Critical Reasoning: Analysing and Assessing Arguments
The anatomy of an argument
An argument is a set of propositions in which some (premises) are offered as support for another (the conclusion). Identify the conclusion first, then the premises, then any unstated assumptions the inference requires. Distinguish deductive arguments, which claim that the premises guarantee the conclusion, from inductive ones, which claim only support or probability.
Validity, soundness, strength
A deductive argument is valid when it is impossible for the premises to be true and the conclusion false; it is sound when it is valid and its premises are true. Validity is about form, not truth, so a valid argument can have false premises. Inductive arguments are strong or weak depending on how well the premises raise the probability of the conclusion; they are cogent when strong and premises are true.
Recognising common forms
Learn to spot modus ponens (if P then Q; P; therefore Q) and modus tollens (if P then Q; not Q; therefore not P) as valid, and their look-alikes affirming the consequent and denying the antecedent as invalid. Watch for fallacies: ad hominem, straw man, false dilemma, begging the question, equivocation, hasty generalisation, post hoc, appeal to authority misused, and slippery slope.
Traps
- Judging an argument invalid because you dislike its conclusion, or valid because you agree with it.
- Confusing an explanation with an argument, or a counterexample to a premise with a counterexample to the form.
- Calling any generalisation a fallacy: hasty generalisation depends on sample size and representativeness.
Worked rule of thumb
"If the policy reduces harm, it is justified. This policy does not reduce harm. So it is not justified." This is denying the antecedent and invalid, because the policy might be justified on other grounds; the first premise states only a sufficient condition. To test any argument, formalise it, then try to describe a consistent scenario in which the premises are true and the conclusion false.