How, then, to avoid a cultural meltdown with profound (and profoundly bad) consequences for democracy? In his debate with Habermas, Ratzinger argued that, while the idea of a "natural moral law" had become "blunt", the idea of human rights (which had itself grown from the claim that there are moral truths we can know by reason) provided a kind of grammar for ordering the public debate about public goods. Within that grammar, real dialogue was possible, and both believers and secular people could once again embark on the adventure of truth as it touched issues of public life.

To the surprise of many, Ratzinger won the argument. Several months later, Habermas took to the pages of the European press to concede that the idea of neutrality between worldviews was too thin a cultural foundation on which to rest the political future of Europe. Something thicker, stronger, more compelling was needed.

Elements of this dialogue with Habermas continue to shape the thinking of Benedict XVI, who must now, of course, speak to a global audience, not simply a European one. Accordingly, at the United Nations in April, Benedict returned once again to the idea of human rights as a grammar for turning the noise that too often characterises exchange within the "international community" into genuine conversation - and perhaps even genuine deliberation. Benedict went further from the green marble rostrum of the General Assembly, however. The protection of human rights, he argued, was the fundamental task of government and the source of government's moral legitimacy. When this "duty to protect" was not met, he implied, the defaulting government in question risked losing its legitimacy - a bold proposal indeed in a UN in which the principle of sovereign immunity is typically cited to preclude action against the likes of Robert Mugabe, the perpetrators of genocide in Darfur and the Burmese junta.

Benedict XVI's most controversial public moment - his Regensburg Lecture of September 12 2006 - deserves to be revisited in this context: the relationship of the moral truths we can know by reason to the tasks of governance.