Preliminary Discourse to the Encyclopedia of Diderot expresses the hopes, dogmas, assumptions, and prejudices that have come to characterize the French Enlightenment. In this preface to the Encyclopedia, d\'Alembert traces the history of intellectual progress from the Renaissance to 1751. Including a revision of Diderot\'s Prospectus and a list of contributors to the Encyclopedia, this edition, elegantly translated and introduced by Professor Richard Schwab, is one of the great works of the Enlightenment and an outstanding introduction to the philosophes.