Here’s a true sentence: The number seven is odd. What’s philosophically odd about the sentence is that it seems to imply that there must be numbers, including the number seven just as the truth of The Statue of Liberty is in New York implies that there is such a statue. But the number seven, unlike the statue, isn’t located anywhere, and we can’t see it or touch it. It is, Plato argued long ago, an abstract entity. But should we think reality includes abstract entities? In his deftly written critical survey Abstract Entities (Routledge, 2017), Sam Cowling provides a sophisticated discussion of what abstract (as opposed to concrete) entities might be, what reasons we have for thinking they exist, and how we might explain mathematical, scientific, and other truths, and our knowledge of such truths, if we don’t think they do. Cowling’s book is an excellent introduction to a fundamental metaphysical debate that brings together main arguments and raises new issues for both sides.