Logic and Critical Reasoning
What to master
Candidates must handle both formal and informal reasoning. In formal logic, know validity versus soundness, the difference between deductive entailment and inductive support, truth tables, and the connectives. Be able to test arguments for validity by counterexample and to identify common valid forms (modus ponens, modus tollens, disjunctive syllogism) and invalid ones (affirming the consequent, denying the antecedent). Understand quantifiers and the square of opposition at a basic level. In critical reasoning, recognise fallacies: ad hominem, straw man, false dilemma, begging the question, equivocation, hasty generalisation, and post hoc. Distinguish necessary from sufficient conditions, and correlation from causation.
Traps to expect
A valid argument can have false premises and a false conclusion; validity concerns form only. Soundness requires validity plus true premises. Do not label a fallacy merely because you disagree with the conclusion. Watch for modal confusion: 'necessary' and 'sufficient' are routinely swapped. In inductive arguments, strength comes in degrees, unlike validity.
Worked example
'If it rained, the ground is wet. The ground is wet, so it rained.' This affirms the consequent and is invalid; the ground could be wet from a sprinkler. Rule of thumb: for any conditional argument, check whether the second premise affirms the antecedent (valid) or the consequent (invalid).