Social thinkers frequently remind us that people differ on what constitutes personal well-being, but that even when they don't differ, they disagree over the extent to which one person's well-being can be permitted to be traded off against another's. They tell us that political differences are to be traced to differences in people's conceptions of personal and social well-being. We are given to understand, in other words, that people's ethics differ.

One response to Derek Parfit's ‘mere addition paradox’ invokes the relation of ‘parity’. Since parity is a form of ‘incommensurateness’ in John Broome's terms, three doubts which Broome raises about accounts involving incommensurateness in Weighing Lives pose a challenge for this response. I discuss two of these. They emerge from a discussion of various intuitions about ‘neutrality’. I argue that an account based on parity may be no less consistent with Broome's intuitions than is his own vagueness view.

Without doubt, Weighing Lives, like its precursor, Weighing Goods, is an excellent and thought-provoking piece of work. In the first place, it addresses a question of the most fundamental importance, namely: how should we aggregate the well-being of past, present and future members of the human race under the various possible states of the world that may, in the event, prevail? This involves, amongst other things, dealing with questions of aggregation across time, people and different states of the world; the issue of what constitutes a “neutral” state of well-being (i.e., one neither better nor worse than not being alive) and what is, in fact, the value of continuing to survive.

It is not the job of philosophy to give direct practical advice either to people or to governments. Nevertheless, moral philosophy is immensely significant in practical matters. It influences the way we think and act, but only slowly as it filters through the process of public debate. I hope Weighing Lives will have a practical influence, but it is not meant to be a directly practical guide.

I am extremely grateful to the five commentators for readingmy book and offering such interesting thoughts in reaction. Shortage of space may make my responses seem brusque. But of course they are not meant to be.