In Book I, Chapter 3 of Prior Analytics, Aristotle says the following:
Whatever is said to be possible, either because B necessarily is A, or because B is not necessarily A, admits of conversion like other negative statements, e.g. if one should say, it is possible that man is not horse, or that no garment is white. For in the former case the one term necessarily does not belong to them other; in the latter there is no necessity that it should: and the premiss converts like other negative statements. For if it is possible for no man to be a horse, it is also admissible for no horse to be a man; and if it is admissible for no garment to be white, it is also admissible for nothing white to be a garment. For if an white thing must be a garment, then some garment will necessarily be white
I am struggling to interpret this passage, though one comes to mind, which has issues that leads me to doubt it. My interpretation is that the possibility in either case converts as "possibly no a is b" to "possibly no b is a" provided there is a necessity of no a being b, in the case of men being horses, and an absence of this in the case of garments being white. However this would seem to contradict Aristotle's definition of possibility as excluding necessity, i.e., neither necessary nor impossible. And further there is the question of why that auxiliary condition is needed in the case of horses and men and not in the case of garments and white. I feel my interpretation is likely incorrect, but then am unsure how the the necessity of B being A, or the absence of that necessity, affects this conversion.
