I wonder what should be done with the tag. Some time ago it has been proposed to be deleted.
A possible worthwhile use of the tag could be using it for questions about the logic behind a proof by contradiction (e.g. this or this), instead of questions about a particular proof by contradiction. But perhaps that would make it a very localised tag.

Shall we delete ?

I have suggested burninating here.

  • $\begingroup$ At least renaming it to proof-by-contradiction IMO, if that's what the tag is about. Before reading the tag excerpt I thought it was a subtag of fake-proofs. $\endgroup$ Commented Apr 6, 2015 at 9:12
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    $\begingroup$ The tag is useless. Proof. Assume towards contradiction that it is useful, but this is a contradiction since the contradiction tag is useless. QED. Drop mic. $\endgroup$
    – Asaf Karagila Mod
    Commented Apr 6, 2015 at 9:12
  • $\begingroup$ @AsafKaragila You assumed both the result and its negation, but you may only assume the negation in a proof by contradiction. $\endgroup$ Commented Apr 17, 2015 at 1:42
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    $\begingroup$ @PyRulez: He is begging the question, but that's doesn't mean the proof is incorrect, just that it is not useful since the result was already known a priori. For example, given the parallel postulate as an axiom I can prove the parallel postulate using contradiction. $\endgroup$ Commented Apr 17, 2015 at 16:13

1 Answer 1


I suggest using (proof-strategy) as the preferred synonym.

I don't think every nuance of a topic has to have a tag of its own; proof-strategy is quite narrow enough. There's no great need for a proof-by-contradiction tag.

  • $\begingroup$ ... and renaming to proof-by-contradiction as suggested by Najib Idrissi. $\endgroup$ Commented Apr 17, 2015 at 7:21
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    $\begingroup$ Though it should be noted that not all contradiction questions fit within proof-strategy, so perhaps we should first re-tag some of them. $\endgroup$ Commented Apr 17, 2015 at 7:24
  • $\begingroup$ I can see a notation question, which only really needs a notation tag (which it has already). $\endgroup$
    – Joffan
    Commented Apr 18, 2015 at 14:38
  • $\begingroup$ I think that before adding synonyms for that tag, the usage of the tag (proof-strategy) should be clarified. It is rather unclear and often users use this tag for any question related to proof. See, for example, this post and other posts linked to it for some discussion of this tag. $\endgroup$ Commented Apr 19, 2015 at 6:56

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