The present age is not merely an epoch of discovery; it is also a period of revision of the various elements of knowledge. Having recognised that there are no phenomena of which the first cause is still accessible, science has resumed the examination of her ancient certitudes, and has proved their fragility. To-day she sees her ancient principles vanishing one by one. Mechanics is losing its axioms, and matter, formerly the eternal substratum of the worlds, becomes a simple aggregate of ephemeral forces in transitory condensation.
            Despite its conjectural side, by virtue of which it to some extent escapes the severest form of criticism, history has not been free from this universal revision. There is no longer a single one of its phases of which we can say that it is certainly known. What appeared to be definitely acquired is now once more put in question.
            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…