Let me end this introduction by mentioning two charms of population ethics as a field of study. First, you can prove theorems. You need not content yourself with sketching out some plausible (though imprecise) premises and drawing a natural (though not inevitable) conclusion. You can lay down axioms and demonstrate that certain claims follow. Better yet, some of the theorems that can be proved are astounding, with nigh-on-undeniable premises together guaranteeing a nigh-on-unbelievable conclusion. Arrhenius’s impossibility theorems are perhaps the best example. In my darker moods, it sometimes feels to me as if philosophy is a magic trick in which the magician is fooled most of all. But even then I figure that, if I am to be fooled, it might as well be with these marvellous tricks. The second charm of population ethics is that it concerns things that are important: life and death, joy and misery, survival and extinction. Not only that, but we find ourselves living at a time where our views on population ethi…