A. Deduction — reasoning from the general From true premises, a valid deductive argument guarantees its conclusion. Test: if the premises were true, could the conclusion be false? If not — valid.
• Valid form (the lawyer's syllogism):
P1: Whoever intentionally kills another commits murder (major premise — the rule). P2: A intentionally killed B (minor premise — the fact). C: A committed murder. This is exactly the judicial syllogism — rule + fact = holding (Legal Method's IRAC is its industrial form).
• Validity vs soundness: valid = the form holds; sound = valid and premises actually true. A perfectly valid argument from a false premise proves nothing — the daily lesson of appellate litigation.
• Formal fallacies (Unit 3.2's full treatment): affirming the consequent, denying the antecedent — seductive invalid forms.
B. Induction — reasoning to the general From observed cases to a probable general claim (or from a sample to a population). Never certain — only more or less strong.
• Inductive generalisation — 40 of 50 audited firms filed false VAT bills → the practice is likely widespread. Strength depends on sample size and representativeness (the classic exam points).
• Causal induction — from correlation to cause (Unit 3.4's cautions: correlation ≠ causation).
• Argument from analogy — this case is like that decided case in material respects; it shouldbe decided alike. Analogy is the common law's engine (and stare decisis's logic — Jurisprudence Unit 2.3.2); its strength turns on relevance of the similarities and absence ofmaterial differences.
• Argument from authority — expert X says so. Legitimate when the authority is competent inthis field and the claim is within the consensus; fallacious otherwise.
C. Abduction — inference to the best explanation From data to the hypothesis that best explains it: the doctors' method (symptoms → diagnosis), the detectives' (evidence → suspect), and the courts' (evidence → the most probable account of events). Criteria of "best" explains more, assumes less, is consistent with background knowledge, testable. The judgment of fact in atrial is abductive: which story best explains the evidence?
D. Conditional reasoning (the hypotheticals of law)If P then Q structures: modus ponens (if P then Q; P; therefore Q — valid) and modus tollens (if P then Q; not-Q; therefore not-P — valid); the fallacy of affirming the consequent (if P then Q; Q; therefore P — invalid: the fire could have another cause) and denying the antecedent (if P then Q; not-P; therefore not-Q — invalid). Statutory and contractual reasoning is thick with such conditionals — test every one.
The patterns in one table:
| Pattern | Moves | Guarantee? | Legal example |
|---|---|---|---|
| Deduction | rule → case | yes (if sound) | the judicial syllogism |
| Induction | cases → rule | probability | precedent by analogy; statistical proof |
| Abduction | evidence → best story | plausibility | findings of fact; diagnosis |
| Conditional | if-then chains | depends on form | statutory conditions; contract terms |