This book is animated by a simple, but important question: If theism is true, then should not the truth of theism penetrate to the core of the correct view of morality?

Mark C. Murphy argues that we should answer ‘yes’ to this question, and begins making his case by using a simple example. He asks us to consider a bowl of water in a locked room that is empty on your return after a week away. It could be that the water has simply evaporated, which would have been the most plausible explanation were it not for the cat present in the room that could not have survived without drinking the water. To settle for this explanation is to use a feature of the situation (i.e. that cats need to drink water if they are to survive for a week) to provide an explanation of what needs explaining. This can be extended by analogy to the theistic explanation of morality, where God is, for want of a better word, a feature of the situation. Just as we need to include the cat at the centre of the explanation of the empty bowl, so we need to include God at the centre of our explanation of morality. This, in effect, becomes a criterion for assessing explanations of morality from a theistic perspective: satisfactory explanations of morality must include our understanding of God at their centre.

What is at issue here is not the role of theistic and religious beliefs informing positions on specific ethical matters (e.g. abortion, the dignity of humanity etc.), but rather with theism playing a central role in explaining morality in general. The obvious candidates for an explanation of morality that allows the truth of theism to penetrate to its core are forms of theological voluntarism, where moral status is determined by acts of God's will. Such theories are, however, open to the serious objections that they sideline the natural order and fail to give an adequate justification of ethics, that an act is right/wrong for no other reason than that God decrees it so. It is because of considerations such as these that Murphy is forced to reject theological voluntarism and his search for a way forward continues.

Whilst natural law accounts, where moral status is determined with reference to concepts such as flourishing, are consistent with theism, Murphy think that they fail to accord God the central and immediate role in moral explanation that he seeks. Murphy clearly has much sympathy for the natural law approach, and by subjecting it to what is a comparatively rare objection in the philosophical context, that natural law is inherently insufficiently theistic, he makes an interesting contribution to the literature. Murphy assesses five lines of defences that the natural law theorist could give, such as that God is in the explanatory background and that God is responsible for moral properties, but marshals an impressive range of arguments to show that natural law theories still fail to provide the sort of theistic explanation of morality he requires.

The way forward, Murphy contends, is a middle position that combines the desirable features of both voluntarist and natural law ethical accounts. Such accounts are perhaps relatively novel in ethics, but Murphy correctly acknowledges that similar issues have arisen in the debates between causal naturalists and occasionalists since the 18th Century. Causal naturalists assume that relations between natural objects can be genuinely causal, and so causal events can be explained without recourse to God. Occasionalists deny this, asserting that what appear to be cause and effect in the natural world are in reality the result of divine intervention to bring about the change. The middle ground between these two positions is concurrentism, where ‘God and creature are complementary causes: in each instance of efficient causation in the natural order, God contributes general, undifferentiated power, while the creaturely agent contributes the specific way that this power will affect other objects; together, these constitute the causing of the unified effect’ (p.145).

The parallels with the moral order are fairly obvious, the causal naturalist paralleling the natural law theorist, and the occasionalist paralleling the voluntarist. It therefore no surprise that the position that Murphy advocates is a form of moral concurrentism. Yet, for all its apparent novelty, I wonder how different this position is from that of Aquinas, generally seen as a paradigmatic advocate of the natural law approach.

Aquinas holds that God is immediately present in the causal order by virtue of being the primary cause of everything, the creator and sustainer of all things in being. Yet there is a genuine causal order in the world, what Aquinas calls ‘secondary causes.’ This seems very close to concurrentism as defined above, the co-presence of general divine power and a particular creaturely power. In the moral context, while the theist and the non-theist can agree that the natural order (including secondary causes) is sufficient for moral explanation in the sense of establishing what is right/wrong etc. (e.g. inflicting pain for pleasure is wrong because it violates human flourishing), the theist advocate of natural law can still claim that God is at the core of his account of morality and that this makes an appreciable difference. The theist's account of the metaphysics and moral significance of human flourishing would be markedly different, presumably of a different order, from that of the non-theist. This need not be limited to allowing theistic considerations to establish what is foundational in the moral theory, but allowing, say, our understanding of the divine attributes to play a major role in the working out of the overall explanatory account. Indeed, assuming theism, a complete account would need to incorporate such considerations. Yet, it seems to me, such an account could reasonably be viewed as a natural law position in being able to give an explanation of morality on the basis of the natural, whilst still asserting that a complete explanation of morality must include our understanding of God at its centre.

There are complex matters here and I have given only a brief outline, but if this line of thought is basically correct, then it seems that moral concurrentism is a less novel position than Murphy seems to suggest. Murphy is, of course, not unaware of such issues, but this is still an area where his claims can be pressed. But even given the reservation I have just outlined and a number of others I have not been able to discuss, this book will be valued for raising important questions and for highlighting the many challenges faced in providing a satisfactory theistic explanation of morality that does not simply acknowledge God but allows theistic beliefs to make a difference.