Forty years ago, when Carlyle wrote, it might really seem to a prejudiced observer as if the encyclopaedic tree had borne no fruit. Even then, and even when the critic happened to be a devotee of the sterile transcendentalism then in vogue, one might have expected some recognition of the fact that the seed of all the great improvements bestowed on France by the Revolution, in spite of the woful evils which followed in its train, had been sown by the Encyclopaedists. But now that the last vapours of the transcendental reaction are clearing away, we see that the movement initiated by the Encyclopaedia is again in full progress. Materialistic solutions in the science of man, humanitarian ends in legislation, naturalism in art, active faith in the improvableness of institutions--all these are once more the marks of speculation and the guiding ideas of practical energy. The philosophical parenthesis is at an end. The interruption of eighty years counts for no more than the twinkling of an eye in the history of the transformation of the basis of thought. And the interruption has for the present come to a close. Europe again sees the old enemies face to face; the Church, and a Social Philosophy slowly labouring to build her foundations in positive science. It cannot be other than interesting to examine the aims, the instruments, and the degree of success of those who a century ago saw most comprehensively how profound and far-reaching a metamorphosis awaited the thought of the Western world. We shall do this most properly in connection with Diderot.