This volume contains the modified and enlarged text of the lectures that the author had the honor of delivering in 1937 at the Institut d'Histoire des Sciences et des Techniques of the University of Paris. He has devoted a considerable portion of the book to expound the motives driving philosophical thought—with a force he believes himself justified in taking as one endowed with logical necessity—in the direction impressed upon it by Husserl. The author has done this to ensure that the book has the character of an introduction, which, as such, should always be as well, and perhaps above all, an outline.