Moreover, seeing that it is difficult to distinguish whatever always follows along with a thing, and is not convertible with it, from its genus, if this follows that universally, whereas that does not follow this universally as eg calm always follows windlessness and divisible follows number, but not conversely (for the divisible is not always a number, nor windlessness calm) you may yourself argue as though the one which always follows is the genus, whenever the other is not convertible with it; if, on the other hand, some one else puts forward the proposition, do not accept it universally.