The Introduction offers a contextual framework for exploring aspects of eighteenth-century thought. In the first section, ‘Ideas and their history’, the question of how to approach the ways of thinking that animated English-speaking peoples some three centuries ago is briefly considered. This is followed by an outline of how knowledge was structured in the eighteenth century, particularly as this is reflected in the pioneering encyclopedias of the period, such as Ephraim Chambers’s Cyclopédia and the French Encyclopédie. The limitations of attempts to label the prevailing intellectual ethos of the period (Age of Reason, of Revolutions, of Enlightenment, of secularization, of progress) or to define its temporal limits (the ‘long’ eighteenth century) are next considered. The introduction concludes with a discussion of the institutional framework and social habits—elements of the sociology of knowledge—that structured intellectual inquiry in the eighteenth century. A short appendix of terms highlights differences between the meanings of key words in the eighteenth and the twenty-first centuries: how they were understood and used then, in contrast with their meaning for us now.