Text 18.  The universal negative necessity proposition converts as itself, that is, as a universal negative necessity. If necessarily no B is A, then necessarily no A is B; were that not the case, then possibly a given A is B—let that be J, such that at a given moment what has become A will have become B, so that it will be B and A, so that that B is an A; this is impossible. (Avicenna Najât [1985] 44–45) 