Practice question · Multiple choice
Why is indirect proof available whatever the shape of the goal?
Hints
- Ask what indirect proof requires of the goal.
- It requires nothing of the goal at all, only that its negation can be assumed.
Show the answer
C. Assuming the negation of any sentence is always permitted, and a contradiction refutes it
Why
Indirect proof makes no demand on the goal’s form: you may assume the negation of anything, and a contradiction refutes it. That universality is why it is the fallback when the goal offers no guidance.
Practise Proof Strategy
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