Does the Kantian idea of interpersonal rationality offer a way out? Can I start with the beliefs and desires I have as an individual, and by subjecting them to the rational constraint of debate with individuals like myself, arrive at a consensus that we all can accept? Is something like the Kantian "kingdom of ends" possible, a society where each of us is respected in his individual autonomy precisely because, and in so far as, he affords similar respect to the autonomy of others?

To end on a negative and perhaps controversial note, I doubt it. Or rather, I doubt if this can be the whole story. For the Kantian apparatus of rational social debate offers us only negative constraints: it prohibits courses of action that cannot be chosen by all, but it does not in itself provide substantive positive values and goals for a worthwhile human life. We are back with the problem that values cannot be conjured out of nothing. The autonomous individual will on its own, even when socially constrained by rational debate with other individuals, cannot determine the right path without a substantive vision of the good to guide it.

For Descartes, the requisite kind of moral vision was generated by Christian metaphysics, the objectivity of whose value system, for all his vaunted programme of doubt, he never seriously questioned. As understood by Descartes, the physical world is one that we can understand and scientifically control as a result of the God-given power of reason. But that same power of reason also enables us to perceive what is objectively good. Despite our many flaws and weaknesses, a benevolent creator has given every single human being the power to dispose their will so as to resolve to pursue that good. This secure metaphysical underpinning for his ethics perhaps accounts, more than anything else, for the pervasive optimism we find in Descartes's moral writings, and his sense that true "tranquillity of soul" was within the grasp of all. Whether our own contemporary worldview, cut adrift from such a sustaining metaphysics, can find a basis for recovering that tranquillity is something that remains to be seen.