In this monograph, Arthur Oosthout lays out meticulously Proclus’ mereology and its integration into his Neoplatonic system, showcasing both its continuity with the Platonic and Aristotelian heritage as well as its relation to modern mereology. Oosthout demonstrates how mereology is an integral part of Proclus’ thought by illustrating how fundamental axioms recur throughout Proclus’ metaphysics, that is, how a theoretical framework is used to explain philosophical problems arising from his exegesis of Plato. The work consists of seven chapters, divided into four parts, which move from a discussion of mereology in general to Proclean mereology specifically and culminate in its application to Proclus’ metaphysical hierarchy.
After a brief introduction that includes a discussion of the status quaestionis and the author’s own work on the issue, Oosthout proceeds in the second chapter to introduce the reader to modern mereological thought, focusing mainly on David Lewis’ concept of ‘unrestricted composition’ and Peter van Inwagen’s life-principle of wholes. In addition, mereology in Plato and Aristotle is discussed from the perspectives of Verity Harte and Kathrin Koslicki, respectively. This background, which, at least in its modern form, is quite different from Proclus’, offers a helpful contrast to what is to follow. More time could have been spent on drawing more concrete conclusions from the discussion of modern mereology, so that the reader knows what to bear in mind when moving on to Proclus (e.g. from the discussion of ‘Classical Extensional Mereology’).
In the third chapter, Oosthout discusses ‘Proclus’ rule’ and its application, whereby beings at both ends of the metaphysical hierarchy have lower complexity than those in the middle. Every level has a number of attributes that it derives from a higher source. This has consequences for mereology, as the attributes are more or less universal (or particular) depending on how fundamental they are, e.g. life is more universal than cognition. Oosthout makes clear that composition is only associated with multiplicity in the intelligible realm; in the sensible, it is opposed to multiplicity, as something more composite is located higher in the metaphysical hierarchy (per Proclus’ rule).
The fourth chapter acts arguably as the heart of the book since it discusses the three different types of wholeness according to Proclus (well illustrated with a figure on p. 92): (1) the whole before the parts; (2) the whole composed of parts; (3) the whole in the part. While most scholars associate the triad with a specific aspect of Proclus’ metaphysics, Oosthout rightly points out that its purpose is ‘rather to establish a mereological basis which informs each and every whole in the Neoplatonic universe’ (87). The Whole itself, i.e. the whole before the parts, is identified by Proclus with a specific entity, namely the second intelligible triad, but manifests itself also at different levels (e.g. in the world-soul or cosmos).
Chapters Five and Six pick this up and treat wholeness and, more specifically, the triad of wholes within both the intelligible realm and the sensible realm. This part of the book is more ‘practical’ and especially useful for the reader of Proclus’ Platonic Theology and Commentary on the Timaeus. Oosthout emphasizes how a certain tension arises between the understanding of a whole as an extensional and relative term (e.g. genus over species) and as an ontologically fixed term (e.g. three manifestations of the whole Intellect). Another major aspect of Proclus’ mereology shines through here, namely its connection with axiology: being whole equates to being perfect, although wholeness is strictly speaking metaphysically prior to perfection, which, like wholeness, is an intelligible principle on its own. When focusing on the sensible realm, Oosthout points out the important mereological role played by Time, which bestows wholeness upon temporal beings in the universe.
The concluding chapter brings together the different strains and reaches more general conclusions. This includes a brief recapitulation of the modern mereological theories and their difference with that of Proclus, which stands out in its connection with axiology, as well as the definitions of wholeness and parthood (which perhaps ought to have been given earlier in the book). Regarding the former, Oosthout shows how Proclus’ mereology bears some similarity to van Inwagen’s insofar as Proclus also considers wholes as alive, though he is willing to extend the meaning of ‘alive’ beyond those things accepted by van Inwagen. One could say that, for Proclus, it is less the whole that is problematic than the part. This is in line with Neoplatonist thought, which problematizes plurality rather than unity. Oosthout concludes that a reconciliation of Proclus’ mereology with its modern counterparts is hardly achievable because of the former’s dependence on Neoplatonist metaphysics specifically and ancient views about the functioning of the universe (e.g. teleology) more generally. Proclus’ conceptualization of wholeness and parthood is Neoplatonist and meant for Neoplatonists.
The bibliography is rich and up to date. The book has helpful indices (locorum and rerum). It is well-presented and includes only a few typos (e.g. p. 35: ‘incited conclude’; p. 87: ‘‘the form of ‘each thing’ (τὸ ἑκάστου εἶδος)’; p. 92: ‘καθ’ ὕπαρξτιν’; p. 101: ‘insofar is it’; p. 111: ‘each kind whole’; p. 125: ‘The second consequence of equation Eternity with Wholeness itself’; p. 209: ‘participation of in their share of the good’; p. 220: ‘in the intelligible realm that it does in the sensible one’; p. 222: ‘in which Proclus the term’). A special mention is merited for the various figures which help the reader navigate through Proclus’ intricate and at times convoluted system.
Oosthout has done a great service to Neoplatonist studies by illuminating an important aspect of Proclean thought. His comparison of Proclus with contemporary philosophy should be taken as an example for further research on Neoplatonist metaphysics.