Since this book is intended to form part of a collection of introductory works on philosophy, we have limited our explanations to the essential points of the principal philosophers of the period. Whenever possible, we have kept the original vocabulary of these philosophers; but when the obscurity of the texts and the absence of adequate philosophical terminology have made it necessary, we did not hesitate to introduce our own interpretation of the doctrine. At all events, it has been our intention to faithfully transmit the central ideas of these philosophers with a view to making their doctrines more comprehensible.