Reductio ad absurdum
Reductio ad absurdum is a type of logical argument where one assumes a claim for the sake of argument, arrives at an "absurd" result (often a contradiction), and then concludes that the original assumption must have been wrong, since it led to this absurd result.
Note that this is a logically valid technique. It is a form of modus tolens, an inference rule which takes this form:
- If P then Q.
- Q is false.
- Therefore P is false.
More formally, a reductio ad absurdum argument typically takes the form:
- Assume P.
- This implies Q.
- It also implies R.
- But Q and R are contradictory (Q iff not R).
- Therefore P is false.
See Can God create a rock so heavy that he can't lift it? for an example in the context of counter-apologetics (the claim being assumed is that God is all-powerful).
The problem with this type of argument is that the "absurdity" one reaches really must be a logical contradiction in order for the argument to be logically valid.
