Three men, each for very different reasons, dominate the modern world, and govern all the problems which torment it: a reformer of religion, a reformer of philosophy, and a reformer of morality –Luther, Descartes, and Rousseau. They are in very truth the begetters of what M. Gabriel Seailles called the modern conscience. I do not speak of Kant, who stands at the meeting of the intellectual streams springing from these three men, and created, so to say, the academic structure of modern thought. I shall consider Luther, not to study him exhaustively and as the founder of Protestantism, but to bring out certain features in the character of that enemy of philosophy which are of consequence to our philosophical battles. After all, it would be astonishing if the extraordinary loss of balance induced in the Christian mind by heresy had not had the most important repercussions in all spheres, particularly in that of the speculative and practical reason. By the very fact that the Lutheran revolution bore on religion, on that which governs all human activity, it was bound to change most profoundly the attitude of the human soul and of speculative thought confronted with reality.