March 26, 2026

“I JUST THINK YOU’RE DUMB”:

Why Fallacies Don’t Exist: (except in textbooks) (Maarten Boudry, Feb 12, 2026)

As the saying goes: correlation does not imply causation. If you think otherwise, logic textbooks will tell you that you’re guilty of the fallacy known as post hoc ergo propter hoc. You can formalize it like this:

If B follows A, then A is the cause of B.

Clearly, this is false. Any event B is preceded by countless other events. If I suddenly get a headache, which of the myriad preceding events should I blame? That I had cornflakes for breakfast? That I wore blue socks? That my neighbor wore blue socks?

It’s easy to mock this fallacy—websites like Spurious Correlations offer graphs showing correlations between margarine consumption and divorce rates, or between the number of people who drowned by falling into a pool and the number of Nicholas Cage films released per year.

The problem is that not even the most superstitious person really believes that just because A happened before B, A must have caused B. Sure, in strict deductive terms, post hoc ergo propter hoc is a fallacy—but real-life examples are almost nonexistent. That’s the first prong of the Fallacy Fork.

So what do real-life post hoc arguments actually look like? More like this: “If B follows shortly after A, and there’s some plausible causal mechanism linking A and B, then A is probably the cause of B.” Many such arguments are entirely plausible—or at least not obviously wrong. Context is everything.

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? Yes, says your logic textbook. No, says common sense—at least if your inference is meant to be probabilistic.

Here, the inference is actually reasonable, assuming a few tacit things:

Some mushrooms are toxic.

It’s easy for a layperson to mistake a poisonous mushroom for a harmless one.

Nausea is a common symptom of food poisoning.

You don’t normally feel nauseated.

If you want, you can even spell this out in probabilistic terms. Consider the last premise—the base rate. If you usually have a healthy stomach, the mushroom is the most likely culprit. If, on the other hand, you frequently suffer from gastrointestinal problems, the post hoc inference becomes much weaker.

Almost all of our everyday knowledge about cause and effect comes from this kind of intuitive post hoc reasoning. My phone starts acting up after I drop it; someone unfriends me after I post an offensive joke; the fire alarm goes off right after I light a cigarette. As Randall Munroe, creator of xkcd, once put it: “Correlation doesn’t imply causation, but it does waggle its eyebrows suggestively and gesture furtively while mouthing ‘look over there.’” The problem with astrology, homeopathy, and other forms of quack medicine lies in their background causal assumptions, not in the post hoc inferences themselves.