From among the eight Aristotelian treatises translated by C. D. C. Reeve thus far, De Caelo belongs, together with Generation of Animals (2019), to the less often rendered. This is the first complete English version since Guthrie (1939), and its virtues are the same as those of previous volumes, including the clarity of translation (here based on Paul Moraux’s 1965 Budé edition) and a thorough Introduction. In this case, the introductory study is especially welcome, since explaining the place of De Caelo in the corpus is no easy task. To this end, Reeve paints a systematic picture that intersects with the Introductions in his other contributions to the series. Instead of introducing the treatise in question by simply following its unfolding, Reeve elects to proceed differently, with the advantages and the risks this choice entails. The advantage is the emphasis on the unity of Aristotelian natural science and metaphysics, supported by considerations of Aristotle’s methodology. The risk consists in sweeping under the systematic carpet, so to speak, some controversial issues where different treatises seem to diverge. Still, I believe Reeve’s choice is justified, since the book is not addressed only to specialists, and the Introduction is there to frame the translation, with some 150 additional pages of endnotes to explain the more intricate issues, conceptual and textual alike. Instead of a tame paraphrasis, we get therefore a strongly held and well-crafted view of Aristotle’s philosophical enterprise as a whole. As Reeve puts it right at the beginning of his Introduction, the latter “is not a comprehensive discussion of every aspect of De Caelo, nor is it, I should add, an expression of scholarly consensus on the issues it does discuss—insofar as such a thing exists—but my own take on them” (p. xi).
On Reeve’s understanding as epitomized in his Introduction, the science of De Caelo is not simply a natural science, but rather a “super-natural” science that encompasses not only the four sublunary elements and the primary body or ether, but also the divine character of the animate primary heaven. Moreover, it has a mathematical dimension (not unlike applied mathematics) so that, all things considered, “the science to which De Caelo contributes is at once a natural-scientific branch of mathematics, a biological science, and a theological one” (p. xxvi). This is not to deny its evidentiary basis, but to stress that to inquire into the superlunary realm (“super-nature”) requires the grasp of the vertical architecture of substances starting with, in descending causal order, the prime unmoved mover. Here the lesson of the Introduction is twofold. On the one hand, Reeve exposes the complex hierarchical order of substances that compose the universe. On the other hand, he is no less emphatic that what enables us to grasp this hierarchy is the role of understanding (νοῦς) in it, since there is a real, not only a methodological, analogy between the divine and our understanding. In other words, the same kind of actuality governs the universe and our understanding of it.
Since Reeve’s reconstruction of the ordering of substances takes seriously Aristotle’s warning against an episodic conception of the universe, it is imperative that it integrate “super-nature” into the causal chain that reaches from the prime mover down to the sublunary substances. As a prerequisite to achieving this aim, Reeve offers a compact explanation of the substances in the superlunary realm. These substances, he insists, are matter-form compounds, but “their matter is primary body (ether),” which is “relevantly similar to intelligible matter” of mathematical objects and makes them “amenable to being studied by applied mathematical science” (p. xxx). If we add that these materially incomposite substances are alive and divine, we obtain “a genuine trans-generic object of study” whose formal aspect allows one to connect the divine yet mobile celestial bodies to the immovable divine substance. De Caelo and Metaphysics XII become parts of a uniform explanatory structure, the former dealing with the matter-form compounds in the superlunary realm, the latter adding the single prime mover that is immovable and separate. This looks like a traditional picture of vertical causal unification, but Reeve presents this picture in new relief that enables us to progressively grasp the whole “vertical and horizontal unification of being” (p. xxxvi).
I cannot do justice to the subtlety and nuance of the interpretation that Reeve builds upon a wide web of quotations from across the corpus. My focus is only on what is relevant to De Caelo, and even more narrowly on the realm inhabited by divine substances. For Reeve, to understand how these substances sustain the cosmic architecture is more than a matter of natural science; it is indeed relevant to our understanding of ourselves. For lack of space, I prefer to quote Reeve’s summary of this crucial point, which nevertheless provokes some metaphysical and some pedestrian copy-editing questions :
When we look at our lives from the outside, so to speak, from the theoretical point of view, if the Metaphysics is right, we see something amazing, namely, that the heavenly bodies, those bright denizens of the starry heavens above, are living beings who, like us, are moved by a desire for the best good—for the god (XII 7). It is the conclusion for which De Caelo II 1, like Physics VII 10, prepares us. When we view our lives from the inside, from that perspective from which “the truth in practical matters” can alone be judged, the Ethics tells us that we will find that we are moved by the same thing—that as the good for the heavenly bodies consists in contemplating the primary god, so too does our happiness. (p. liv)
(Reeve continues with quotation from NE X 8 1178b21-23.) This mutual articulation of two perspectives on human life is brilliant. However, there are two catches in it. The first one is the double typo that may confuse the reader, since the correct references to De Caelo and the Physics are II 12 and VIII 10 respectively. This error blemishes the synoptic presentation Reeve intends. The result is less dangerous in the case of the Physics since there simply is no Physics VII 10, but the case of De Caelo will mislead non-specialist readers who may puzzle over the denial, in the wrongly indicated II 1, of there being a soul of the heaven: how could this denial support the argument that the “denizens of the starry heavens” are moved by “a desire for the best good”? Another instance of this problem occurs on p. xxxiv, where we are referred to Metaphysics II 8 instead of XII 8; again, a specialist spots the typo immediately, but other readers may look in vain for a nonexistent chapter (though, fortunately, Bekker numbers are supplied this time).
The mention of Metaphysics XII 8 brings us to a more serious issue. In the passage with the erroneous reference to this chapter, Reeve relies on the fact that his Aristotelian translations form a unified whole so that his new Introduction can quote from previous volumes—for instance, when addressing the coordination of many celestial motions into a single heaven. Borrowing from the Introduction to his Metaphysics, Reeve explains this unity by quoting Met. XII 8 1074a31-38 and concluding that, despite some puzzling features of this text, its intention is sufficiently clear: “what accounts for the unity of the heaven is that the movements in it are traceable back to a single cause—the prime mover” (Metaphysics p. xxxvi = De Caelo p. xxxiv). The new book accompanies the quotation of Met. XII 8 1074a31-38 with the addition of “also Cael I 8”. This extension is not an innocent one. In De Caelo I 8, Aristotle does indeed argue for a single heaven, but he introduces no fewer than seven arguments, none of which is the one that we find in the quoted text of MetaphysicsXII. In fact, only the brief sixth argument mentions the possibility of arguing for a unique heaven by recurring to primary philosophy, and this mention receives no further development. The lack of development may be a detail, but it brings us to the heart of the matter: the quoted conclusion about the clear relation between “the unity of heaven” and the primary mover that guarantees the unity of many celestial motions is quite optimistic as far as De Caelo is concerned. Whether we are willing to share it depends, among other things, on how we read various analogies and comparisons with ensouled sublunary beings and their desire. In other words, a lot depends on Reeve’s understanding of De Caelo II 12.
I return to this chapter because it is crucial for Reeve’s interpretation which, therefore, relies on a text with a rather special status within De Caelo. Analogizing the varied motions of the planets with human exercises aiming at “the good state”, this chapter can be read as accepting that the aporia concerning the complexity of planetary motion can only be solved “reasonably” (εὐλόγως), or it can be taken as suggesting that the celestial bodies are indeed animals striving for their good. With some modest caveats, Reeve follows the latter option and combines it with Metaphysics XII 8 (and XII 10) so as to arrive at a strong conclusion about the cosmic hierarchy of substances, including the divine ones, which can be grasped thanks to the analogy between celestial souls and human souls. Again, this is a non-trivial and attractive reading, although one wonders if the explanatory analogies are not mixed with scientific statements too easily. After all, even the analogical explanation elaborated in De Caelo II 12 refrains from explicitly ascribing to the celestial movers some sort of desire, and the rest of the treatise is silent on this issue, just as it omits any real account of immaterial divine substances. In fact, it contains only one explicit reference to an immaterial mover: in II 6, Aristotle offers six possible arguments in favor of the regularity of celestial motion, and the second of these introduces such a mover as a guarantee of the regularity in question (288b5-6).
Naturally, silence provides ample space for conjecture, and Reeve’s reading is entirely legitimate. So why all this nitpicking instead of a simple praise for a successful translation? Not in the least because De Caelo talks to us differently than, for instance, the Nicomachean Ethics or even De Anima. It is trivial to say that, today, one can hardly believe in Aristotelian cosmology in the same way as, for instance, Aristotle’s conception of virtue. Philosophically, however, both are equally relevant, even if the cosmology’s value consists in exposing the difficulties that arise, even on Aristotle’s own premises, if we hang the whole system of the world on the notion of an actually perfect being. In fact, it is precisely these difficulties that are perennially fascinating, and Reeve’s translation (if not always his Introduction and Notes) conveys them most efficiently. All things considered, De Caelo adds another building block to Reeve’s admirable Aristotelian edifice.
 Reeve quotes Metaphysics XII 8 1074a31-38 again at 188 n. 319, connecting it to De Caelo I 9 279a11-28, which he takes as saying that “all the immovable movers except the primary one are inside the sphere of the primary mover.” From this he derives the association of immaterial celestial movers with the superlunary material element and concludes that “[t]he fact that these spheres, as celestial animals, are analogous in structure to ourselves helps us to make some sense of what this association might be” (189, italics mine). On De Caelo I 9, one can compare Fabienne Baghdassarian, “Aristote, De caelo, I, 9: l’identité des ‘êtres de là-bas’”, Philosophie antique 11, 177-203.
 It matters how we read the much-disputed ὡς at 292a20. Reeve translates the sentence in question “we should conceive [the stars] as participating in action and life.” But cf. Leggatt’s “we ought to conceive of [the stars] as though they partook of action and life.” On the role of ὡς see Stuart Leggatt, Aristotle, On the Heavens I and II (Warminster 1995), 248-9; Catherine Dalimier and Pierre Pellegrin, Aristote, Traité du ciel (Paris 2004), 438-9 n. 5. On the context of this issue, see e.g. David Charles, “Teleological Causation”, in Christopher Shields (ed.), The Oxford Handbook of Aristotle (Oxford 2012), 250-3 (on De Caelo and Metaphysics XII).