Aristotle Rhetoric 3.17.10

[10] Deliberative speaking is more difficult than forensic, and naturally so, because it has to do with the future; whereas forensic speaking has to do with the past, which is already known, even by diviners, as Epimenides the Cretan said; for he used to divine, not the future, but only things that were past but obscure. ¹ Further, the law is the subject in forensic speaking; and when one has a starting-point, it is easier to find a demonstrative proof. Deliberative speaking does not allow many opportunities for lingering—for instance, attacks on the adversary, remarks about oneself, or attempts to arouse emotion. In this branch of Rhetoric there is less room for these than in any other, unless the speaker wanders from the subject. Therefore, when at a loss for topics, one must do as the orators at Athens , amongst them Isocrates, for even when deliberating, he brings accusations against the Lacedaemonians, for instance, in the Panegyricus, ² and against Chares in the Symmachikos (On the Peace). ³