Subcontraries, which medieval logicians represented in the form 'quoddam A est B' (some particular A is B) and 'quoddam A non est B' (some particular A is not B) cannot both be false, since their universal contradictory statements (every A is B / no A is B) cannot both be true. This leads to a difficulty that was first identified by Peter Abelard. 'Some A is B' seems to imply 'something is A'. For example, 'Some man is white' seems to imply that at least one thing is a man, namely the man who has to be white, if 'some man is white' is true. But, 'some man is not white' also implies that something is a man, namely the man who is not white, if the statement 'some man is not white' is true. But Aristotelian logic requires that necessarily one of these statements is true. Both cannot be false. Therefore, (since both imply that something is a man) it follows that necessarily something is a man, i.e. men exist. But (as Abelard points out, in the Dialectica) surely men might not exist?