Rhetoric and Logic: Friends and Foes

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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

Gil
Guillory [send him mail]
is a chemical engineer in Houston.

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