For instance, if our conceptions of the universe have profoundly changed since the time of the Revolution, it is because astronomical discoveries and the application of experimental methods have revolutionised them, by demonstrating that phenomena, instead of being conditioned by the caprices of the gods, are ruled by invariable laws. Such revolutions are fittingly spoken of as evolution, on account of their slowness. But there are others which, although of the same order, deserve the name of revolution by reason of their rapidity: we may instance the theories of Darwin, overthrowing the whole science of biology in a few years; the discoveries of Pasteur, which revolutionised medicine during the lifetime of their author; and the theory of the dissociation of matter, proving that the atom, formerly supposed to be eternal, is not immune from the laws which condemn all the elements of the universe to decline and perish. These scientific revolutions in the domain of ideas are purely intellectual. Our sentiments and beliefs do not affect them. Men submit to them without discussing them. Their results being controllable by experience, they escape all criticism. 3. Political Revolutions. Beneath and very remote from these scientific revolutions, which generate the progress of civilisations, are the religious and political revolutions, which have no kinship with them. While scientific revolutions derive solely from rational elements, political and religious beliefs are sustained almost exclusively by affective and mystic factors. Reason plays only a feeble part in their genesis. I insisted at some length in my book Opinions and Beliefs on the affective and mystic origin of beliefs, showing that a political or religious belief constitutes an act of faith elaborated in unconsciousness, over which, in spite of all appearances, reason has no hold. I also showed that belief often reaches such a degree of intensity that nothing can be opposed to it. The man hypnotised by his faith becomes an Apostle, ready to sacrifice his interests, his happiness, and even his life for the triumph of his faith. The absurdity of his belief matters little; for him it is a burning reality. Certitudes of mystic origin possess the marvellous power of entire domination over thought, and can only be affected by time. By the very fact that it is regarded as an absolute truth a belief necessarily becomes intolerant. This explains the violence, hatred, and persecution which were the habitual accompaniments of the great political and religious revolutions, notably of the Reformation and the French Revolution. Certain periods of French history remain incomprehensible if we forget the affective and mystic origin of beliefs, their necessary intolerance, the impossibility of reconciling them when they come into mutual contact, and, finally, the power conferred by mystic beliefs upon the sentiments which place themselves at their service.