Unit 3: Reason, Logic and Argument in Practice
What you must understand
Unit 3 deepens formal and informal reasoning. You must handle propositional logic (negation, conjunction, disjunction, conditional), truth tables, and the standard valid forms - modus ponens, modus tollens, disjunctive syllogism - alongside the classic fallacies: affirming the consequent, denying the antecedent, begging the question, ad hominem, equivocation, and false dilemma. You also need to assess inductive reasoning, including generalisation, analogy, and the problem of induction, and to recognise cognitive bias and rhetorical manipulation.
Traps that appear in questions
- Denying the antecedent. 'If P then Q; not P; therefore not Q' is invalid, but it looks like modus tollens at speed.
- Confusing correlation with causation. Inductive generalisations require representative samples and relevant variables.
- Mistaking a counterexample for a refutation of the form. One bad instance weakens an inductive generalisation but does not invalidate a deductive form.
- Equivocation. A term shifts meaning mid-argument; check that key terms stay constant.
Worked example
'If the policy is just, it treats everyone equally. The policy does not treat everyone equally. Therefore it is not just.' This is modus tollens and is valid: P → Q, ¬Q, therefore ¬P. Contrast with: 'The policy is not just. Therefore it does not treat everyone equally.' That is denying the antecedent and invalid.
Rule of thumb
Write the argument in symbols. If the conclusion is the negation of the antecedent, you have almost certainly committed a fallacy.