Mortality, then, is neither a permanent attribute—much less an essential one—for in the Hereafter the human being remains fully human while living forever.

Grant, if you will, that some people doubt or even deny this. Is it not nevertheless a notion the mind can conceive? Should one picture a person in the state of which the messengers—peace be upon them—have informed us, he would still be a complete human being, yet he would not die.

One may add that, according to many—indeed most—Muslims, Jews, and Christians, an angel dies; and even if we concede the position of those, within these faiths and beyond, who maintain that an angel does not die, this is not something known to the addressee of the definition. All the more so given that, in their own scheme, the rational soul belongs to the very genus they call “angels,” namely the celestial intellects and souls. This exposes the frailty of the precaution advanced by al-Fārābī, Abū Ḥāmid, and others.

They may reply that the term “animal” in their usage is confined to what grows and takes nourishment, thereby excluding the angel. If so, “animal” rules the angel out, while “rational” is broader than “human,” for it can apply to humans and non-humans alike, just as “animal” is broader than “human.”

Let us even accept their proposed differentia, “mortal.” Mortality, too, is not peculiar to man; it is a property shared by all animals.

Thus it has become clear that none of these predicates—animal, rational, or mortal—is exclusive to the species Homo. Their contention that a differentia must consist solely of properties unique to the species, to say nothing of being essential, therefore collapses. Distinction is achieved only by citing the ensemble: the two qualifiers together, or all three.

This holds when the differentia is meant to place a mental image of the thing in the mind of one who has not yet conceived it. If, however, the differentia is intended merely to set the thing apart from everything else, then without doubt it will consist of properties unique to it.