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What is a word or phrase for argument that shows the presupposition is false?

E.g.:

Someone said writing requires big hands. I showed them that someone can write well with small hands. Therefore, I have argued against what he said.

What is this method of argument known as?

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    A counter-argument? – David Garner Jun 16 '15 at 8:37
  • Those voting to close, please show how the "research" may be done. – Kris Jun 18 '15 at 6:49
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That's a counterexample.

From Oxford:

An example that opposes or contradicts an idea or theory.

Merriam-Webster has it stronger than that:

an example that refutes or disproves a proposition or theory.

(I personally would have written it as counter-example, but all the dictionaries I looked at excluded the hyphen, so it looks like the non-hyphenated version is preferred.)

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  • Why do you not use the OED? It is vastly more extensive than Merriam Webster. If you are a UK Council Tax payer you will very likely have free on-line access to the OED (If your local authority library has a subscription). – WS2 Jun 16 '15 at 9:43
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    @WS2 Thanks for the tip - I do actually have an old printed one, but I'm sure the online version will be more up-to-date. I will look into it. Here though, I'm trying to provide links another reader can click through to without any subscription... which is a good idea, surely? In this specific case I looked at a few alternatives to check whether they differed substantively, and (to my surprise) they did, so I posted two sample definitions to illustrate the range of opinion. – Morton Jun 16 '15 at 9:56
  • If you have a 'printed' OED it runs to about 30 volumes! I think what you may mean is the Shorter Oxford Dictionary, which is two volumes. But the online OED is vastly more than a English dictionary. It is a dictionary of the history of English. It is a major source of etymology, and provides examples of the use of every single word from earliest times. The Americans who use the site do not have access unless they buy a subscription. Are you a member of your local library. If so look at your library card. – WS2 Jun 16 '15 at 10:13
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    @WS2: Those who are unfortunate enough not to belong to a local library and not rich enough to afford a subscription (which means most users of this site) cannot use your links. – Tim Lymington supports Monica Jun 16 '15 at 11:17
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    @WS2: Many ELU users do not enjoy the felicity of living in Britain. Many of those who do have not joined a local library, for whatever reason (or have done so, but have not logged on to oed.com) I do not think you will succeed in changing either of these facts, and until you do, links to the OED are not helpful. I meant no more than that. – Tim Lymington supports Monica Jun 16 '15 at 12:11
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rebuttal, n

rebut verb (rebuts, rebutting, rebutted)
[with object]

1 Claim or prove that (evidence or an accusation) is false:
he had to rebut charges of acting for the convenience of his political friends

syn: refutation, denial, countering, invalidation, negation, contradiction "now that you've heard the accusations, have you a rebuttal?" (source: Google Search)

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I think that the term you are looking for is disprove by contradiction. However, I think that Morton's counterexample is also a very good answer.

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If you wish to convey the idea that your colleague was attempting to falsely convey the notion of truth in his argument you could say:

A colleague argued speciously that writing requires big hands. I rebutted with my own evidence which exposed his artifice.

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