Graham Oppy's new book makes a general defense of a no‐frills naturalism. It is a part of the book series Investigating Philosophy of Religion and stands as something of a black sheep among other titles, which approach modern religions such as Islam, Buddhism, and Judaism from a philosophical perspective. Naturalism and Religion, by contrast, seeks to make a philosophical case for naturalism over all such religious explanatory frameworks. This book would be of benefit to any scholar who wishes to understand what naturalism is, and whether it can provide a coherent, plausible, and satisfactory answer to the “big questions” typically thought to lie within the magisterium of religion.

According to Oppy's brand of naturalism, reality is exhausted by “natural causal entities with none but natural causal powers” (p. 25). Oppy accepts that “well‐established science is our touchstone for identifying natural causal entities and natural causal powers” (p. 2). So, our guide for determining what will count as a natural entity or power is current established science (current established science is, roughly, the set of scientific claims about which there is at present, and into the foreseeable future, widespread expert agreement). The book's most general aim is to demonstrate that the very best naturalistic “big pictures” (something akin to a worldview) can be defended against attacks from the very best religious ones. To be clear, this book does not seek primarily to critique religious belief (although it does make the odd jab), but instead seeks to defend naturalism from recent philosophical attacks. In order to mount such a defense, Oppy must juggle several hot potatoes at once, the three hottest being the very concepts of naturalism, religion, and science, all of which are heavily contested.

For this reason, Oppy is careful to draw a circle around the precise notion of naturalism, which he seeks to defend. The circle he draws is minimal, as he hopes to make something of a big tent for naturalists of all stripes to be included. Thus, he accepts that some naturalists are committed to the existence of some abstract entities, such as numbers, so long as such entities are taken to be noncausal. He accepts also that some naturalists may be committed to the existence of mental causation, where mental causal powers are identified with neural causal powers, for example. Oppy is also careful to distinguish naturalism from some closely related ideas with which it is often conflated, such as ontological physicalism (some naturalists accept some emergent phenomena) scientism, humanism, and empiricism (pp. 12–14).

Equally careful is Oppy in outlining how he has arrived at his particular definition of religion. Following the work of Atran and Norenzayan (), Oppy defines religions as communal displays of costly commitments to the satisfaction of nonnatural causal beings and/or the overcoming of nonnatural causal regulative structures (p. 31). Such an account of religion sets belief in nonnatural causal agents and/or regulative structures as a necessary feature. Thus, it appears, by definition, that naturalism and religion are opposed, and that one cannot be a logically consistent “religious naturalist.” Interestingly, Oppy charges that there is no prospect of giving an “insider” definition of religion, since typically any believer belongs only to a tiny fraction of the world's religious traditions. It follows that nobody could be suitably well‐placed to give an insider definition which did justice to the commitments of the adherents of other traditions. “Unless we wish to say that there is really only one religion,” says Oppy, “it seems that the requirements of definition will force us to give an outsider account” (p. 33). However, certain pluralist and perennialist conceptions of religion are what might be thought of as “insider” accounts of religion. Yet, such accounts often manage to extend to a great many contrary religious traditions. Perhaps more convincingly, Oppy charges that common insider accounts that stress the role of “transcendence” or “religious experience” fail to include traditions that stress, say, orthopraxy over orthodoxy (p. 34).

In the fourth chapter, Oppy considers the prospects for developing a “naturalistic religion” and he surveys some possible candidates. It becomes clear that there are, roughly speaking, two ways to attempt to develop a naturalistic religion. One could attempt to “naturalize” the nonnatural commitments of some existing religious tradition (e.g., one could identify God with the sum total of human loving relationships). Conversely, one could attempt to bring communal, ritual displays to bear on the commitments of naturalistic big pictures (e.g. one could develop something like the “Sunday Assembly” movement). Of those committed to the first approach, Oppy assesses pantheism, panentheism, and John Bishop's “euteleological” view. Of those committed to the latter, Oppy considers religious naturalism, the “religion of humanity,” and religious humanism. In brief, Oppy concludes that the former views are either inconsistent with naturalism or ultimately “inadequate foundations for religion” (p. 55). The latter views are found to be either nothing at all like religions, or impossible to manufacture in a naturalistic setting successfully, given that the success of religious belief can be explained as the result of “non‐functional products of human cognitive mechanisms [which] become the locus of emotionally motivated self‐sacrifice that stabilizes in‐group order and assuages existential anxiety” (p. 53).

The next four chapters are the crucial chapters in which Oppy considers the relationship between naturalism, science, and religion. In the course of these chapters, Oppy considers arguments against naturalism, giving detailed treatment to arguments from Alvin Plantinga, Michael Rea, and Thomas Aquinas. Subsequently, he asks whether science defeats religion. That is to say, he asks whether there is a conflict between scientific and religious claims, which ought to be settled on the side of science. Somewhat surprisingly in a book that leans toward scientism, Oppy concludes that since religious claims need not contradict scientific ones, one can avoid the conclusion that science defeats religion. Surely, a stronger claim could be made here. Elsewhere, for example, Oppy says that “all religions include reports of miracles” (p. 169). Either this last claim is hyperbole, or science and religion actually do conflict in a way that would imply, by Oppy's lights, that science defeats religion at least in common practice.

The eighth chapter of this book is the most important. It argues that naturalism indeed defeats religion (even if science can't). Oppy's argument is a simple appeal to old Ockham and his razor. The best naturalistic big pictures are minimal, insofar as their theoretical commitments are a subset of the theoretical commitments of all other best big pictures. Moreover, the best naturalistic big pictures are maximal, insofar as their depth, breadth, and adequacy of explanation is concerned (p. 151). In short, the extra theoretical commitments of best religious big pictures are straight‐up costs for no extra benefit. Oppy discusses some of the phenomena routinely presented as best explained by nonnatural entities or causal powers (e.g., miracles, consciousness, religious experience) and takes his examples from an admirably wide set of religious traditions. The case that Oppy makes in this chapter is simple and persuasive, and so it is surprising to read, by way of conclusion, “I do not claim that the case that I have set out is successful; I do not claim that anyone who fails to be persuaded by this case is irrational… So long as there are intelligent, sensitive, well‐informed people who do not agree with what I've said, I have the best of reasons for supposing that the case that I have set out is not successful” (p. 184). While humility may be a virtue, self‐deprecation is not. Perhaps Oppy is right that his case is unlikely to persuade anyone, since “it cannot persuade anyone who already accepts it, and is unlikely to persuade anyone who does not already accept it” (p. 185). However, it seems unnecessarily cautious (and rhetorically awry) to conclude that one, therefore, has the best of reasons to think one's own argument unsuccessful.

Indeed, this is an important theme in Oppy's book, which is left unanalyzed. The very notion that expert agreement provides good grounds for rational belief is not expanded upon. He describes science as “a collective enterprise of data‐driven description, prediction, and understanding in which universal expert agreement functions as regulative ideal” (p. 127), but little more is said about the evidential value of this kind of agreement. Of course, universal expert agreement is a very useful thing to find, but where we find such agreement, we have found only further evidence that the proposition agreed upon is true (or empirically adequate, or accurate, or whatever). This is just because such an agreement signals to insiders and outsiders alike that the explanation successfully accounts for the phenomena in question (at least, according to the principles of evidence and justification accepted within that knowledge community). Put briefly, when experts disagree, they do not get relationship counseling. They criticize and test the competing hypotheses. If that's because criticism and testing are good ways to eliminate error, then given the careful and critical approach that Oppy has adopted in arguing his case, he should perhaps be a little more confident that his arguments are successful, whether or not they are capable of persuading his critics.