D'Holbach met all these theories by pointing out that human development, from the "state of nature" to social life and the ideas and commodities of civilisation, is itself natural, given the innate tendency of man to improve his lot. To return to the simpler life of the forests--or to any bygone stage--would be denaturer l'homme, it would be contrary to nature; and if he could do so, it would only be to recommence the career begun by his ancestors and pass again through the same successive phases of history. [Footnote: Syst. soc. i. 16, p. 190.] There was, indeed, one question which caused some embarrassment to believers in Progress. The increase of wealth and luxury was evidently a salient feature in modern progressive states; and it was clear that there was an intimate connection between the growth of knowledge and the growth of commerce and industrial arts, and that the natural progress of these meant an ever-increasing accumulation of riches and the practice of more refined luxury. The question, therefore, whether luxury is injurious to the general happiness occupied the attention of the philosophers. [Footnote: D'Holbach, ib. iii. 7; Diderot, art. Luxe in the Encylopaedia; Helvetius, De l'esprit, i. 3.] If it is injurious, does it not follow that the forces on which admittedly Progress depends are leading in an undesirable direction? Should they be obstructed, or is it wiser to let things follow their natural tendency (laisser aller les choses suivant leur pente naturelle)? Voltaire accepted wealth with all its consequences. D'Holbach proved to his satisfaction that luxury always led to the ruin of nations. Diderot and Helvetius arrayed the arguments which could be urged on both sides. Perhaps the most reasonable contribution to the subject was an essay of Hume. 4. It is obvious that Rousseau and all other theorists of Regress would be definitely refuted if it could be proved by an historical investigation that in no period in the past had man's lot been happier than in the present. Such an inquiry was undertaken by the Chevalier de Chastellux. His book On Public Felicity, or Considerations on the lot of Men in the various Epochs of History, appeared in 1772 and had a wide circulation. [Footnote: There was a new edition in 1776 with an important additional chapter.] It is a survey of the history of the western world and aims at proving the certainty of future Progress. It betrays the influence both of the Encyclopaedists and of the Economists. Chastellux is convinced that human nature can be indefinitely moulded by institutions; that enlightenment is a necessary condition of general happiness; that war and superstition, for which governments and priests are responsible, are the principal obstacles.