identify and explain common informal fallacies in arguments, including fallacies of relevance, ambiguity and presumption

What this dot point is asking

QCAA wants you to spot the reasoning errors that make an argument persuasive but defective. A fallacy is a mistake in reasoning. You need to distinguish formal from informal fallacies, name the major informal fallacies, and explain why each one fails, then apply this to real or stimulus arguments. This is examined across IA1, IA2 and the external exam, and is the analytical backbone of strong essays.

The answer

Formal versus informal fallacies

A formal fallacy is an error in the structure of an argument: the form is invalid regardless of content (for example affirming the consequent, or the undistributed middle in syllogisms). An informal fallacy is an error in the content or context of reasoning, where the words may sound logical but the premises do not properly support the conclusion. Informal fallacies are usually grouped as fallacies of relevance, presumption and ambiguity.

Fallacies of relevance

These offer premises that are psychologically persuasive but logically irrelevant to the conclusion.

• Ad hominem ("against the person"): attacking the arguer instead of the argument. "Her climate claims are worthless because she failed science once." The person's history does not bear on whether the claim is true.
• Straw man: misrepresenting an opponent's position to make it easier to attack, then refuting the distortion rather than the real view.
• Appeal to authority (illegitimate): citing an authority outside their field, or a fake consensus, as proof. Appeal to a genuine, relevant expert is acceptable; appeal to a celebrity on a medical question is not.
• Appeal to emotion / appeal to the people (ad populum): substituting fear, pity or popularity for evidence. "Everyone believes it, so it must be true."
• Appeal to ignorance (ad ignorantiam): claiming something is true because it has not been proven false, or vice versa.

Fallacies of presumption

These smuggle in an unwarranted assumption.

• Begging the question (petitio principii): assuming the conclusion within the premises, so the argument moves in a circle. "The Bible is true because it is the word of God, and we know it is the word of God because the Bible says so."
• False dilemma (false dichotomy): presenting only two options when more exist. "Either we ban the app entirely or accept total surveillance."
• Hasty generalisation: drawing a broad conclusion from too small or unrepresentative a sample.
• Slippery slope: claiming one step must inevitably lead to an extreme outcome without justifying each link in the chain.
• False cause (post hoc ergo propter hoc): treating mere sequence or correlation as proof of causation.

Fallacies of ambiguity

These exploit unclear or shifting language.

• Equivocation: using a word in two different senses within one argument. "Only man is rational; no woman is a man; therefore no woman is rational" trades on two meanings of "man".
• Amphiboly: ambiguity arising from sentence structure or grammar rather than a single word.
• Composition: inferring that what is true of the parts must be true of the whole. "Each player is excellent, so the team is excellent."
• Division: the reverse, inferring that what is true of the whole is true of each part.

Applying fallacy analysis

In a QCAA response you should: (1) name the fallacy precisely, (2) point to the exact move in the argument that commits it, and (3) explain why that move fails to support the conclusion. Naming alone earns little; the explanation of why the reasoning breaks is what is assessed.

Where fallacies sit in philosophy

Fallacy analysis goes back to Aristotle, whose Sophistical Refutations catalogued deceptive arguments used by the Sophists. In Unit 4 social and political philosophy, you will read thinkers such as Thomas Hobbes and John Stuart Mill; their critics often allege fallacies (for instance, that an argument begs the question about human nature). Spotting these moves is the same skill QCAA assesses in Unit 3.

Try this

Q1. Distinguish formal from informal fallacies with one example of each. [3 marks]

• Cue. Formal: structural error, e.g. affirming the consequent. Informal: content or context error, e.g. ad hominem.

Q2. Identify and explain the fallacy: "We cannot prove ghosts do not exist, so they must be real." [3 marks]

• Cue. Appeal to ignorance; absence of disproof is not evidence of truth.

Q3. Explain why "Each brick is light, so the wall is light" is fallacious. [2 marks]

• Cue. Fallacy of composition; a property of the parts is wrongly transferred to the whole.