This chapter expounds some technical philosophical notions that figure prominently in the rest of the book. Among the phenomena elucidated are moral conflicts, strong permissibility versus weak permissibility, overtopping obligations versus non-overtopping obligations, weak absoluteness versus strong absoluteness, physical freedom versus deontic freedom, the Hohfeldian analysis of legal positions (claim-rights and duties, liberties and no-rights, powers and liabilities, and immunities and disabilities), and the distinction between causal relationships and constitutive relationships. Although all of these concepts and distinctions are of great significance in debates over the principle of freedom of expression, they also figure saliently in many other debates. Hence, the purpose of this introductory chapter is to provide the basic philosophical preliminaries that are needed for a rigorous engagement with the principle of freedom of expression. The chapter sets the stage for the subsequent chapters, where that principle is the focus of attention.