We crave contrast in the dramatic point of view.

We seek occasional change of key.

That serious comedy should move a sympathetic tear is reasonable enough; but it is hard to find that it grudges us a single smile.

The result of Diderot's method is that the spectator or the reader speedily feels that what he has before him substitutes for dramatic fulness and variety the flat monotony of a homily or a tract.

It would be hard to show that there is no true comedy without laughter--Terence's _Hecyra_, for instance--but Diderot certainly overlooked what Lessing and most other critics saw so clearly, that laughter rightly stirred is one of the most powerful agencies in directing the moral sympathies of the audience,--the very end that Diderot most anxiously sought.
It is mere waste of time to bestow serious criticism on Diderot's two plays, or on the various sketches, outlines, and fragments of scenes with which he amused his very slight dramatic faculty.