David Armstrong's book is a contribution to the philosophical discussion about possible worlds.

Preface; Part I. Non-Naturalist Theories of Possibility: 1. The causal argument 2. Non-Naturalist theories of possibility; Part II. A Combinatorial and Naturalist Account of Possibility: 3. Possibility in a simple world 4. Expanding and contracting the world; 5. Relative atoms; 6. Are there de re incompatibilities and necessities?; 7. Higher-order entities, negation and causation; 8. Supervenience; 9. Mathematics; 10. Final questions: logic; Works cited; Appendix: Tractarian Nominalism Brian Skyrms; Index.