“What Is X?” has been described as “a cross between a Platonic dialogue and ‘The Price Is Right.’” It combines dialectical inquiry of the sort perfected by Socrates and his interlocutors with a distinctly ludic spirit. Here’s how it works: For each episode, host Justin E. H. Smith invites on a guest distinguished in their field (or occasionally a “regular” person who really likes to talk). Smith asks the guest to answer a question of the form “What is X?” (for example, “What is beauty?” “What is nature?” “What are dreams?”), after which the two partners in dialogue undertake a Socratic...