Devin Henry’s excellent book takes on Aristotle’s theory of substantial generation. Substantial generation is the sort of “unqualified” change in which a substance comes to be: it is what happens when Socrates comes to be, rather than when he grows a centimetre taller (1). Henry’s overarching argument is that “Aristotle employs a single model of generation throughout the corpus”: the hylomorphic model.
This argument comes in two stages. Chapters 2-4 introduce the three principles of the hylomorphic model: matter, form, and efficient cause. Chapters 5-8 consider these principles in Aristotle’s account of animal generation. This discussion is framed by an introduction and two chapters that consider the wider context: Aristotle’s reaction to his intellectual inheritance and his cosmological perspective on generation.
The result is a thoughtful, systematic account of Aristotle’s theory and the biological details that fill it out in practice. Its central moves are philosophically rich, well-argued, and responsive to a variety of texts. Indeed, a significant strength is its sustained attention to not only to the implications of the Physics and GC’s more abstract framework for our understanding of the biological details, but also the puzzles those details raise for the theoretical account. Moreover, its careful analysis and formulations of existing debates point up important questions.
Chapter 1 begins by sketching Aristotle’s responses (in Physics 1.8 and GC 1.3) to his Eleatic inheritance, which raised the worry that substantial generation is not possible. Chapters 2-3 inaugurate Henry’s main project, beginning with Physics 1.7’s account of form as the “positive state that is acquired” (53) and matter as the “subject” (hupokeimenon) of the change. As Henry notes, this chapter is sometimes read as claiming that all subjects satisfy two conditions. As Henry formulates it, the “Subject Condition” (SC) claims: “There must always be a subject from which a thing comes to be non-incidentally.” The “Persistence Condition” (PC) adds: “That subject persists through the change to become a constituent of the finished compound” (42)—as Henry notes later, as “numerically the same subject” (63). This refined formulation of SC—in terms of a pre-existing subject—allows Henry to claim that “in this context the concept of a hupokeimenon does not entail being a continuant” (42). He argues that, while Physics 1.7 holds that all coming-to-be satisfies SC (42-3), its approach to PC is “undeveloped”: it does not consider whether PC holds for substantial generation, nor does it develop a connection between SC and PC (46-7, cf. 65-7). Chapter 3 draws on GC 1.1-4 to argue that what distinguishes substantial generation is precisely that it does not satisfy PC (60).
Henry’s claim that SC does not entail PC—coupled with his observation that Aristotle does not seek a persisting subject for animal generation (71-4)—offers a valuable perspective on what drives Aristotle’s thinking. If his reason for insisting that substantial generation has a hupokeimenon is not to avoid treating it as “sheer replacement” or to ensure the “continuity of a substance over time” (65), we must look elsewhere to understand his motivations (cf. 25 ff.). Still, there are questions about how Aristotle develops the refined conception of SC (as not entailing PC). Physics 1.7’s prefatory remarks do not mention a hupokeimenon, but do describe a case where something remains (190a9-13). Its next step is to introduce SC and PC with a transitional remark: “Once these distinctions have been made one can grasp the following from all cases of coming-to-be” (190a13-4). Henry observes that this comment (and thus PC) may apply only to “all cases” just described: of qualified coming-to-be—not “all cases” whatsoever (48-9). Even so, there is a question about the implications of introducing SC in this way. If SC is grasped by reflecting on examples that satisfy PC, what drives Aristotle towards the refined SC—and how? Are those prefatory remarks, understood with care, enough to guide him to it, or is additional work needed?
Chapter 4 offers three refinements to the hylomorphic model, drawing on GC 2.9. First, matter is not just a subject, but is so because it is “a capacity to be and not be”—say, “to be the substance at the end of the change” (80). Second, form is not just the positive state acquired, but also “serves as” its goal (80-2). Third, generation requires another principle: a “goal-directed moving cause” (82).
Chapters 5 and 6 introduce Henry’s second focus: how Aristotle uses this three-principle framework to explain animal generation. They focus on his “reproductive hylomorphism”: the view that the male is the efficient cause that provides form whereas the female provides the matter (109, 117). Henry suggests that what counts as matter and form in animal generation depends on the “explanatory context” (117). He identifies two contexts (117-8): the process of embryogenesis (GA 1) and the completed substance (2.4-5).
Chapter 5 focusses on the first. Appealing to GA 1’s interest in the “initial formation of the embryo” (119) and its analogy to how rennet acts on milk (120), Henry argues that here the male provides form “in the purely mechanical sense of imposing a limit or boundary (dioriountos) on the relatively indeterminate fluid causing it to set” (119). That menstrual fluid, provided by the female, is matter in that it plays the role of “the passive subject of change that is potentially the substance that comes to be from it” (123). Henry emphasizes that, while this material role is a passive one, Aristotle’s accounts of parthenogenesis and heredity treat menstrual blood as “complex”, “dynamic”, and “active” (122-3). This distinction between blood and role (cf. 95-103) offers a promising way of untangling these issues. Moreover, it clarifies the task any alternative interpretation faces: showing that the material role is active.
Chapter 6 takes on the second context: the completed substance. Henry argues that GA 2.4 presents the male as providing soul and the female body (131-2), but GA 2.5 complicates the picture. It reveals that the female provides some nutritive soul-capacities and the male the sensory ones (133-7). This move is controversial, and scholars will appreciate Henry’s careful statement of the evidence and responses to objections. Especially valuable is his argument that his position does not imply that these soul parts can exist independently. Drawing on GA 4.3, Henry argues that what exists in the parental contributions is motions—not soul parts—“in virtue of which” these contributions transmit powers to generate parts (nutritive and sensory organs). While the resulting organism must have the corresponding soul parts integrated appropriately, the motions need not (141, 159-60). This is an elegant solution. Henry’s distinction between blood and its material role also pays off here: blood can be passive in its role as matter, while also having other features, including motions that transmit active powers (to make nutritive parts). Nevertheless, further argument for distinguishing the parental “motions” for parts along nutritive/sensory lines may be needed. Since Aristotle does not say this explicitly, an outline of the considerations that favour connecting taking GA 4.3’s “motions” with 2.4-5’s hylomorphism would help fill the gap.
A highlight of Chapters 5-6 is their attention to the variations in Aristotle’s applications of hylomorphism. Still, this perspective raises questions about what weight to assign those differences. While Henry is right to ask what sort of hylomorphism the rennet analogy can support, the resulting “deflationary” (119) interpretation may come at a cost. As Henry notes (123-5), what suits the menstrual blood to receive the male’s motion is that it is potentially all the parts (741b7-5; cf. 734b19-24). If these are the capacities actualized in embryogenesis, why think this transmission of form is a “purely mechanical” imposition of a boundary? Indeed, as some interpreters have emphasized, what allows the semen to “set” female residue is that it is “moved with the same motion according to which the body grows” (737a19-21). Such “setting” looks less mechanical and more like the early stage of a growth-like process that gets more sophisticated as it proceeds—and likewise the form it transmits. More details are needed: What makes the imposition of a boundary “mechanical”? How does it figure in generation/development as a whole?
Chapter 7 turns to the efficient cause. The puzzle is that Aristotle’s view that the male is the efficient cause suggests that that cause is external, but Physics 2.1 appears to claim that natural things have internal efficient causes. Henry convincingly resolves the tension by arguing that, even if the father is the external efficient cause of embryogenesis, the process of morphogenesis that follows (the “development and organisation of the offspring’s new body”) has as its internal cause the “formal nature in the embryo” (155). This discussion is compelling: it preserves the traditional reading of Physics 2.1, while offering insight into the GA.
With these three principles—matter, form, and efficient cause—in place, Chapter 8 considers their interaction. Henry argues that Aristotle “naturalises” the model sketched in Plato’s Timaeus and Politicus. Aristotle’s version of this “architectonic” model explains how natural things come to be in a “good and beautiful condition” (176) by appealing to “goal-directed” efficient causes that control the “choreography” of the necessary behaviours of the simple bodies (heating, cooling) (182)—determining their “location, timing, and degree” (180) and reducing them to “co-causes” (178). Chapter 9 takes a cosmological perspective, arguing that continuous animal generation exists in part because it contributes to the “good state of the universe” (209).
The appendix to Chapter 8 deserves mention. Mariska Leunissen has distinguished between “primary” teleology (which actualizes a “pre-existing potential for form” and produces “vital and essential parts”) and “secondary” teleology (which makes good use of “resources that are present owing to material necessity alone” and produces “subsidiary and luxury parts”) (188). The appendix argues convincingly that these categories do not capture “two distinct kinds of teleological causation” (188): they group together cases where the formal nature co-opts (for functions) parts generated by material necessity and those where it co-opts (to generate parts) raw materials present by material necessity (189). Instead, we should distinguish “variations” on the architectonic model (where “formal nature uses material necessity as a co-cause”) from cases where material necessity alone generates the part (191-2). This discussion offers a clear orientation to the issues and a judicious assessment of the evidence. Henry’s use of his “architectonic” model successfully re-sets the debate: if he is right that that model is central to Aristotle’s teleological explanations of generation, variations in those explanations should be variations in the application of the model.
Overall, Henry’s interpretation is philosophically fruitful and well-motivated. This excellent book is essential reading for students of Aristotle’s metaphysics and biology.
 Translation from the book.
 David Charles suggests these lines “tie the relevant use of the term ‘to underlie’ … to what remains” (“Physics I.7” in D. Quarantotto, ed., Aristotle’s Physics Book I, Cambridge, 2018, 180). David Bostock’s remarks may point in a similar direction (“Aristotle on the Principles of Change in Physics I” in Space, Time, Matter, and Form: Essays on Aristotle’s Physics, Oxford, 2006, 5). On Aristotle’s interest in persistence, see Benjamin Morison, “The Complexity of the Subject in a Change: Physics I 7, Part 1” in V. Karasmanis, P. Kalligas, and K. Ierodiakonou, eds., Aristotle’s Physics Alpha, Oxford, 2019, 246.
 Cf. Henry’s discussion of 195b5-9 (66-7).
 For the next moves: Sophia Connell, “Nutritive and Sentient Soul in Aristotle’s Generation of Animals 2.5,” Phronesis65 (2020), 324-54.
 As Connell notes (“Nutritive and Sentient Soul”, n. 4 above, p. 349).
 Diana Quarantotto argues that semen transmits (presumably comparatively sophisticated) “living activity” (“Aristotle on the Order of Embryonic Development and the Homonymy Principle” in S. Föllinger, ed., Aristotle’s Generation of Animals. A Comprehensive Approach, Berlin, forthcoming). Also helpful: Ignacio De Ribera-Martin, “Movement (Kinēsis) as Efficient Cause in Aristotle’s Generation of Animals,” HOPOS 9 (2019), 296-326; Jessica Gelber, “Aristotle on Seed” in C. Cohoe (ed.), Aristotle’s De Anima: A Critical Guide (Cambridge, forthcoming).
 Mariska Leunissen, Explanation and Teleology in Aristotle’s Science of Nature (Cambridge, 2010).
 I thank Robert Howton and David Charles for valuable comments on a draft, and Devin Henry and David Charles for discussion of the issues.