Logical fallacies are patterns of reasoning that appear valid but are not. They matter because they are everywhere, in political rhetoric, advertising, online debate, and our own thinking, and because recognizing them is a skill that requires deliberate practice.
Fallacies divide into formal and informal:
- Formal fallacies are errors in the logical structure of an argument. They produce invalid deductive arguments regardless of the content. Example: affirming the consequent: "If it rains, the ground is wet. The ground is wet. Therefore, it rained." This is structurally invalid, the ground might be wet for other reasons. - Informal fallacies are errors in reasoning that arise from the content, the misuse of language or evidence, irrelevant appeals, or hidden and false assumptions. An argument can be formally valid and still commit an informal fallacy.
The most important informal fallacies fall into three clusters:
Relevance fallacies, premises that are psychologically persuasive but logically irrelevant:
- Ad hominem: attacking the person making an argument rather than the argument itself. "You can't trust her analysis, she's a known partisan." (The source's motivation may be relevant to assessing their credibility, but it does not refute the argument's content.)
- Appeal to authority: citing an authority as evidence in a domain where they have no special expertise. - Appeal to the crowd (ad populum): "Everyone believes this, so it must be true."
- Tu quoque ("you too"): deflecting a criticism by pointing out the critic's inconsistency rather than addressing the substance.
Assumption fallacies, arguments that assume what they need to prove:
- Begging the question (petitio principii): the conclusion is hidden in the premises. "Men are better at logical reasoning because men are more rational." If "better at logical reasoning" just means "more rational," the argument says nothing. - False dilemma: presenting two options as exhaustive when there are others. "Either you're with us or against us."
- Slippery slope: claiming that one step inevitably leads to an extreme consequence without demonstrating the causal chain.
Ambiguity fallacies, arguments that exploit the flexibility of language:
- Equivocation: using a term with two different meanings as if it had one. "Only man is rational. No woman is a man. Therefore, no woman is rational." ("Man" shifts from "human" to "male.")
- Amphiboly: exploiting grammatical ambiguity in a premise.
A particularly important and slippery fallacy is the argument from ignorance (ad ignorantiam): claiming that something is true because it hasn't been proven false, or false because it hasn't been proven true. "No one has proven that ghosts don't exist, so they must." This conflates absence of evidence with evidence of absence, a confusion with enormous consequences in scientific reasoning, law, and everyday life.
Recognizing fallacies is not the same as refuting the conclusion of the argument that commits them. An argument can commit a fallacy and still have a true conclusion, the fallacy means the argument doesn't establish the conclusion, not that the conclusion is false. This distinction is critical: the correct response to a fallacious argument is "your argument doesn't prove your conclusion", not "your conclusion is therefore false."