Penelope Maddy tackles philosophical problems in a concrete way. Her most effective arguments are tied to specific examples drawn from esoteric regions of research mathematics, such as descriptive set theory. With remarkable clarity she reports on and contributes to mathematics in the making. In previous work she was keenly sensitive to its philosophical content, noting how debates about, say, the Axiom of Choice or impredicative definitions have involved deep issues about the reality and accessibility of abstract entities. Philosophers, of course, found this heartwarming.