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…
Here’s the nub of the problem: arguments that are deemed ‘fallacious’ according to the standard approach are always closely related to arguments that, in many contexts, are perfectly reasonable. Formally, the good and bad ones are indistinguishable. No argumentation scheme can succeed in capturing the difference, separating the wheat from the chaff.
That’s Maarten Boudry (Ghent), arguing against the value of emphasizing fallacies (e.g., ad hominem, ad ignorantiam, ad populum, begging the question, post hoc ergo propter hoc, affirming the consequent, argument from authority, and the like).
In a post at his blog (which will be published as an article in Skeptical Inquirer), he summarizes an argument from a paper he co-authored with Fabio Paglieri and Massimo Pigliucci, called the “fallacy fork,” which poses a dilemma for fallacy fans.
The first horn of the dilemma, or prong of the fork, is that if we’re going to be strict about it, in order for a piece of reasoning to suffer from one of the typical fallacies, the reasoning must be explicitly or implicitly in a tightly deductive form, and, as it turns out “we hardly ever find such clear-cut errors, presented in deductive form, in real life.”
The second prong is this: if we’re not going to be strict about it, then we need to formulate our fallacies in a way such that they apply to more than just instances of tightly deductive reasoning; but once we add “some qualifiers and nuances” to capture more instances of reasoning, it is no longer clear that those instances of reasoning are fallacious.
Here’s an example of how the fallacy fork works, with the post hoc ergo propter hoc fallacy:
Every skeptic is familiar with the saying: correlation does not imply causation. To think otherwise is to commit the post hoc ergo propter hoc (or cum hoc) fallacy. The website Spurious Correlations has collected some outrageous examples, with fancy graphs: there is a clear correlation between margarine consumption and divorce rates, and between the number of people who drowned by falling in a pool and the number of films featuring Nicholas Cage (per year). Is there a mysterious causal relationship between these events? If I was ill yesterday and feel better today, to which of the myriad possible earlier events should I attribute my improved condition? That I had cornflakes for breakfast? That I watched a movie with Nicholas Cage? That I was wearing my blue socks? That my next-door neighbor was wearing blue socks?
Not even the most superstitious person believes that correlation automatically implies causation, or that any succession of two events A and B implies that A caused B. There are just too many things going on in the world, and not enough causal connections to account for them. In its clear-cut deductive guise, the post hoc ergo propter hoc inference is a fallacy, to be sure, but hardly anyone makes it in real life. This is the first prong of the Fallacy Fork. So what about the kind of post hoc arguments that people do use in real life? (Pinto 1995) As it turns out, many of those are not obviously mistaken. It all depends on the context.
Imagine you eat some mushrooms you picked in the forest. Half an hour later you feel nauseated, so you put two and two together: “Ugh. That must have been the mushrooms”. Are you committing a fallacy? Not as long as your inference is merely inductive and probabilistic. Intuitively, your inference depends on the following reasonable assumptions: 1) some mushrooms are toxic 2) it’s easy for a lay person like you to mistake a poisonous mushroom for a healthy one 3) nausea is a typical symptom of food intoxication 4) you don’t usually feel nauseated. If you want, you can show the probabilistic relevance of all these premises. Take the last one, which is known as the base rate or prior probability. if I am a healthy person and don’t usually suffer from nausea, the mushroom is most probably the culprit. If, on the other hand, I suffer from a gastro-intestinal condition and I often have bouts of nausea, my post hoc inference will be less strong.
Indeed, almost all of our everyday causal knowledge is derived from such intuitive post hoc reasoning. For instance, my laptop is behaving strangely after I accidentally dropped it on the floor; some acquaintances un-friended me after I posted that offensive cartoon on Facebook; the fire alarm goes off after I light a cigar. As Randall Munroe (the creator of the web comic xkcd) once put it: “Correlation doesn’t imply causation, but it does waggle its eyebrows suggestively and gesture furtively while mouthing ‘look over there’.” Most of the time these premises remain unspoken, but that cannot be a problem per se. Practically every form of reasoning in everyday life, and even in science, contains plenty of hidden premises and skipped steps.
So how about the post hoc arguments that we hear from quack therapists and other pseudoscientists? Someone takes a dose of oscillococcinum (a homeopathic remedy) for his flu, and he feels better the next day. If he attributes this to the pill, is he committing a fallacy? Not obviously, or at least not on formal grounds. It all depends on the plausibility of a causal link, the availability of alternative explanations, the prior probability of the effect, etc. Dismissing any such inferences as post hoc ergo propter hoc fallacies is just a knee-jerk reaction. The real problem with homeopathy is that there is no possible physical mechanism, because of the extreme nature of the dilutions, and because randomized clinical trials have never demonstrated any effect whatsoever. But appealing to post hoc reasoning by itself is not fallacious. We do it all the time when we’re taking real medicine and conclude that it “works for us”.
Professor Boudry provides several other examples in his post. He adds, “None of this is to suggest that people don’t use bad arguments. But lazy and sloppy arguments are much more common than cut-and-dried fallacies.”
The whole post is here.