 Fârâbî says that what is one in the strongest [aḥrâ] sense is what is circumscribed by a quiddity (#78). He says equally that what is circumscribed by a quiddity is “circumscribed by its portion [or allotment] of existence [qisṭ al-wujûd]” (#17), i.e., by its being what it is: this being-what-it-is is described in the Book of Letters as the existence [wujûd] of the thing, and On the One and Unity argues that it is also the thing’s unity [waḥda]. Fârâbî uses this analysis to try to make good on Aristotle’s programmatic claim in Metaphysics IV,2 that being and one are mutually entailing and in a way equivalent, or that there is a sense of “being” and a sense of “one”, or a family of senses of “being” and a corresponding family of senses of “one” which are mutually entailing and in a way equivalent. Thus here in On the One #17 he says that “one” in this meaning, i.e., the meaning “circumscribed by a quiddity”, is mutually entailing or equivalent [musâwiq] with being. So “one” in this sense needs to apply to things in all categories, not just substances, and indeed Fârâbî says that in #17, and he adds that it also applies to “other things, if there are any outside the categories” (#17, p.51, compare the Book of Letters on a possible ascent to a first cause of being to substances which would itself be outside all categories including substance). Again, if “one” in this sense is necessarily coextensive with being, Fârâbi needs to say that not only species and genera but also individual substances (and individual qualities and so on) are each circumscribed by their own quiddity—not merely by a shared quiddity and an individual place or other accidental attributes—so that there are many things in each species, in the sense of “many” arising from this sense of “one”.