I. Introduction; 1. What this book is about; II. The never-ending debate; 2. The innateness hypothesis; 3. Philosophy of mind; 4. Cognitive science; 5. Modularity; III. The mental grammar, language universals and language change; 6. The organization of the mental grammar; 7. Language universals; 8. Language change; IV. Language acquisition; 9. Language acquisition: the road from input to mental grammar; 10. Stages of language acquisition; 11. Critical period effects; 12. How children create new languages; V. Language in a different modality; 13. Sign languages; VI. Winding up; 14. Evaluating the arguments: a forum discussion.