (and then let our claims be tested against the evidence). We may not agree with what controversialists say about these figures but we must defend to the death their right to say it. There should, for obvious reasons, be limits to what we are free to say about living people, but these limits must be very tightly drawn.

Among the liberal essentials is equality before the law, including equal rights for men and women. Among the essentials is also freedom of religion. Since a core liberal notion is that we must be free not just to pursue our own version of the good life but also to question and revise it, it follows that we must be free to propagate, question, change or abandon our religion. In a free society, proselytisation, heresy and apostasy are not crimes. This - and apostasy in particular - is not accepted in many versions of Islam, but it is a liberal essential on which there can be no compromise.

In order to secure these freedoms, we need a secular public sphere. But what exactly do we mean by that? To say "Enlightenment values" begs the question, "which Enlightenment"? The Enlightenment of John Locke, which claimed freedom for religion, or that of Voltaire, which aspired rather to freedom from religion? (I deliberately simplify a complex history.) A liberal order in which the devotees of all Gods are free to try their hand in the public square, on an equal footing with those who insist - correctly, in my view - that there is no God? Or a liberal order in which all gods are kept as far as possible out of the public square? (The French republican understanding of laïcité is closer to the latter, the United States' first amendment tradition to the former.) I'm more of a Lockean myself, but I don't think this debate is best pursued at the abstract, theoretical level of "which Enlightenment"? Better to tackle specific issues: faith schools, new mosques, the teaching of evolution, the hijab, Muhammad cartoons and so on.

We do, however, need to be clearer about the difference between secularism and atheism. Secularism, in my view, should be an argument about arrangements for a shared public and social life; atheism is an argument about scientific truth, individual liberation and the nature of the good life. Today's debate around Islam is bedevilled by a confusion between the two. Atheists must be free to say to Muslims, Christians or Jews: "Your mind would be much more free if you gave up your ridiculous belief in God." Believers must be free to argue back: "You would have a more profound sense of personal freedom if you did believe." But neither is entitled to demand that of the other as a condition for participating as a citizen in a free society. The public policy argument about freedom for religion and the private conviction argument about freedom from or in religion should operate on different levels.

That distinction would, of course, no longer hold if being a devout Muslim were in fact incompatible with being a full citizen of a free society. I feel this is what quite a few participants in the current debate, both atheist and Christian, really believe, while seldom spelling it out so clearly. Yet the thought keeps peeping through, for example in the formula "Islam is incompatible with democracy". But as a non-Muslim I can only agree with the author Edward Mortimer who, in his book Faith and Power, concluded that there is no single, unchanging Islam, "there is only what I hear Muslims say, and see them do". What Muslims say and do in the name of Islam has varied enormously through history, and varies enormously today. Yes, of course, there is the Qur'an and the Hadith, just as there is the Bible. But, as in all great religions, these are complex texts, subject to diverse interpretations.

When a Muslim letter-writer in yesterday's Guardian tells us, with the aid of Qur'anic references, that Islam, properly understood, supports "the vital principle of freedom of speech", what possible interest have we non-Muslim liberals in arguing against him? If a Christian supports the rule of law, as we understand it in a 21st-century secular liberal state, we don't cry: "But your Old Testament says 'life for life, eye for eye, tooth for tooth'!" Unless, of course, an atheist agenda - to show that religion is not just nonsense but dangerous nonsense - trumps the secular liberal agenda, which is to find the ways in which people with different beliefs can live together peacefully in freedom.

Well, I have run out of space, and I have barely begun. There is so much else that needs saying. All comments are welcome and let's continue this vital conversation.

Timothy Garton Ash