Practice question · Sort into groups
Sort each move by whether it is a legitimate step in a natural deduction proof.
Groups: Legitimate · Not permitted
- Using a premise at any point in the proof
- Assuming the negation of the goal to seek a contradiction
- Using a line from a subproof still open around you
- Concluding while an assumption remains undischarged
- Using a line from a subproof that has been closed
Hints
- The restrictions all concern the scope of assumptions.
- Anything derived under a withdrawn assumption is no longer supported.
Show the answer
Legitimate: Using a premise at any point in the proof, Using a line from a subproof still open around you, Assuming the negation of the goal to seek a contradiction
Not permitted: Using a line from a subproof that has been closed, Concluding while an assumption remains undischarged
Why
Premises and still-open subproofs are always available; closed subproofs never are. Concluding with an assumption still open would leave the result conditional on a supposition rather than on the premises.
Practise Proof Strategy
The app has 5 more questions on this lesson, and keeps your place in the course. Philosophy I is free to start.
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