One of the primary interests of skepticism is to counter forms of sloppy thinking and fallacious reasoning that are so prevalent in the culture at large. But in doing this, and compiling lists of “common logical fallacies,”^* there is a danger of interpreting a proper skeptical position too rigidly. Upon closer inspection, many of the so-called logical errors turn out to be based on useful rules of thumb in inductive reasoning. Moreover, skeptics and scientists regularly rely on these, and legitimately so.

As an example, let me discuss ad hominem and authority-invoking arguments. Broadly speaking, these are similar: they attempt to influence the credibility of an assertion by bringing in information about its source. At first, this appears to be obviously fallacious, since the validity of an argument is independent of the name it is presented under. This is entirely correct; however, it does not follow that information about the source is irrelevant in evaluating claims with empirical significance.

Imagine a controversial claim, something like “strong new evidence for ESP!” Say also that the formal quality of the argument used to establish this conclusion is impeccable; the statistics are flawless, and the data do indeed support the claim. Would we then take this more seriously if it were published in Nature by a team including James Randi, or, if the identical paper was the product of a lab infamous for sloppy work and perhaps outright dishonesty?

Any time a reference is made, or when data are presented, an element of trust in the competence of the claimant comes into play. Thus any empirically connected assertion is unable to stand alone like a result in pure mathematics might be able to, regardless of a reputation of the mathematician for displacing minus signs. Outside information is relevant, and this is implicitly acknowledged whenever a reference is made to a physics textbook in discussing the influence of constellations on persons, rather than a manual of Black Magic. Practically no one is able to check all physical assertions, but we may have a rational basis for substantial trust in scientific institutions—a legitimate authority. And arguments from fraud against “psychical research,” expressing lack of such institutional trust, are analogous to ad hominem arguments.

It is the improper use of external information that skeptics should be concerned with, rather than hunting down seeming fallacies. If someone claims that a skeptics’ position on Tarot divination can be discounted on the basis of membership in the Beer Drinker’s Party, the response to it would be to point out the irrelevance of that particular bit of information to the issue—but it is not automatically made so because of the personal attack form of the argument. Or, even if it is granted that membership in such a party is an irrational act, it can be said that knowing this would have at best a very weak effect on the evaluation of the argument.

I propose that the prevalence of certain forms of “fallacious” reasoning would not be explained best by a perverse irrationality, but as a misapplication of otherwise useful rough rules. They represent tools in plausibility reasoning, not in strict logical proof or disproof. Abuse of such tools is most often by overstatement, treating weak support as strong or even overriding of other considerations.

Skeptical arguments castigating seeming fallacies should also be understood on a practical, nonabsolute basis, indispensable for a rational process in a complex and uncertain environment. This goes for many principles mistakenly seen as ironclad, not only arguments invoking authority. The requirement of falsifiability for scientific theories, for example, also cannot be interpreted strongly: almost no empirically referent statement of interest is logically falsifiable. Take “all crows are black,” an ad hoc extension is always available to save face—the white crow said to be observed may have been a mistake, or somebody might have painted it white. Nevertheless, insistence on a practical sense of testability has its merits.

Interpreting certain “fallacies” as transgressions against logic is a form of thinking in terms of deductive proof instead of inductive support. The interest of skeptics and scientists is in the latter, trying to obtain support rather than strict proof. And the process involved is invariably approximate and comparatively inelegant. It may be superficially more effective to classify fallacies and lay down the laws of proper reasoning when engaging in public education. But ultimately, it is also desirable to foster at least an implicit understanding of the complexities involved in real life inference.

* : The skeptical literature contains many examples, e.g. the section on “Common Fallacies” in W.D. Gray’s Thinking Critically About New Age Ideas, Wadsworth Publishing Co., 1991.