Post Hoc

In plain terms

Post hoc ergo propter hoc is Latin for "after this, therefore because of this." The fallacy treats sequence as causation: B happened after A, so A must have caused B.

Sequence is necessary for causation but nowhere near sufficient. Two events can follow each other through coincidence, through a shared third cause, or because the second would have happened no matter what came before.

Why it's fallacious

Causation requires more than timing. It requires a mechanism (some plausible way A could produce B) and usually some comparison case (what happened in situations where A didn't occur). Post hoc reasoning skips both. The before-and-after is taken as the whole story, and everything else that might explain the outcome is quietly ignored.

The fallacy is everywhere. Human brains are wired to find patterns, and the temporal pattern of "this, then that" is almost automatic. Noticing the pull toward it is the first step in not being ruled by it.

Canonical example

"I started taking elderberry syrup in October. I didn't get a cold all winter. The syrup works."

The cold-free winter followed the syrup, but so did a lot of other things. The person might have washed their hands more. They might have avoided a sick coworker. The flu season might have been mild. They might have gotten colds in other years without syrup anyway. One winter, from one person, tells us almost nothing about whether the syrup is causally doing anything.

Counter-example (not a fallacy)

"Infant mortality in the city dropped 22% in the two years after the water system was overhauled. Follow-up studies traced the reduction to lower rates of waterborne illness, which matched the specific contaminants the overhaul removed. The timing is part of the argument, but the mechanism is separately shown."

This isn't post hoc. The claim of causation is supported not just by the timing but by a mechanism (contaminant removal) and by follow-up investigation that tied the drop to the specific change. Sequence plus mechanism plus evidence of the mechanism operating is how causation is established. Sequence alone is just noticing.

How to respond when you see it

Ask two questions. First: what else happened at the same time that could also explain this? Second: is there a plausible way the first thing produced the second? Most post hoc claims don't survive the first question. The ones that do are often still just shy of the second.