Logic / Unit 1 Logic 021 short lesson, about 20 seconds
The Counterexample Method
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The idea
To prove invalidity, build an argument with the same form whose premises are obviously true and conclusion obviously false.
In practice
Form: All A are B; c is B; so c is A. Substitute: All dogs are mammals; my cat is a mammal; so my cat is a dog.
Where people slip
A counterexample proves invalidity. Failing to find one proves nothing — you may just lack imagination.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
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