Tuesday, May 24, 2011

Fallacies (a New Series in the Manner of Cognitive Biases) - Part 1

As a follow-up to my series on cognitive biases, here’s the start of a new one: a description of classical fallacies. For the purposes of this discussion, a fallacy is an opinion or position based on invalid reasoning. The conclusion itself may not be wrong, but the argument offered in support of it is wrong.

“He plays a doctor on TV; he endorses this medical product; therefore, the medical product is good” is fallacious not because the medical product being touted is necessarily not good, but because it’s ad verecundiam, an appeal to an authority outside the authority’s area of expertise. If the same authority made a statement about the profession of acting, however, it wouldn’t be ad verecundiam because the actor presumably has standing on the subject.

Just because the actor has standing, however, doesn’t constitute a final proof that the proposition is true. At most, it counts as an evidence point — but it’s a logically valid one.

For the list of fallacies, I’m starting with the list on Wikipedia. There’s another extensive list at The Nizkor Project.  The examples and discussion are my own.

Fallacies come in two basic flavors.

Formal fallacies, as the name suggests, are errors of form: regardless of the contents of the argument, or the truth of any of the statements, the argument is invalid on its face.  An appeal to probability is one such fallacy: the idea that if something could happen, it necessarily will happen.

There are subsets of formal fallacies. Propositional fallacies are structural errors in logic.  “It’s raining or it’s Tuesday; it’s not raining, therefore it’s Tuesday” is a propositional fallacy (affirming a disjunct). It may not be raining, but that doesn’t make it Tuesday.

Syllogistic fallacies also fall into the category of formal fallacies. “Some cats are black; some black things are televisions; therefore some cats are televisions” involves an illicit treatment of the minor term (you can’t assume that the set of televisions and the set of cats have common members because they share a particular color).

Informal fallacies are not fallacious because of their structure, but usually because of their content. Ad verecundiam and its kissing cousin ad hominem both use a personal characteristic to prove or disprove a proposition without establishing that the personal characteristic in question actually determines whether the conclusion is true or false.

I learned a lot doing the cognitive biases series, and I hope to learn a lot on this project as well. If you’ve seen any great examples of any of these fallacies, please let me know.


  1. Michael, I love your series on cognitive biases and look forward to this series. They are clear and well-written. They have helped me to articulate my own thoughts more accurately and have a good resource to pass on to others.

    Our schools should include critical thinking skills as a subject. When I am queen, we will have fun & funny public service announcements on TV and radio teaching critical thinking skills. Only when we think clearly and accurately will we be able to have real democracy and protect ourselves from charlatans who would exploit our ignorance.