It could be argued--and esteemed historian of ideas Russell Jacoby does so here--that the less diversity there is, the more we talk about it. But what does the term actually mean? Where does it come from? What are its intellectual precedents? Moreover, how do we square our recognition of the importance of diversity with the fact that the world is becoming more and more, well, homogeneous? In fine prose and lucid argument, Jacoby puts our volatile...