Identify and explain common formal and informal fallacies and diagnose them in given arguments without committing the fallacy-fallacy

What this dot point is asking

SEAB wants you to recognise, name and explain the recurring patterns of bad reasoning, and to spot them in real arguments. Fallacies are the standard failure modes of argument, and a large part of the critical thinking paper is detecting them. Equally important is the discipline of not over-diagnosing: labelling an argument fallacious shows it fails to support its conclusion, not that the conclusion is false.

The answer

Formal versus informal fallacies

A formal fallacy is an error in the logical form of an argument: the conclusion does not follow from the premises whatever their content. Affirming the consequent (if P then Q; Q; so P) and denying the antecedent (if P then Q; not P; so not Q) are the standard examples. An informal fallacy is an error in content or context that cannot be read off the form: a failure of relevance, clarity, or evidence. Most everyday fallacies are informal, which is why diagnosing them requires reading the argument, not just its shape.

Fallacies of relevance

These offer considerations that do not bear on the conclusion. The ad hominem attacks the arguer instead of the argument (their character, circumstances or motives), as if that refuted their reasons. The straw man misrepresents an opponent's position as a weaker one and attacks that. The appeal to authority cites a source whose authority is irrelevant, unqualified, or outside their field (a legitimate appeal to a genuine expert in their field is not fallacious). Appeals to popularity (everyone believes it) and to emotion (fear, pity) substitute feeling or numbers for reasons.

Fallacies of presumption

These smuggle in an unwarranted assumption. The false dichotomy presents two options as if they exhausted the possibilities when they do not. Begging the question assumes the conclusion among the premises, so the argument is circular. The slippery slope claims that one step will inevitably lead to an extreme outcome without showing the intervening steps are likely. Hasty generalisation draws a broad conclusion from too small or unrepresentative a sample.

Fallacies of ambiguity

These exploit unclear language. Equivocation shifts the meaning of a key term between premises, so the argument only appears valid (a famous toy example trades on two senses of a word). Amphiboly trades on ambiguous grammar. These connect to the language area: vague or shifting terms are a frequent source of bad argument, which is why precise definition matters.

Diagnosing fallacies fairly

Two cautions. First, charity: reconstruct the argument in its strongest form before declaring it fallacious, because a charitable reading sometimes dissolves the apparent error. Second, the fallacy-fallacy (argument from fallacy): from the fact that an argument for a claim is fallacious, it does not follow that the claim is false. A true conclusion can be defended by a bad argument. So the correct response to a fallacy is to withhold endorsement of that argument, not to assert the opposite of its conclusion.

Examples in context

Example 1. The false dilemma in debate. A speaker says, "We either accept mass surveillance or we surrender to terrorism." This presents two options as exhaustive, ignoring middle paths such as targeted, warrant-based monitoring with oversight. Naming it a false dichotomy and pointing to the omitted alternatives is the correct diagnosis; it shows the argument fails without claiming surveillance is definitely wrong.

Example 2. Equivocation on a key term. "Laws require a lawgiver. There are laws of nature. So nature has a lawgiver." The word "law" shifts from a prescriptive rule (which does require a legislator) to a descriptive regularity (which does not). The argument looks valid but trades on two meanings, so it equivocates. Spotting the shifting term is the heart of diagnosing fallacies of ambiguity, and it ties to the importance of stable definition.

Try this

Q1. Distinguish a formal from an informal fallacy with one example of each. [6 marks]

• Cue. Formal: an error of form regardless of content, such as affirming the consequent. Informal: an error of relevance, presumption or ambiguity, such as ad hominem or false dichotomy, detectable only by reading the content.

Q2. Explain the fallacy-fallacy and why it matters. [8 marks]

• Cue. It is the error of concluding that a claim is false because an argument for it is fallacious; it matters because a true claim can be supported by a bad argument, so a fallacy defeats the argument, not the conclusion.

Q3. Identify the fallacy: "Of course exercise is good for you. Everyone knows it." [6 marks]

• Cue. Appeal to popularity (the bandwagon): that a belief is widely held is not by itself a reason that it is true; the claim may be true, but this gives no genuine evidence.