When one asks for explanation of something, the other give an equivalent explanation. Examples: "the weather is hot because it is not cold", "I am smart because I am not stupid". So what is the common idiom to describe this case?

  • Neither of those sentences necessarily convey an equivalence. Just because it's not cold doesn't mean it's hot. It could simply be mild. Similarly, somebody could be of average intelligence, neither smart nor stupid. Are you asking about this particular form of error in deductive reasoning? (If it's not one thing, it must be the opposite?) Or are you asking about a tautology—even though your sentences really aren't? (If you're poor, you must not have any money.) – Jason Bassford Nov 17 '18 at 5:59
  • @JasonBassford at some extend they are not equivalent. By the way, I am not native english speaker so these are just examples to describe the idea. Do you know some idioms about an equivalent explanation ? Maybe I mean tautology – Dat Nov 17 '18 at 6:03
  • It still depends on what you're trying to express. Are you looking for an idiom that captures it's true because it's not false? (Where there can only be two possibilities?) Is it known that it's not one thing—or is it just being asserted without evidence? – Jason Bassford Nov 17 '18 at 6:06
  • I think I go to tautology (the saying of the same thing twice in different words,) do you know any idiom about tautology? – Dat Nov 17 '18 at 6:08
  • Perhaps you’re thinking of “circular reasoning”? – Scott Nov 17 '18 at 7:23

I can think of no idiom that directly captures a tautological utterance itself. Not unless you want to imbue the tautology with a nonsensical meaning. In which case, there's A nod is as good as a wink (to a blind horse / to a blind bat).

But there is an expression that captures a response to a tautology.

"It's hot because it's not cold."
"I don't care what you call it—I still have to wear a T-shirt."

The response can be captured with: A rose by any other name would smell as sweet.

This is the common quotation from William Shakespeare's Romeo and Juliet. The actual two lines, as quoted from BookBrowse, are:

What's in a name? that which we call a rose
By any other name would smell as sweet

In comments below your question, you said you weren't interested in anything related to reasoning or fallacy, just an idiom that expressed the idea of "the saying of the same thing twice in different words."

This expression can have a broader interpretation than just a tautology, but it can be used in the case of a tautology.

  • You make me so confused. Do you have any idiom that mocking about "circular reasoning"? – Dat Nov 17 '18 at 11:10
  • @dat I can think of idioms that mockingly tell somebody they aren't making sense (the first one I gave does that, although I think differently than you want)—but none that say they aren't making sense because of circular reasoning specifically. – Jason Bassford Nov 17 '18 at 15:04
  • @Jason Bassford - Note that the link you give re 'blind bat' gives an incomplete explanation and the phrase isn't in itself nonsense. A blind creature can't see a nod or a wink so they are indeed equivalent to that creature. Sorry this is going off at a tangent! – chasly from UK Dec 17 '18 at 18:09

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