IMMANUEL KANT argued with great force and subtlety that metaphysics, which claimed to provide knowledge of a supersensible world, was impossible. Theologians were understandably alarmed. The Church's creeds were audaciously metaphysical. Besides affirming the existence of God and post-mortem survival, they made claims about the internal structure of the Godhead. If metaphysics was impossible, theologians had better shut up shop - or find another line of goods to sell.

Kant suggested the alternative: ethics. Ethics was the one hope left in Pandora's box after all the other functions of religion had been secularised. Churches no longer had a monopoly on literacy or education. People did not consult the Bible for information about cosmology, or the origin of species, or even Middle Eastern history. What was left for religion? Nothing, it seemed, in the world of fact. If religion was worth anything, concluded, it must be in the world of "value".

Theologians, therefore, seized on ethics as a more respectable alternative to metaphysics - and were hoist with their own petard.

In the late 20th century, religious practice plunged precisely because people rejected Churches' moral authority, and what they saw as Christians' attempts to force their values on others. In the United States, there was a pushback against the Religious Right (originally styled the Moral Majority).

But if the Religious Right's attempt to impose its socially conservative agenda on the general public was offensive, the Religious Left's arrogance was insulting. Church leaders on the Left assumed that they were qualified to promulgate doctrine on all manner of social and political issues about which informed citizens believed that they were competent to decide for themselves.

So, for example, the Episcopal Church in the USA, at its General Convention last year, established a commission to study the ethics of genetically modified food crops in order to "guide church public policy advocacy" on this.Who were they to make recommendations on the development of GM crops, or any other complex issues? Church leaders had no special technical expertise; so why should the nation look to them for guidance, or support their advocacy efforts?

Church leaders who were watching religious affiliation decline assumed that the problem was metaphysics. But it was not the metaphysics that put people off - it was the ethics. And it was the principle of the thing: not just the untenable code of sexual conduct that conservatives promoted, or liberals' reflexive political correctness, but church leaders' assumption that they knew better than others did, and were in a position to teach.

Churches have ceded a range of projects to secular experts: cosmology to scientists, Middle Eastern history to historians, education to secular schools, social services to secular charities and the welfare state. There seems no reason why they should keep a hold on ethics or social policy.

Nowadays, metaphysics is booming. There is a renewed interest in philosophical theology, and enough metaphysical work for any theologians who want it. It might be time to give metaphysics another look. It is, in any case, time to get out of the ethics business.

Dr Harriet Baber is Professor of Philosophy at the University of San Diego, USA.