  One other piece of subject-matter bears on the interpretation of the O form. People were interested in Aristotle’s discussion of “infinite”  negation,[9]  which is the use of negation to form a term from a term instead of a proposition from a proposition. In modern English we use “non” for this; we make “non-horse,” which is true of exactly those things that are not horses. In medieval Latin “non” and “not” are the same word, and so the distinction required special discussion.  It became common to use infinite negation, and logicians pondered its logic. Some writers in the twelfth and thirteenth centuries adopted a principle called “conversion by contraposition.” It states that