The rhetorician Quintilian, in his study of the orator’s education (IV.2.52), stated that there were four factors affecting the credibility of a forensic argument.  By forensic argument, we mean a speech or writing that examines past events and evidence in order to evaluate culpability.

Credibility, according to Quintilian, turns on four factors, which are as follows:  (a) an evaluation of our inner feelings to determine what we believe is right and natural; (b) proving the listener with reasons and motives for the events that have taken place; (c) setting up personalities consistent with those deeds which we want to be believed; and (d) offering specific places, dates, and times for the actions under consideration. 

Letter (c) in the previous sentence needs some additional explanation.  What does Quintilian mean by “setting up personalities consistent with those deeds which we want to be believed” (personas convenientes iis quae facta credi volemus constituerimus)?  He means that the actions of the person we are prosecuting or defending must be described in ways that advance our arguments.  A thief, therefore, should be characterized by a prosecutor as greedy, an adulterer as libidinous, and a murder as impetuous; the lawyer defending such accused individuals would of course adopt opposing characterizations.  These, then, are the four factors that contribute to the success or failure of an argument’s credibility. 

We may see how these factors operate by considering a recent case which progressed to a jury trial.  The defendant was charged with stalking and harassment of his ex-wife, with whom he had several children.  The complaint against him specified that he had a pattern of unwanted telephone and text message contact, and that he had installed an Apple Air Tag device in the trunk of the victim’s car to track and monitor her movements.  To explain the defendant’s behavior, the prosecutor stated that he was motivated by jealousy and revenge.  According to this argument, the defendant could not accept that the relationship with his ex-wife was over, and nurtured an obsessional curiosity about whom she was dating and visiting. 

On cross-examination, the prosecutor grilled the defendant about his messages and calls to the victim, as well as the tracking of the victim’s movements.  Since many of the text messages were of an uncomfortably salacious nature, the presentation of such messages to the jury did a great deal to shape their negative perceptions of the defendant.  Specific times, places, and dates additionally enhanced the credibility of the prosecutor’s argument.

Against all this, the attorney for the defense offered his own narrative, relying on Quintilian’s factors as described above.  The defendant had several children with the alleged victim, and had every right to communicate with her to discuss child visitation and other related issues.  The defendant’s motive in installing an Air Tag device in the trunk of the victim’s car, it was argued, was simply a way for the defendant to monitor the collateral (the vehicle) which he was responsible for, and which he was paying for.  In other words, he was tracking the car as property, and not the victim.  While it was true that the defendant sent a number of sexually explicit messages to the victim, it could be argued that the defendant and the victim had engaged in this sort of talk before, and that it was not unusual. 

So while the prosecutor sought to portray the defendant as jealous and unstable, the defense sought to portray him as a distressed, harried single father doing his best to monitor property he was responsible for, whose conduct did not rise to the level of unwanted contact.  In the end, of course, the decision on whose characterization to accept was in the hands of the jury.  But in enhancing the credibility of an argument, I have found it very productive to weigh each of Quintilian’s four factors.  They are useful not only to lawyers and composers of speeches, but to anyone seeking to buttress the credibility of a sustained persuasive effort. 

I will also say that his first factor (the evaluation of our inner feelings to determine what we believe is right) is far more important than one might think.  Credibility can be sensed on an unconscious level; it must come from the soul.  You must believe in your argument; it must spring from your soul’s innate sense of right and wrong.  This kind of inner conviction radiating from a speaker is contagious, and can overcome the hesitations of even the most skeptical auditor.       

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Read more essays on various topics in history, character, and morals in the collections Digest and Centuries.