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Rhetoric and Logic: Friends and Foes

The formal instruction of both logic and rhetoric receives spotty coverage at best in modern curricula. A well-rounded education should include both, and in heavy doses. Especially for those of you that find yourself arguing with others, or attempting to persuade others, you may be interested in some of my favorite resources for logic and rhetoric:

It is this last resource that drew my attention to van Eemeren and Gootendorst's pragma-dialectical theory of argumentation, which demonstrates an interesting tension between rhetoric and logic that should be kept in mind by those who wish to deploy and critique arguments.

As Hayek and Popper stressed, we are always in a position of imperfect knowledge, and social institutions often evolve to economize on the transmission of knowledge. A favorite foil of the neoclassical economist is the application of Bayes's theorem to general problems of knowledge and certainty. While this analogy is sometimes overdrawn (much as the second law of thermodynamics and the statistical thermodynamic interpretation of entropy as "disorder" are overdrawn on occasion), there is truth to the proposition that there exists in the mind of a target audience member a good number of "priors". Priors are commonly incomplete pictures of reality with some degree of truth content. Working against this backdrop, rhetorical work must often employ strong tactics to persuade.

Many of the strong rhetorical moves are logical fallacies, and so are avoided by some writers. In this Hayekian/Bayesian framework, this strategy must be rethought. Sometimes it is useful and even valid to use an ad hominem attack, or an appeal to authority, or other moves that are, strictly speaking, logical fallacies. I will demonstrate this surprising method.

Ad Hominem

Does it matter whether a person advocating the observance of the 7th Commandment cheats on his wife regularly? Here, we can invoke the charge of performative contradiction.

Does it matter whether a person takes his funding from a party in whose cause he has enlisted? Yes. We know that the lure of money can sometimes cloud judgment and blind one to the pursuit of truth. Virtually no man will change his mind in public, but the pressure brought to bear on a man not to abandon falsehoods even in private due to a pecuniary tie can be strong.

What is important to see here is that ad hominem, while a logical fallacy, can convey useful information to the audience about the possible prejudices, commitments, and motivations of a disputant. While not decisive, such information can, and perhaps should, condition the audience's reception of claims.

There are a multitude of people in the world that attempt to persuade us. Given constraints on resources, any sort of information about someone can inform our decision of whether or not to give someone the benefit of the doubt, or to listen with skepticism, or to reject someone's opinion as not worthy of even entertaining. This bleeds into the next topic.

Appeal to Authority

An authority is someone in a position to know, or likely to know, and who is immersed in an incentive structure that tends not to bias answers. So, without even knowing anyone on the Code Committee of the American Society of Mechanical Engineers, I readily pick up ASME B31.3–2004 and look up that the allowable yield stress of A106 carbon steel at 300F is 20,000 psi. If a mechanical engineering colleague were to show me a lab book of his own yield tests that were at odds with the value in the ASME code, I would still be more persuaded by the ASME number. I fully realize that he may be right, and that they may be wrong; and yet, this appeal to authority is proper.

If two professors are arguing an empirical point, and you haven't the time to invest to run the issue to ground, whom do you believe? The professor with the most authority, as you perceive him to be. In my case, that might be someone with better free-market credentials, or someone whose main area of research is the subject of the dispute, or someone that I personally know and regard to be of high integrity. These are not wrong ways of deciding questions – rather, they are appropriate to the human condition.

The Upshot

There are other "exceptions" to virtually every logical fallacy. We should not revel in these rhetorical flourishes. They should be used with care, in the right circumstances. Generally, context will show where a particular logical misdemeanor is both rhetorically persuasive and morally acceptable.

There are innumerable rhetorical strategies well-covered by Cloud, Zarefsky, and others. One can get hide-bound in restricting one's rhetorical playing ground. Don't do it. If there's one paramount lesson from Cloud, it is this: when the rhetoric you employ does not persuade, do something different. Change it and change it and change it, until it works – that is, it persuades.

Rhetoric and logic are best learned by practice. Read arguments. Critique the bad ones and imitate the good ones. Try them on people, then return to your texts on logic and rhetoric. Where did you go wrong? How can you alter your approach? Rinse and repeat. Have fun!

January 24, 2006