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
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.