Logical Fallacies
A fallacy is a defect in the shape of an argument. The conclusion may still be true — but the reasoning given for it doesn't support it. Knowing the names lets you point at the broken part precisely.
Argument, not opinion
An argument has two parts: premises (the reasons given) and a conclusion (what they're meant to support). An argument is valid when accepting all premises forces you to accept the conclusion. A fallacy is a recurring way that structure breaks.
A fallacious argument can have a true conclusion. The fallacy is about whether the reasons earn it — not about the conclusion itself.
Two families: formal and informal
Formal fallacies are broken regardless of content — they fail the logic schema (affirming the consequent, undistributed middle).
Informal fallacies — most of this catalog — are broken by what's missing or misdirected in the content: relevance, ambiguity, presumption, emotion. Aristotle's Sophistical Refutations (4th c. BCE) collected the first 13; the modern list runs into the hundreds.
Filter the catalog
Argument deconstructor
Diagnose the argument
Read the argument. Identify what type of fallacy (if any) the move commits. The premises and conclusion are split out for you, the way you'd diagram it in a logic textbook.
Spot the fallacy
Read each real-world style argument. Pick the fallacy most clearly at work.
Connect the dots
Quiz
15 questions on logical fallacies.
Flashcards
Tap to flip. Twenty-four classic fallacies.
Teacher mode
Lesson outline, quick-reference card, and a printable worksheet with answer key.
Lesson outline (45 min)
- 5 min · Hook — Read aloud a short paragraph of campaign or ad copy. Students raise hands every time they hear a fallacy. Tally on the board.
- 10 min · Concept — Premises and conclusion. Valid vs. sound. Why a true conclusion can ride on bad reasoning. Brief history (Aristotle's Sophistical Refutations).
- 10 min · Catalog — Walk the six families: relevance, structural, ambiguity, presumption, causal, emotional. One example each.
- 15 min · Practice — Students diagnose 5 short arguments from the Spot-the-fallacy section. Defend their pick to a partner.
- 5 min · Wrap — Homework: find one fallacy in any media this week, name it, write the rebuttal in one sentence.