Informal fallacies

Core idea

An informal fallacy is an argument that fails for reasons of content rather than form. The structure of the inference may be impeccable; the failure is in what the premises mean, what they conceal, or what psychological effect they are exploiting on the audience. Informal fallacies are the diseases of real-world reasoning — the moves that win debates without earning the conclusion.

The catalogue breaks into two families. Fallacies from the abuse of language exploit instability of meaning (the topics on meaning sets these up). Fallacies of relevance appeal to features of the audience or situation that have no logical bearing on the conclusion. Naming a fallacy is not itself a refutation — the conclusion may still be true — but naming it isolates where the bad reasoning sits and lets you ask the speaker to do the missing work.

Why it matters

Why the catalogue is open-ended

Logicians count anywhere from a dozen to a hundred named informal fallacies. The list keeps growing because new rhetorical tricks emerge and new contexts (advertising, social media, AI-generated text) surface old tricks in new clothes. The remedy is not to memorize every name, but to internalize the two diagnostic questions that detect any fallacy in either family.

Naming is not refuting

A common mistake is to think "you committed an ad hominem" is itself an argument. It is not. It is a flag: the offered support does not support the conclusion. The speaker still owes you an argument that does support it.

Mental model

The two families of informal fallacy

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The diagnostic flow

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The flagship fallacies you will see every day

A few are worth memorizing because they appear constantly in media, politics, and corporate reasoning:

• Equivocation — using one word in two senses across the argument. (A feather is light; what is light cannot be dark; therefore no feather is dark.)
• Slippery slope — claiming that a small concession must cascade into catastrophe, without arguing for the intervening steps.
• False dilemma — presenting two options as exhaustive when others exist. ("You're either with us or against us.")
• Straw man — refuting a crude caricature of the opponent's position rather than the position itself.
• Ad hominem — attacking the speaker's character or circumstances rather than their argument.
• Tu quoque — dismissing a claim because the speaker does not act on it. ("You drive a car, so you can't criticize fossil fuels.")
• Appeal to ignorance — concluding that something is true because it has not been proved false, or false because it has not been proved true.
• Begging the question — assuming in the premises the very thing the conclusion is supposed to establish.

Practical application

Once a piece of reasoning is suspected of containing a fallacy, run a three-step diagnosis:

1. Identify the conclusion the speaker wants you to accept. Strip rhetoric. Write it as a single declarative sentence.
2. Diagnose the family. Apply the two diagnostic questions in order. Does any term shift? Is any premise about the speaker, audience, or emotion?
3. Name the specific fallacy and say what missing work the speaker now owes. "You attacked her character; the argument that wages should rise is still on the table. Address it."

Step 3 is the critical move. Naming a fallacy without saying what the speaker now owes leaves you sounding pedantic. Naming it and pointing to the unaddressed substance moves the conversation forward.

Example

A real online exchange:

• Anna: "I think we should adopt remote work full-time. Studies from MIT, Stanford, and Microsoft all show productivity is stable or higher with no commute."
• Ben: "Of course you think that — you live two hours from the office. You'd say anything to not have to come in."
• Anna: "That's a personal attack. Address the studies."
• Ben: "Look, if we go fully remote, in five years no one will come to the office at all, and then we have no company culture, and then the company collapses."
• Carol: "We have to either stay fully on-site or go fully remote. Pick one."

Three fallacies in three short turns:

• Ben's first response: circumstantial ad hominem — Ben dismisses Anna's claim because of her personal circumstances (the commute), not because of the studies she cites. Even if Anna's motives are mixed, the studies stand or fall on their own merits.
• Ben's second response: slippery slope — Ben asserts that a small concession (some remote work) must cascade into catastrophe (company collapse) without arguing for any of the intervening steps. Why would full remote eliminate culture? Many fully remote companies have strong culture; many on-site companies don't.
• Carol: false dilemma — Carol presents two options (fully on-site, fully remote) as exhaustive when a hybrid is the obvious middle ground actually used by most companies. The framing rules out the most likely answer before the discussion starts.

The right move now is not to walk away in disgust. It is to name each fallacy and point at the substance the speakers now owe: address the studies, justify each step of the cascade, and explain why a hybrid is off the table. If they can do those three things, the conversation moves forward. If they can't, that is the diagnostic — they had no argument, only rhetoric.

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