LSAT Conditional Reasoning: Contrapositives, Chains, and Common Errors

Conditional statements are one of the LSAT's most reliable scoring tools. Once you can diagram them fluently, entire question categories — Must Be True, Sufficient Assumption, parts of Parallel Reasoning — become mechanical.

The basic form

'If A, then B' means A is sufficient for B, and B is necessary for A. The only automatically valid inference is the contrapositive: 'if not B, then not A.' Both the converse ('if B, then A') and the inverse ('if not A, then not B') are invalid.

Common trigger words

Sufficient triggers: if, when, whenever, all, any, every, each, in order to. Necessary triggers: then, only, only if, must, requires, unless (special: 'unless X' means 'if not X'). The LSAT layers these into dense stimuli to test whether you can extract the underlying conditional.

Chains and the 'no way out' rule

When two conditionals share a term, they chain: A → B, B → C yields A → C. Chains let you make multi-step inferences that answer options often test. The 'no way out' rule: if a chain leads to a contradiction, the antecedent must be false.