The true answer to Holbach is to be found in a different order of ideas from any that Voltaire was prepared to accept. Yet Locke might have taught him that if there is no logical reason to believe in the existence of mind, there is as little to believe in the existence of matter. Experience might have shown him that men do not always seek the thing which they believe most useful to themselves. The old and favorite doctrine of utility labors under the disadvantage that it has never shown, nor ever can show, an adequate reason why any man should care for another or for the race. And as for the existence of God,--that can no more be proved by argument than the existence of matter, mind, or the _non-ego_. Helvetius and Holbach had worked out the theories of the school to their last philosophical conclusion. A younger writer in the last years of the reign of Louis XV. was to furnish the complete application of them. The Chevalier de Chastellux is well known in America by the book of travels which he wrote when he accompanied the Marquis of Rochambeau in the Revolutionary War. Chastellux was just then at the height of his reputation. He had published in 1772 a book which, although now almost forgotten, is still interesting as a link between the thought of the last century and that of a large school of thinkers to-day. The title is "Of Public Felicity, or considerations on the fate of men in the different Epochs of History," and the motto is _Nil Desperandum_. "So many people have written the history of men," says Chastellux; "will not that of humanity be read with pleasure?" And again: "Several authors have carefully examined if such a Nation were more religious, more sober, more war-like than another; none has yet sought to discover which was the happiest."