Through the character Caius Julius Caesar and through this character's conversation with another character, Antonius, Cicero speaks in De Oratore of five aspects of laughter (humor)--the nature of laughter, the source of laughter, the appropriateness of laughter in oratory, the limits of laughter, and the classification of laughable things:
*As recorded in De Oratore lviii, 236: “…merriment naturally wins goodwill for its author; and everyone admires acuteness, which is often concentrated in a single word, uttered generally in repelling, though sometimes in delivering an attack; and it shatters or obstructs or makes light of an opponent, or alarms or repulses him; and it show the orator himself to be a man of finish, accomplishment and taste; and, best of all, it relieves dullness and tones down austerity, and, by a jest or a laugh, often dispels distasteful suggestions not easily weakened by reasonings.”
*The orator restricts the use of humor according to the subject matter, the context, and the hearers. As regards topics, the following is to be ruled out: “…neither outstanding wickedness, such as involves crime, nor, on the other hand, outstanding wretchedness is assailed by ridicule, for the public would have the villainous hurt by a weapon rather more formidable than ridicule; while they dislike mockery of the wretched, except perhaps if these bear themselves arrogantly. And you must be especially tender of popular esteem, so that you do not inconsiderately speak ill of the well-beloved (De Oratore lviii, 237).
Such then is the restraint that, above all else, must be practised in jesting. Thus the things most easily ridiculed are those which call for neither strong disgust nor the deepest sympathy. This is why all laughing-matters are found among those blemishes noticeable in the conduct of people who are neither objects of general esteem nor yet full of misery, and not apparently merely fit to be hurried off to execution for their crimes; and these blemishes, if deftly handled, raise laughter. In ugliness too and in physical blemishes there is good enough matter for jesting, but here as elsewhere the limits of licence are the main question (lix, 238-9).
*If the mimicry is overly done, “it becomes the work of buffoons in pantomime, as also does grossness. It behoves the orator to borrow merely a suspicion of mimicry, so that his hearer may imagine more than meets his eye; he must also testify to his own well-bred modesty, by avoiding all unseemly language and offensive gestures.
[…] both in narrative and in mimicry, all likeness to buffoons in pantomime is to be avoided, so in this latter case the orator must scrupulously shun all buffoonish raillery (lxix, 242-lx, 243).