This is adduced on p. 34, in Thomas Morris's Philosophy For Dummies (1999 1 ed).
The dangers of argument: A short survival guide
How can you actually use argument well and maybe even make progress in an argument with another person — or at least not get your shorts all twisted and wind up with an intellectual wedgie? You can find all sorts of advice on this subject from throughout the centuries. A bit of this advice is philosophical, some of it is psychological, and part of it is just plain pragmatic.
First is the pragmatic advice, such as, “In arguing, answer your opponent’s earnest with a jest and his jest with earnest” — Leontinus Gorgias (as quoted in Aristotle’s Rhetoric). [emboldening mine] In other words, keep the person with whom you’re arguing off balance. But this practice is indeed a subset of rhetoric, which is the art of persuasion, and not of philosophy, which is a search for the truth. Most of the pragmatic advice available about argument presupposes precisely that you’re after a win and not after the truth.
The psychological advice warns us most often about the limits of argumentation in dealing with another person and the truth at the same time. Sir Thomas Browne, for example, warned, “In all disputes, so much as there is of passion, so much there is of nothing to the purpose.” Debate, you often hear, typically generates more heat than light. Know that likelihood going in. Passion clouds reason. And in the context of an interpersonal argument, or debate, people sometimes are willing to do anything to save face. Joseph Addison once observed, “Our disputants put me in mind of the scuttlefish, that when he is unable to extricate himself, blackens the water about him till he becomes invisible.” So, as Publilius Syrus concluded long ago, “In a heated argument, we are apt to lose sight of the truth.”
Finally, some modest philosophical advice of a practical bent: Protagorus did affirm that every question has two sides. And Henry Fielding added in the 18th century that “Much can be said on both sides.” Whenever you see sincere, intelligent people supporting a cause or arguing a point of view, you can expect as a maxim of common sense to find more than sheer foolishness in that position or cause. By extrapolation, I think I can say that, in all the history of philosophy, with all the competing schools of thought and opposed points of view, you’re never going to come across large numbers of sincere, intelligent, and relatively well-informed people who are just completely wrong in every way. So always try to remain open-minded and look for the truth that any opposing view may capture. On the other hand, Oscar Wilde warned us, with more than a bit of hyperbole, that “The man who sees both sides of a question is a man who sees absolutely nothing at all.” Neither reason nor common sense dictate or even advise that we aspire to balanced indecision anywhere in life.
And, of course, life consists of much more than argument. Socrates once remarked, “You are fond of argument, and now you fancy that I am a bag of arguments.” You don’t want to avoid argument, and yet neither do you want to constantly seek it out as the only thing in life worth your time. Tell your undergraduate philosophy-major friends: Not even Socrates was a bag of arguments.