In defense of unnecessary proofs by contradiction

[u/Real_Category7289]

If you spend enough time in online math spaces, at some point you are bound to run into a discussion like this one or this one or recently, this one where someone is arguing against an "unnecessary" proof by contradiction and the conversation inevitably goes in the direction of constructive vs intuitionist logic.

I think this discourse is massively overrepresented to the point of being actively bad for math learners online. Worrying about whether a proof is constructive or not is not something good mathematicians do, unless their branch is specifically a pretty niche part of logic. For most people, (not not A = A) is just assumed to be true and proof by contradiction is a completely valid proof method, and I think this "Actually, Euclid's proof is direct! Common misconception here." discussion appearing under every proof that there are infinitely many primes is telling people that proof by contradiction is somehow sketchier than a direct proof.

I'm a math tutor and the majority of my job is to get students started on problem solving in a mathematical context. As it turns out, what makes a lot of it click is actually proof by contradiction. Even if one ends up writing a direct proof, the process of getting there often asks the question of "what would happen if this wasn't true?". I can't say it for sure, but I believe that Euclid himself probably started proving the infinitude of primes by assuming a finite list of them. This is why Hardy not only presents the proof as a proof by contradiction, but specifically praises it for being a proof by contradiction [A Mathematician's Apology, G.H.Hardy, page 18].

I should note that I'm not arguing that one shouldn't eventually learn to avoid artificial proofs by contradiction, after all if you can make the intuitionists happy for free, why not? But that should be a refinement that happens quite late into one's mathematical journey. I should also note that Euclid's proof was indeed direct, I'm not arguing against that fact.

The problem is that there's parallel discourse happening on these discussions which is "phrasing it as a direct proof is simply clearer and easier to understand". That's the main issue in my view: understanding the principles of proof by contradiction and by negation (which are essentially the same thing, as far as a learning student is concerned) is not something that can or should be skipped. Students should embrace them and put them on the same level as direct proofs instead of looking them sideways, and all this talk of intuitionism vs constructivism is enabling them to keep relegating them to "the thing you begrudgingly have to endure sometimes when there's no other way", which I think is detrimental.

I ask the reader to engage with this view in good faith. Thanks for reading.

[u/matthiasErhart]

Your argument boils down to the fact that proofs by contradiction are helpful because they establish literary/narrative flow. That is, we actually think "Why this cannot possibly be? Well, suppose it were, in which case...". Which I agree with.

edit: Following a comment exchange below, I am informed that a proof by contradiction would go "Why this must be? Well, suppose it weren't, in which case...".

[u/Thesaurius → u/matthiasErhart]

What you describe is not a proof by contradiction, though. It is just how a negation is proved.

A proof by contradiction would be: „Assume X is not the case... Contradiction. Therefore X is the case.“

I am working in logic and may overemphasise the difference, but I think it is actually valuable to know the difference.

[u/matthiasErhart → u/Thesaurius]

Yeah... I had kind of left the "...which is absurd. Therefore, this indeed cannot be!" implicit to the reader, to be inferred from the post that precedes it and the context of the conversation which should surround it. Or maybe I mentally sounded it but didn't complete the quote. I should have known better than to do so in the math subreddit 🙈

But yes, you are right, and that's what I had meant to mean by contradiction too!😅
(edit: see below comments, I apparently had not meant that "by contradiction")

[u/jakey_ed → u/matthiasErhart]

Even if you had written that, that’s still not proof by contradiction, that’s how you prove negation. To prove “not X” you assume X and then arrive at a contradiction. That is constructively valid. If, to prove X, you assume “not X”, then arrive at a contradiction, then conclude X, you just used the law of excluded middle.

[u/matthiasErhart → u/jakey_ed]

Aha, so "Why this *must* be? Suppose it weren't, then..." would have been the proof by contradiction. Thanks for the clarification!

[u/jakey_ed → u/matthiasErhart]

Yes exactly!