 [ii] But if one says that inference A is not valid—on the contrary: If inference A is not valid, it is possible that B be true while C is false. But that is false, which I prove as follows. For if A is not valid, then C is true, and so A is valid, since B is not true unless C is true. So if A is not valid, A is valid. The premise [‘If A is not valid then C is true’] is clear, for if A is not valid, things are as C signifies, because C signifies that A is not valid; and, consequently, C is true. The first inference is evident, for in order that A be valid it is enough that B cannot be true without C, if it is formulated. The last inference holds “from the first to the last”.