All these researches, which are here summed up in a few lines, demanded long years for their accomplishment. Despairing of completing them, I abandoned them more than once to return to those labours of the laboratory in which one is always sure of skirting the truth and of acquiring fragments at least of certitude. But while it is very interesting to explore the world of material phenomena, it is still more so to decipher men, for which reason I have always been led back to psychology. Certain principles deduced from my researches appearing likely to prove fruitful, I resolved to apply them to the study of concrete instances, and was thus led to deal with the Psychology of Revolutions--notably that of the French Revolution. Proceeding in the analysis of our great Revolution, the greater part of the opinions determined by the reading of books deserted me one by one, although I had considered them unshakable. To explain this period we must consider it as a whole, as many historians have done. It is composed of phenomena simultaneous but independent of one another. Each of its phases reveals events engendered by psychological laws working with the regularity of clockwork. The actors in this great drama seem to move like the characters of a previously determined drama. Each says what he must say, acts as he is bound to act. To be sure, the actors in the revolutionary drama differed from those of a written drama in that they had not studied their parts, but these were dictated by invisible forces. Precisely because they were subjected to the inevitable progression of logics incomprehensible to them we see them as greatly astonished by the events of which they were the heroes as are we ourselves. Never did they suspect the invisible powers which forced them to act. They were the masters neither of their fury nor their weakness. They spoke in the name of reason, pretending to be guided by reason, but in reality it was by no means reason that impelled them. ``The decisions for which we are so greatly reproached,'' wrote Billaud-Varenne, ``were more often than otherwise not intended or desired by us two days or even one day beforehand: the crisis alone evoked them.'' Not that we must consider the events of the Revolution as dominated by an imperious fatality. The readers of our works will know that we recognise in the man of superior qualities the role of averting fatalities. But he can dissociate himself only from a few of such, and is often powerless before the sequence of events which even at their origin could scarcely be ruled. The scientist knows how to destroy the microbe before it has time to act, but he knows himself powerless to prevent the evolution of the resulting malady. When any question gives rise to violently contradictory opinions we may be sure that it belongs to the province of beliefs and not to that of knowledge.