He was impressed by the difference between written history and(though he did not use the phrase) what Charles A. Beard called"history as actuality"

It is also good to use metaphorical words; but the metaphors must not be far-fetched, or they will be difficult to grasp, nor obvious, or they will have no effect. The words, too, ought to set the scene before our eyes; for events ought to be seen in progress rather than in prospect. So we must aim at these three points: Antithesis, Metaphor, and Actuality.

Three chief features of these clever, pointed sayings are: (1) antithesis, (2) metaphor, and (3) actuality or vividness (i.e. the power of "setting the scene before our eyes").