In Meta. Λ 8, Aristotle argues that the heaven (οὐρανός)–and, thus, the cosmos – is numerically unique on the grounds that its first unmoved mover is numerically unique. The latter is numerically unique because it is ‘essence’ (τὸ δὲ τί ἦν εἶναι) and does not have matter. “But whatever is many in number has matter.” I refer to this inference as Aristotle’s metaphysical argument for the uniqueness of the cosmos. A problem arises: (A) If the subsidiary unmoved movers of the planetary spheres are, like the prime unmoved mover, immaterial substances and belong to the same species of unmoved mover (qua “thinkers thinking themselves”), it seems that they could not be numerically distinct from that first unmoved mover – while Aristotle maintains that they are, in fact, numerically distinct. That is, as immaterial substance(s), it/they could not be individuated by matter. However, (B) if they do, as souls or soul-like forms, inform matter (namely, that of the celestial spheres which they move), it seems that there is no reason why the first unmoved mover, which moves the sphere of fixed stars or outermost, celestial sphere, should not similarly inform the matter of that celestial sphere. In the latter case, Aristotle’s argument for the uniqueness of the heaven and cosmos would be vitiated. The first unmoved mover would be the form of the outermost celestial sphere; and there would evidently be no metaphysical reason why that form could not be materially instantiated by other outermost celestial spheres, each enclosing its own cosmos distinct from our own. I argue that neither of the two salient options for resolving this problem with Aristotle’s metaphysical argument for the uniqueness of the cosmos is satisfactory. The result, so I maintain, is Aristotle’s diremption of divinity into two essentially unrelated aspects: divinity as autonomous, ‘hermetic’ cognition and divinity as ultimate cause(s) of cosmic motion and change.