First: Our condition is that one must choose the nearest genus. But how can the student be sure he will not overlook it, selecting instead a genus he merely thinks is nearest while an even closer one exists? He might define wine as “an intoxicating liquid,” unaware that “beverage,” which lies directly beneath it, is a closer genus; or he might define man as “a mortal rational body,” overlooking “animal” and the like.
Second: We also stipulate that every differentia be essential. Yet an inseparable accident—something that never departs from the thing either in actuality or in the mind—can easily be mistaken for an essence; distinguishing the two is among the most elusive of tasks. How, then, can he be sure he will not overlook this and substitute an inseparable accident for a differentia, imagining it to be essential?
Third: We further require that he include all the essential differentiae so that none is omitted. But how can he be certain that one of them will not escape him—especially if he has already found a single differentia that yields clear distinction and uniformity in the predication of the name, as in the expression “a body possessing a sentient soul,” whose wording matches “animal” while leaving out “self-moving by volition”? This, too, is among the hardest things to discern.
Fourth: The differentia constitutes the species and divides the genus. If the rule of orderly division is not observed, one will introduce, within the division, differentiae that are not primary to the genus, and this is unacceptable in definitions. Just as body is divided into animate and inanimate by a primary essential differentia, so too it is divided into sentient and non-sentient, and into rational and non-rational. Whenever one says, “Body is divided into rational and non-rational,” the genus has been divided by something that is not the first-level dividing differentia. The proper procedure is first to divide body into animate and inanimate; then animate into animal and non-animal; then animal into rational and non-rational. Likewise, animal is divided into two-legged and multi-legged, but this is not by primary differentiae. Instead, animal should first be divided into walkers and non-walkers, and the walkers then into two-footed and so on.