Among the events whose study seemed completed was the French Revolution. Analysed by several generations of writers, one might suppose it to be perfectly elucidated. What new thing can be said of it, except in modification of some of its details? And yet its most positive defenders are beginning to hesitate in their judgments. Ancient evidence proves to be far from impeccable. The faith in dogmas once held sacred is shaken. The latest literature of the Revolution betrays these uncertainties. Having related, men are more and more chary of drawing conclusions. Not only are the heroes of this great drama discussed without indulgence, but thinkers are asking whether the new dispensation which followed the ancien regime would not have established itself naturally, without violence, in the course of progressive civilisation. The results obtained no longer seem in correspondence either with their immediate cost or with the remoter consequences which the Revolution evoked from the possibilities of history. Several causes have led to the revision of this tragic period. Time has calmed passions, numerous documents have gradually emerged from the archives, and the historian is learning to interpret them independently. But it is perhaps modern psychology that has most effectually influenced our ideas, by enabling us more surely to read men and the motives of their conduct. Among those of its discoveries which are henceforth applicable to history we must mention, above all, a more profound understanding of ancestral influences, the laws which rule the actions of the crowd, data relating to the disaggregation of personality, mental contagion, the unconscious formation of beliefs, and the distinction between the various forms of logic. To tell the truth, these applications of science, which are utilised in this book, have not been so utilised hitherto. Historians have generally stopped short at the study of documents, and even that study is sufficient to excite the doubts of which I have spoken. The great events which shape the destinies of peoples-- revolutions, for example, and the outbreak of religious beliefs-- are sometimes so difficult to explain that one must limit oneself to a mere statement. From the time of my first historical researches I have been struck by the impenetrable aspect of certain essential phenomena, those relating to the genesis of beliefs especially; I felt convinced that something fundamental was lacking that was essential to their interpretation. Reason having said all it could say, nothing more could be expected of it, and other means must be sought of comprehending what had not been elucidated. For a long time these important questions remained obscure to me. Extended travel, devoted to the study of the remnants of vanished civilisations, had not done much to throw light upon them.