What Makes a Piece of Reasoning Fallacious

An argument has two separate jobs: its premises have to be true, and the path from premises to conclusion has to actually carry that truth across. A fallacy is a failure of the second job. The premises can be perfectly accurate, the conclusion can even turn out to be correct, and the argument can still be fallacious, because the reasoning connecting them does no real work. That's why bad arguments feel convincing in the moment: your attention is on whether the pieces are true, not on whether they're wired together. An hour later, you notice the wiring.

Fallacies split into two families. Formal fallacies break on structure alone: the shape of the argument would turn true premises into a false conclusion regardless of subject matter, the way affirming the consequent does. You can spot these by testing the pattern with any content and watching it fail. Informal fallacies keep a valid-looking shape but go wrong on content: a premise that is true gets smuggled in as if it were relevant to the conclusion when it isn't. A person's wealth, job history, or unpopularity can be completely true and completely beside the point of whether their argument holds.

This is also why a fallacious argument doesn't prove its conclusion false. Calling out bad reasoning only shows that this route to the conclusion fails; the conclusion might still be true for other reasons. And the same sentence can switch sides: pointing out a conflict of interest is a fallacy if it's used to declare a conclusion false, but sound if it's used only to lower how much weight the conclusion deserves. Context decides which job the sentence is doing.