The history of science is partly the history of an idea that is by now so familiar that it no longer astounds: the universe, including our own existence, can be explained by the interactions of little bits of matter. We scientists are in the business of discovering the laws that characterize this matter. We do so, to some extent at least, by a kind of reduction. The stuff of biology, for instance, can be reduced to chemistry and the stuff of chemistry can be reduced to physics. Thomas Nagel has never been at ease with this view.