 To Fârâbî’s “general metaphysics”—his study of being and its universal attributes, rather than of God as the first cause of being—belongs not only the Book of Letters but also On the One and Unity. Fârâbî in On the Aims of the Metaphysics had included unity alongside being in the object of metaphysics; and when Book of Letters I follows the agenda and many of the subject-headings of Aristotle’s “lexicon” in Metaphysics V, it is strange that it leaves out unity, which Aristotle treats in Metaphysics V,6. But the treatment of unity, missing from the Book of Letters, is supplied by On the One and Unity [henceforth On the One]. Since the Book of Letters as is incompletely transmitted, it is not entirely out of the question that On the One is a missing part of the Book of Letters. Perhaps more likely, On the One was originally intended as a part of the Book of Letters, but got too long and complicated, and was spun off as a separate treatise. The treatise is too complicated and too little-studied (with major editorial problems) to treat here in full detail, but we can describe some major themes. The treatise is helpful in understanding the notion of circumscription by a quiddity, which is thematized both in the Book of Letters and On the One and used without explanation in the Perfect City and Political Regime, and also in understanding Fârâbî’s explanation of Aristotle’s claim in Metaphysics IV,2 that being and unity are coextensive and mutually entailing, and his account in the Perfect City and Political Regime of how unity is predicated of God.