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As English is not my native language, I have a trouble to find the correct way to express the following:

Two negations are equivalent to an affirmation

properly.

This is how it translates from my native language (Greek) if it is translated word by word: "δύο αρνήσεις ισοδυναμούν με μία κατάφαση", and it does not sound good to me.

Is there a standard English expression (an appropriate cliché, for example) with this meaning?

UPDATE. Let me give an example of what this expression really means:

"There is NO student in this classroom who DOES NOT HAVE a pencil."

is logically equivalent to:

"Every student is this classroom HAS a pencil"

So the first sentence has two negations - a negation of a negation - while the second one has affirmative nature.

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1  
Two wrongs don't make a right, but three lefts do. –  SF. Feb 4 at 11:59
    
Two Wrights make an airplane. –  David Schwartz Feb 4 at 12:37
    
Every student has at least ONE pencil –  mplungjan Feb 4 at 13:17
    
Grammatically, your example sentence makes sense, but stylistically I was taught to avoid it because it can lead to awkward constructions. Instead, either state things in the positive (as in your second example), or use a word that expresses the negative, eg, No student lacks a pencil. –  James Kingsbery Feb 4 at 14:51
    
It would help for you to provide more context about how you want to use this expression. English teachers give students a common admonition: "Don't use double negatives!" If you're a math teacher you could make a joke that math is better than english because you can use double negatives all you want - as long as you can keep it straight that what you're saying is what you think you're saying and what others think your saying is what you're really saying. –  CoolHandLouis Feb 4 at 15:27

2 Answers 2

Two negatives make a positive. Minus times minus is plus. Also, two wrongs (don't) make a right.

Edit to address your update: the term in logic for the situation you describe is double negation. Not to be confused with the linguistic term double negative, where two (or more) negatives might or might not resolve to a positive, depending on language and context.

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Two wrongs don't make a right, but three lefts do. ;-) –  TecBrat Feb 5 at 1:32

'Two minuses make a plus'. That's how it was explained to me, over fifty years ago!

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I corrected the title of my question: minus-> minuses. –  Yiorgos S. Smyrlis Feb 4 at 11:23

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