Laudator Temporis Acti

Polybius 2.56.10-12 (tr. Ian Scott-Kilvert):

It is not a historian's business to startle his readers with sensational descriptions, nor should he try, as the tragic poets do, to represent speeches which might have been delivered, or to enumerate all the possible consequences of the events under consideration; it is his task first and foremost to record with fidelity what actually happened and was said, however commonplace this may be.

For the aim of tragedy is by no means the same as that of history, but rather the opposite. The tragic poet seeks to thrill and charm his audience for the moment by expressing through his characters the most plausible words possible, but the historian's task is to instruct and persuade serious students by means of the truth of the words and actions he presents, and this effect must be permanent, not temporary.

Thus in the first case the supreme aim is probability, even if what is said is untrue, the purpose being to beguile the spectator, but in the second it is truth, the purpose being to benefit the reader.