⚖️ What Is a Fallacy?
A fallacy = an error in reasoning.
Two main categories:
- Formal Fallacies – structural/logic errors
- Informal Fallacies – flaws in content or relevance
🧩 Formal Fallacies
| Fallacy | Invalid Form | Description / Example |
|---|---|---|
| Affirming the Consequent | If A → B; B; ∴ A | “If it rains, the ground’s wet; ground’s wet → it rained.” |
| Denying the Antecedent | If A → B; ¬A → ¬B | “If I study, I pass; I didn’t study → I won’t pass.” |
| Exclusive Or Fallacy | Reads inclusive OR as exclusive | “I’ll go to a movie or do homework → did one so not the other.” |
| Illicit Predicate Instantiation I | All S → P; x is P; ∴ x is S | “All humans are mortal; Socrates mortal → Socrates human.” |
| Illicit Predicate Instantiation II | All S → P; ¬S → ¬P | “All humans are mortal; rocks not human → rocks not mortal.” |
| Illicit Universal Syllogisms I–III | Mixes class relations wrongly | e.g., “All dogs are mammals, all mammals animals → all animals dogs.” |
🧠 Informal Fallacies
🔹 Fallacies of Irrelevance
Premises irrelevant to conclusion.
| Type | Description | Example |
|---|---|---|
| Genetic Fallacy | Attacks source/origin | “That claim comes from a liberal paper.” |
| Ad Hominem (to person) | Attacks arguer not argument | – Abusive: “He’s corrupt.” – Circumstantial: “She benefits from her claim.” – Tu Quoque: “You do it too.” |
| Equivocation | Term used ambiguously | “Perfect will = infinite will.” |
| Argument from Ignorance | True ↔ not disproven | “No one disproved aliens → they exist.” |
| Red Herring | Diverts topic | “Other companies pollute worse!” |
| Straw Man | Misrepresents opponent’s view | “Universal health care = socialism.” |
| Appeal to Emotion | Plays on feelings not logic. – Force (ad baculum): threat – Pity (ad misericordiam): sympathy | |
| Appeal to Popularity (ad populum) | “It’s true because everyone agrees.” | |
| Appeal to Tradition | “It’s right because it’s always been done.” | |
| Composition | True of parts → true of whole | |
| Division | True of whole → true of parts |
🔹 Fallacies of Problematic Premises / Insufficiency
| Fallacy | Description | Example |
|---|---|---|
| Begging the Question | Premise = conclusion | “God exists because there’s a providential order.” |
| False Dilemma | Only two options given | “You’re with us or against us.” |
| Slippery Slope | Unproven causal chain | “Allow X → will cause Y → end of freedom.” |
| Hasty Generalization | Small/unrepresentative sample | “Two friends like Jags so repairs are cheap.” |
| Faulty Analogy | Weak comparison | “Mind like a computer → has total recall.” |
🔹 Additional Informal Fallacies (From Week 2 - Theory of Definitions)
| Fallacy | Core Error | Example |
|---|---|---|
| Accident | Over‑applying a general rule | “Everyone has free speech → so hate groups should spread hate.” |
| False Cause | Faulty causal inference | – Post Hoc: “A→B because A preceded B.” – Oversimplification: ignores multiple causes. – Cause ↔ Effect: reverses causality. |
| Complex Question | Traps answer with bias | “Were you running when you stole the purse?” |
| Suppressed Evidence | Ignores relevant data | “Teachers make more now → so shouldn’t complain.” |
| Subjectivism / Wishful Thinking | Belief = truth / Want = truth | “Company won’t fail because I need it not to.” |
| Two Wrongs | Wrong justified by another | “Murderer deserves death for killing.” |
| Inconsistency | Self‑contradictory reasoning | “Follow the law—but defendant should be exempt.” |
| Irrelevant Conclusion (Non Sequitur) | Premise supports irrelevant claim | “Schools underfunded → raise taxes now.” |
⚗️ Identifying Fallacies in Arguments
| Scenario | Likely Fallacy |
|---|---|
| All rats are rodents; all gerbils rodents → gerbils are rats | Illicit Universal Syllogism |
| “Whenever I think of a friend, I run into them.” | Post Hoc / False Cause |
| “I know my friend is innocent—it’s too hard to believe otherwise.” | Wishful Thinking / Subjectivism |
| “Pensions low → cut military spending.” | Irrelevant Conclusion |
| “Ben Franklin said ‘a stitch in time’ → change oil every 1000 miles.” | False Analogy / Faulty Application |
| “Don’t do as I say, do as I do.” | Inconsistency / Self‑contradiction |
| “Draft fixed by sealing ceiling crack.” | Oversimplified Cause |
| “All renates are cordates; fish are cordates → renates are fish.” | Illicit Universal Syllogism |
| “I cheated on insurance—they cheat too.” | Two Wrongs |
| “Free speech → hate groups okay.” | Accident |
🧩 Quick Recognition Summary
| Category | Core Feature |
|---|---|
| Formal | Invalid logical form |
| Irrelevance | Premises don’t connect to conclusion |
| Problematic Premise | Missing, circular, or false assumption |
| Insufficiency | Evidence too weak or incomplete |
| Causal | Unjustified causal link |
| Emotive/Persuasive | Appeals to emotion or prejudice |
✅ How to Evaluate an Argument
- Identify type – deductive or inductive.
- Test form – valid pattern? any formal fallacy?
- Test content – relevance? evidence quality? definition clarity?
- Detect emotional language – manipulative appeals?
- Name the fallacy – classify & explain.
🧾 Final Recap
- Formal Fallacies = structural logic errors.
- Informal Fallacies = relevance, sufficiency, or content defects.
- Fallacy detection depends on context — not all ad hominems, force, or pity appeals are wrong (e.g., legal/political contexts).
- Key goal: Evaluate why reasoning fails, not merely label.