THE FALLACY FORK

Fallacy theory is popular among skeptics, but it is in serious trouble. Every fallacy in the traditional taxonomy runs into a destructive dilemma that I call the “Fallacy Fork”: either it hardly ever occurs in real life, or it is not actually fallacious.

Why do people believe weird things? Why is there so much irrationality in the world? Here’s one common answer from the skeptic’s playbook: fallacies. Fallacies are certain types of arguments that are common, attractive, persistent, and dead wrong. Because people keep committing fallacies, so the story goes, they end up believing all sorts of weird things.

IN popular books about skepticism and in the pages of skeptical magazines such as this one, one commonly finds a concise treatment of the most common types of fallacies. The traditional classifications are widely known, often by their Latin names: ad hominem, ad ignorantiam, ad populum, begging the question, post hoc ergo propter hoc. Some of them are more obscure, such as ignoratio elenchi, affirming the consequent, secundum quid, and ad verecundiam, better known as “argument from authority.” Most of them date back to the days of Aristotle; others are relative newbies, such as the slippery slope fallacy, the genetic fallacy or—for obvious reasons— the reductio ad Hitlerum.

Such lists serve a pedagogical purpose. By learning the most common types of reasoning errors, you will help avoid making them yourself and become better at spotting them when others do. It’s a kind of inoculation against irrationality. If only people would learn the list of fallacies, the world would be a far more rational place!

Except this neat little story is wrong.

Skeptical about Fallacies

I used to teach a course in critical thinking at Ghent University. As behooves a good skeptic, I first presented my students with the usual laundry list of fallacies, after which I invited them to put the theory into practice. Take a popular piece from the newspaper or watch a political debate and try to spot the fallacies.

I no longer give that assignment. My students became paranoid! They began to see fallacies everywhere. Rather than dealing with the substance of an argument, they just carelessly threw around labels and cried “fallacy!” at every turn. But none of the alleged “fallacies” they spotted survived a close inspection.

Were my students to blame? I had to confess that when I did the exercise myself and looked for clear-cut fallacies in real life, I came away mostly empty-handed. Perhaps because my students didn’t find any clear instances of fallacies, they started to make them up? So I turned to the classics. The Demon-Haunted World (1996) by Carl Sagan, perhaps the most celebrated work in the skeptical library, has a special section on reasoning fallacies, like many other books in the genre. But although Sagan duly lists all the usual suspects, he never puts them to work in the rest of the book. His treatment comes across as perfunctory, and he hardly gives any examples from real-life pseudoscience. Like many other skeptics, Sagan just invents some toy examples, which are easy to knock down but don’t actually correspond to real-life arguments. It seems that Sagan is paying lip service to fallacy theory but has no use for it in his actual debunking work.