Limits of naturalism in ethics

Philippa Foot's Natural Goodness is a restatement of a naturalistic theory of ethics. It is an interesting book for many reasons, since the author changes her position from a broadly Humean position to a more thoroughly Aristotelian one. Foot criticizes the non-cognitivist stance and various forms of expressivism and utilitarianism. She now declares that there are categorical imperatives, a thesis she used to deny. These imperatives are based on the idea of "patterns of natural normativity" which depend on the forms of life we are talking about. Human life needs practical rationality in order to flourish, and this means to have a rational will, without "volitional defect". Yet, this strategy confines practical rationality to an instrumental value and, more importantly, it can't stand the objection that, after all, being human or flourishing as human may not be a normative ideal, e.g. from a Nietzschean point of view. Only if the will has an intrinsic side constraint as pure will, the Nietzschean objection can be faced: even a Will of Power cannot will against itself. But it can still will against nature. Naturalism, at least in this version, cannot face such a challenge.