Logic / Unit 6 Logic 280 short lesson, about 20 seconds
The Finite-Universe Method
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The idea
To prove invalidity: build a small universe (one or two members) where universals become conjunctions, existentials become disjunctions — then find an assignment with true premises and a false conclusion.
In practice
A two-member world is a truth-table row for predicate logic.
Where people slip
This proves invalidity only. Proofs certify validity; models certify invalidity; between them, the system is complete.
Your place on the trail
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