Lytton Strachey describes the Philosophes of late-eighteenth-century France:
So far as the actual content of their thought was concerned, they were not great originators. The germs of their most fruitful theories they found elsewhere—chiefly among the thinkers of England; and, when they attempted original thinking on their own account, though they were bold and ingenious, they were apt also to be crude. In some sciences—political economy, for instance, and psychology—they led the way, but attained to no lasting achievement. . . . In their love of pure reason, they relied too often on the swift processes of argument for the solution of difficult problems, and omitted that patient investigation of premises upon which the validity of all argument depends. They were too fond of systems, and those neatly constructed logical theories into which everything may be fitted admirably—except the facts. In addition, the lack of psychological insight which was so common in the eighteenth century tended to narrow their sympathies; and in particular they failed to realize the beauty and significance of religious and mystical states of mind. These defects eventually produced a reaction against their teaching—a reaction during which the true value of their work was for a time obscured. For that value is not to be looked for in the enunciation of certain definite doctrines, but in something much wider and more profound. The Philosophes were important not so much for the answers which they gave as for the questions which they asked; their real originality lay not in their thought, but in their spirit. They were the first great popularizers. Other men before them had thought more accurately and more deeply; they were the first to fling the light of thought wide through the world, to appeal, not to the scholar and the specialist, but to the ordinary man and woman, and to proclaim the glories of civilization as the heritage of all humanity. Above all, they instilled a new spirit into the speculations of men—the spirit of hope.
From Landmarks in French Literature, 1912.