Writing the Book of the World is masterful work. Sider offers a careful and insightful treatment of metaphysical structure (or naturalness, or fundamentality), developing a framework for metaphysical inquiry while defending the centrality of metaphysics to philosophy as a whole. There is much to admire and much to discuss. I expect Sider's book to become a touchstone for further discussion of the nature and viability of metaphysical inquiry. It is required reading.

Sider's project is a radical extension of Lewis's (1983) project in “New Work for a Theory of Universals.” Lewis—building on Armstrong's (1978) defense of universals—identifies theoretical roles for a metaphysical posit of natural properties, while pointing out that one need not posit universals to fill these roles: tropes will do, as will a primitive status of naturalness for elite sets of possibilia. Sider's (vii, 8, 85) core innovation is to extend...

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