A single powerful idea, that of progress, dominated the nineteenth century and became its main symbol—or so it seemed to Renan when he measured “the enormous strides that the science of man has made during the last one hundred years.” Since the waning of the Middle Ages, intellectual progress had gone hand in hand with the rejection of the appeal to authority. Francis Bacon had assigned to this kind of progress a practical goal, and Descartes had provided it with an effective method. It seemed, therefore, that it was capable of going on, for some time at least, by additions to, and a systematic completion of, already existing knowledge. It is in such terms that Pascal described it in the preface to his Traité sur le vide, assimilating the development of mankind to that of “a single individual who lives forever and learns continually.” This picture, which became common to all later doctrines of progress, did not specify whether the human species would continue to progress indefinitely, as Condorcet said it would, or whether it would reach a state of old age and decrepitude and eventually become extinct, as Fourier imagined it. When reason, in the eighteenth century, ceased to be the gift of providence to the established political authorities, it became the property of all men, and the standard by which their conduct was to be judged. The progress of the intellect was thus socialized. In Turgot's words, “the entire mass of men is on the move to greater perfection, though a period of calm may alternate with one of agitation, and good with evil, and no matter how slow the movement may be.” This move took place against the background of an immovable nature, and as a result of the spread of enlightenment throughout society; and according to Turgot and his contemporaries, it could not be stopped by ancient institutions that stood in its way. The framework of nature was soon to be shaken when, at the turn of the century, the theories of Kant and Laplace divested the Newtonian order of its appearance of eternity, and when advances in geology and, finally, the transmutationist theories which culminated in Lamarck's Philosophie zoologique (1809), seemed to show that both inorganic. matter and living species were in the process of transformation. In biology, the criterion of progress was an increase in the specialization of the organism. It is this criterion which Saint-Simon wanted to apply to human societies, reminding us with satisfaction of what he owed to the anatomist Vicq d'Azyr. In the beginning of the nineteenth century, embryological discoveries seemed to show that the biological evolution of man was continuous with that of the lower animals. This gave rise to the idea of a continuous progress, and to the conviction that no valid reason could be found why progress should be confined to the present. The perfection of the individual (sketched tentatively by Condorcet) and the perfection of society found in Saint-Simon, and later on in Spencer, their systematic interpreters; and so did the idea that individual and society were to achieve, either spontaneously or by design, a dynamic equilibrium. Pascal's purely speculative picture was thus transformed into an organic theory of social evolution.