Aristotle developed an account of four causes (or aitiai) to explain things and processes in the natural world. It is very common to explain this causal scheme with reference to a single example: given a statue, the material cause is the bronze out of which the statue was made; the formal cause is the form or shape which the statue came to have; the efficient cause is the source or maker of the statue (or, more specifically, the maker’s craft); and the final cause is that for the sake of which it was made, its purpose. However, in his book Aristotle’s Four Causes, a revised version of his Habilitationsschrift submitted to the University of Leipzig, Boris Hennig sets out to challenge this received understanding in significant ways. The result is a new and philosophically sophisticated account of Aristotelian causation, why there are four of them and how they relate to one another.

For Hennig the four causes form a system of two co-ordinated pairs. Put briefly, the material cause (or matter) of a natural thing is to potentially be this thing, while the formal cause (or essence) is what the matter potentially is, or what the natural thing becomes according to its typical course of development. Correspondingly, the efficient cause (which is the only one of the four causes to produce effects, and so be a cause in the modern sense of the term) of a natural process is that which potentially is this process, whereas the final cause of a natural process is the essence or limit of this process so long as all goes well. Material causes relate to formal causes roughly as efficient causes relate to final causes. Further, whereas material and formal causes are concerned with things, as opposed to properties which belong to things, efficient and final causes are concerned with processes, even though these processes can ultimately be reduced to things which (potentially or actually) change or act in a certain way.

Hennig’s book consists of a general introduction, ten chapters and a conclusion. The ten chapters divide into five pairs of two chapters each, with the first (‘Aristotle’s Four Causes’) providing a detailed summary of the book as a whole and the four remaining pairs of chapters being devoted to each of the four causes (chapters 3–4 on matter; chapters 5–6 on form; chapters 7–8 on causation; chapters 9–10 on teleology). Chapter 2 (‘Two Epistemic Directions of Fit’) discusses J.L. Austin’s distinction between two ‘directions of fit’ (‘How to Talk’, ch. 6 of Philosophical Papers (Oxford 1979)) with a view to showing how, according to Aristotle, natural things must be approached in a certain way in order that they might be analysed causally. Ifound the inclusion of this chapter slightly jarring after the introductory steps provided by the first two sections, especially since the philosophical resources which it employs are not used again until chapter 6. The main chapters begin and end with Aristotle but contain extensive discussions of other thinkers, such as Kant, Hume and Anscombe, except for the two chapters on matter, which follow Aristotelian texts more closely.

Hennig’s approach is tirelessly philosophical. Near the beginning he outlines his analytic aim to ‘repeat’ what Aristotle said rather than merely ‘report’ it, stating that ‘[i]n general, Iendorse the claims that Iattribute to Aristotle, so that Iam as accountable for their intelligibility and truth as Ithink Aristotle is’ (9). This bold strategy pays great dividends in many contexts. For instance, in chapter 7 Hennig defends as Aristotelian the view that causation should not be understood as a relation between two distinct things, that is cause and effect, as Hume maintained, but as a process within which cause and effect are parts. Chapter 5 also sees Hennig develop the notion of a ‘type’ to demonstrate that, for Aristotle, the formal cause (or essence) of a natural thing is subject to certain standards of typicality, which may not always be reflected by particular instances of that thing (understood as a ‘type’). In both of these cases, Hennig’s insights not only overturn the simplistic traditional picture of the efficient and formal causes as the agent and shape, respectively, but also seem deeply Aristotelian (compare, for example, the notion of the formal cause as a generic type with its description as a ‘paradigm’, Ph. 2.3, 194b26).

In sum, Hennig’s book is a demanding read but it offers a rewarding and impressively coherent study of Aristotle’s four aitiai that, at least for this reader, is superior to the traditional way in which they are understood. It certainly deserves to be read (and reread) by anyone interested in Aristotle’s philosophy of nature or causation in general.