In spite of its modest length, Philip Clayton's Mind and Emergence is a remarkably ambitious book; its scope can best be conveyed by brief summaries of each of its five chapters. The first chapter, ‘From Reduction to Emergence’, narrates the history of the emergence concept from the late nineteenth century down to the present. The second, ‘Defining Emergence’, discusses different conceptions of emergence and lays the conceptual foundations for what follows. ‘Emergence in the Natural Sciences’ presents a broad range of scientific phenomena that can be identified as emergent, ranging from physics to biology. ‘Emergence and Mind’ picks up the story from there and develops in detail the central, crucial case for philosophies of emergence, the emergence of mind and of persons. The final chapter, ‘Emergence and Transcendence’, develops the broader metaphysical implications of emergence, including the notion of deity as emergent, which Clayton seriously entertains but ultimately fails to embrace. So in a little over two hundred pages we have a history of emergence, conceptual clarification of the various notions of emergence, a detailed overview of emergence as it appears in the natural and human sciences, and the application of emergence to the most profound and far-reaching questions of metaphysics.

Accomplishing these tasks imposes major demands upon the author, both in literary terms and in the breadth of information that must be assimilated and synthesised. The literary demands are met with apparent ease. Some of the material is quite detailed, but one never feels bogged down in detail; technical concepts are explained when necessary but there is no sense that technicalities are being flaunted for their own sake. The exposition is fairly dense in spots but manages to flow well nonetheless.

Clayton's mastery of the tradition of rational theology (revealed theology plays no substantive role) is evident; he has written extensively on this in the past. His knowledge of a broad range of scientific work is also impressive. Possibly specialists in the various sciences might find things to fault here, but he considers, and where necessary lucidly explains, a wide range of examples from many different areas of science. Where philosophy is concerned, however, the picture is mixed. Clayton has read much of the philosophical literature that is relevant to the emergence debate and makes extensive reference to it, but at crucial points a philosophically informed reader will recognise that his comprehension of the material is not as accurate as it needs to be. One example of this (among others which could be cited) is found in his discussion of mind-body supervenience (pp. 124–7). Clayton borrows from Jaegwon Kim the distinction between weak and strong supervenience, but actually he fails to follow Kim's distinction; all of the varieties of supervenience discussed by Kim would be ‘strong’ in Clayton's classification. The ‘type–type supervenience’ which Clayton incorporates into his own theory would not be recognised as supervenience at all by Kim or by other philosophers who have written on the topic. Philosophically knowledgeable readers are bound to find this material perplexing; others will simply be misled.

The central conceptual question, of course, concerns the definition of emergence itself. Clayton writes, ‘If forced to give a one-sentence definition, I would say that emergence is the theory that cosmic evolution repeatedly includes unpredictable, irreducible, and novel appearances’. But he immediately goes on to say that ‘simple definitions fail to satisfy: either they combine features of multiple theories at the cost of superficiality, or they present one particular viewpoint without argument while passing silently over all others’ (p. 39). Among emergence theories, the most important distinction to be made is between ‘weak’ and ‘strong’ emergence. Roughly speaking, weak emergence is the view that novel properties emerge at higher levels of organisation, but these properties do not affect what goes on at the lower levels; causation remains ‘bottom-up’. (For the body–mind issue, this view results in something like epiphenomenalism; mental properties and events occur all right, but have no causal influence.) Strong emergence, on the other hand, is the view that the novel, emergent properties do affect what happens at lower levels; thus ‘top-down causation’ is crucial for strong emergence.

At this point the scientific examples cited by Clayton come into play. It is beyond question, once one has abandoned the more extreme varieties of reductionism, that science affords numerous examples of (at least) weak emergence. For some of the examples it is evident that only weak emergence is occurring; others (according to Clayton) are ambiguous as between weak and strong emergence. In the end Clayton leaves it undecided how many cases of strong emergence there may be, but he insists there must be at least two, with the strongest candidates being mind and biological life. (If mind were the only instance, we would be in danger of falling into the pit of mind–body dualism, a view which functions throughout the book as a bugbear to be avoided at all costs.)

As noted, top-down causation is crucial for strong emergence as Clayton understands it. Unfortunately, the concept of top-down causation he is appealing to (and it is not a concept that is unique to Clayton) is deeply confused, even contradictory. This generates an incoherence at the very heart of his position, one that needs to be corrected before the position is even a possible candidate for acceptance. We now proceed to justify these assertions.

The key claim of the view in question is that higher levels of organisation exert a ‘top-down’ influence without creating modifications of or exceptions to the laws that govern the lower levels. As Roger Sperry says, in a passage quoted and endorsed by Clayton, ‘Higher-level phenomena in exerting downward control do not disrupt or intervene in the causal relations of the lower-level component activity. Instead they supervene, in a way that leaves the micro interactions, per se, unaltered’ (p. 139). As an example of this, Sperry cites a rolling wheel, in which an individual molecule retains its usual inter-molecular relations within the wheel, but is nevertheless carried through a particular space-time trajectory as a result of the movement of the wheel as a whole. This example is all too effective; it illustrates perfectly the fatal flaw in the entire proposal. The ‘wheel as a whole’ is itself made up of individual molecules, and it is precisely the laws of physics, operating on those molecules in their relations to the proximate environment (itself made up of similar molecules, governed by the very same laws) that govern the motion of the wheel and therefore of the individual molecules within it. There is absolutely nothing here that can elude the constraints of ‘bottom-up’ causation; we have at most an example of weak, rather than strong, emergence. The same principles apply, and the same flaw appears, if we are speaking of life as emerging from a suitable organisation of molecules, or of consciousness as emerging from the functioning of neurons in the brain. That is, the flaw occurs so long as the micro-laws are invariant and hold without exception. Only by challenging the latter assumption (as has been done, for example, by Timothy O'Connor) is it possible to craft a viable notion of top-down causation. This, however, is something Sperry, Clayton and many others are unwilling to do, because it would compromise the hegemony of physics, which they fear to challenge on pain of becoming ‘unscientific’. But physics so understood exacts a high price from its devotees; the cost of paying homage to it is complete causal reductionism, a view that emergence theory was supposed to deliver us from. Finally, in the last chapter, Clayton says, ‘In the end . . . it makes sense to defer to science only in the domain of its particular epistemic authority’ (p. 181). But this reservation comes too late, and is given too narrow an application, to save him from the problem noted above.

Something needs to be said also about Clayton's final chapter – his explorations of the possibility of a transcendent level beyond the natural order. The trajectory of his thought creates some momentum in the direction of a view like Alexander's in which deity, or godhood, is a property (or set of properties) of the natural order itself, emerging in the course of the development of the universe. In the end, however, he rejects this, preferring to posit a divine ‘being, force, or ground’ that was present from the very beginning. In so far as this move is made, he admits, ‘the framework of emergence has been left behind’ (p. 179). In fact, the view results in a sort of ‘theological dualism’, which Clayton argues is more acceptable than the mind-body dualism he has scorned. And this much at least is true: any view which even distantly approximates the traditional theistic conception of God is unable to avoid such a dualism of God and the world. But Clayton's own theism is far from traditional, as is shown by his account of divine action. He admits that ‘an infinitely powerful being (if one exists) could do anything it wanted in and with the world it created’. However, this assertion ‘does not count as a solution to the problem of divine action’, because actions of this sort would imply the ‘falseness of physics’ (p. 188). The physicist, then, has apparently been granted authority to determine either that God is prohibited from acting in ways that go beyond the laws of nature, or else that if God were to attempt to do so the attempt would fail! As Clayton also states, ‘a God who would be doing things within the cosmos subsequent to the big bang would be encroaching on the territory for which the sciences are responsible’ (p. 185). We certainly would not want God to be guilty of encroachment!

In fact, no sort of divine action in the physical world below the level of persons (and possibly some of the higher animals) is deemed credible – so after the initial creation, God was forced to wait for some billions of years before any further opportunity for divine action presented itself. (God on Clayton's view has considerably less capacity to act in the world than the God of process theism, which has itself taken heavy criticism on just this point.) But how is it that God can act so as to affect the lives of persons? For various reasons, Clayton excludes both divine manipulation of quantum indeterminacies and direct transfer of ideas from the divine to the human mind. In the end, he admits that ‘The theist will be unable to explain in human-scientific terms how it is that God affects the person as such’ (p. 198), while insisting that his view does allow for divine influence, and revelation, in the form of a ‘differentiated lure of the divine on humans’.

As usual, this review has focused on difficulties and objections at the expense of a deserved appreciation for the positive qualities of Clayton's book. His richly detailed discussions contain many valuable insights, and the volume as a whole serves as an extremely useful guide to the full range of literature on emergence. He has, furthermore, presented a carefully worked out theory of his own, which deserves careful consideration (provided that the major flaw noted above can be remedied) as a possible account of the role of emergence in our world.