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.
But he attempted to do what none of his masters had done, to test the question methodically from the data of history.