We have 18th century mathematician and grammarian Robert Lowth to thank for the received wisdom that two negatives are ungrammatical. In his 1762 book A Short Introduction to English Grammar, Lowth, who was enamoured of Latin-derived logical models, decreed that two negatives cancel each other out and create a positive; until that time, two negatives were taken to reinforce each other. Lowth's rule, though arbitrary and invented, has become "truth", as has his insistence that sentences should not end in prepositions, that split infinitives are ungrammatical, and that "they" cannot be used as a gender-neutral pronoun. Not all languages consider a double negative to be not correct.