The bounds of credibility change across time and place. What was believable in one historical period is no longer in another; what one culture finds utterly incredible is an article of faith in another. There is perhaps no more conspicuous instance of this principle than the process of secularisation – the decline in religious beliefs and practices in the modern West. In the context of world history this development has been both momentous and anomalous. Almost all cultures, in almost all historical epochs, have been religious, in the sense of assuming the existence of some extra-mundane reality. Not surprisingly, then, the phenomenon of secularisation has attracted a considerable amount of scholarly attention. Theorists have grappled with its multiple causes, passed judgement on whether or not it has been a good thing, and pondered whether it is an inevitable process that represents the ultimate destiny of all societies. This book offers a new perspective on secularisation that focuses on one of its central aspects – the phenomenon of modern naturalism, understood as denial of, or disbelief in, the existence of supernatural entities and powers. It proceeds by way of a history of the core categories in terms of which this denial is typically expressed, ‘belief’ and ‘supernatural’, and considers how and when these notions became embedded in Western self-understandings.

It is commonly assumed that ‘belief in the supernatural’ is constitutive of religious commitment. However, as this book seeks to establish, the two conceptions that appear in this definition were not available to most of the subjects to whom they are now routinely applied. As we shall see, in the West the expression ‘supernatural’ first came into use in the thirteenth century. Even then, the initial medieval usages are rather different from our present understandings. Only in the nineteenth century do we get the settled idea of a disjunction between natural and supernatural, accompanied by the respective ‘-isms’ of ‘naturalism’ and ‘supernaturalism’. The historically late arrival of a formal notion of the supernatural prompts the question of what adherents of pre-modern religions imagined themselves committed to. Christianity, for example, seemed to have managed quite well for a least a millennium without possessing a conception that is supposedly fundamental to its identity as a religion. Much the same seems to be true for other religious cultures. Paradoxically, then, it looks as though the concept of the supernatural is more important for the self-image of modern naturalism than it had been for those with traditional religious commitments.

The puzzle is deeper than this because the other key notion in the definition of religion – ‘belief’ – turns out to be characteristically Western and historically late as well. Unlike the case of ‘supernatural’, the belated appearance of a modern idea of belief is disguised by the existence of an older terminology of faith/belief that seems to map reasonably well on to modern understandings.Footnote ² Yet, as we will see, these modern understandings of belief locate it within a new epistemological framework that is discontinuous with its past in crucial ways. This is especially true when it comes to the question of whether faith commitments were voluntary and what kinds of justification were called for in order to hold beliefs. As Charles Taylor has noted, a distinctive feature of secular modernity is that belief in the supernatural has become ‘one option among others’.Footnote ³ The historical possibility of disbelief was accompanied by a change in what belief itself was thought to consist in.

In short, the concepts of ‘belief’ and ‘supernatural’ as we presently understand them were not prominent in pre-modern Christianity nor, indeed, in other cultures. To recount the circumstances that led to the emergence of these conceptions is to tell the story of how Christianity came to assume a form that made its denial possible. As it turns out, these circumstances relate to neither philosophical critique of the rationality of theistic commitment, nor scientific assaults on the plausibility of religious beliefs, but mostly concern the downstream effects of historical developments internal to Christianity itself. This points to an ironic pattern of modern naturalism’s indebtedness to a particular version of monotheism. Modern religion, understood as belief in the supernatural, and modern naturalism, understood as its denial, are two sides of the same historical trajectory.

A related theme of the book addresses the fact that, viewed in the light of our own history and considered in relation to that of other cultures, the dawn of a secular age and the rise of a naturalistic outlook is something of a historical aberration. When, in the eighteenth century, the Scottish philosopher David Hume wrote his sceptical account of miracles he declared that reading the records of ancient nations was akin to encountering ‘some new world’. For Hume, the miraculous relations of these times and places suggested an alien nature that operated on entirely different principles to those with which he and his readers were familiar. But from a more dispassionate perspective it is the naturalistic world of secular modernity that is new and strange. Sustaining the intellectual foundations of naturalism thus requires not only an assumption of the universality of what are contingent and culturally unique historical categories – ‘belief’ and the ‘supernatural’ – it also needs a normative theory of history, tacit or otherwise, that lends legitimacy to the unique historical turn that the secular West has taken. That will usually be some theory of historical progress that places the West in the vanguard of civilisational advance. As will become apparent, eighteenth- and nineteenth-century theories of social evolution that purported to show how societies inexorably progress from some magical, mythical, religious stage of development to a more enlightened scientific one, continue to undergird contemporary naturalism despite this dependence rarely being acknowledged. The more overt assumption seems to be that the singular naturalism of the modern West is licensed by the success of the natural sciences. But in a sense, this is just a doubling down on the older stories of progress, but with an apparently larger and more defensible evidence base. The modern sciences, it can hardly be denied, do represent something unique and special, and for the most part in a good way. The achievements of science serve double duty in this discussion because they are argued to be the very embodiment of naturalism. Close examination of the relevant history reveals something different, however. As in the case of naturalism’s conceptual indebtedness to theology, narratives of the uniquely progressive direction of Western history can be understood as repackaged theological notions of providence or eschatology presenting themselves in a secular guise. Moreover, the marriage of science and naturalism is a late nineteenth-century invention, advanced to counter the then standard assumption that the regularities of nature upon which science was premised were to be explained in terms of divinely instituted laws of nature. Viewed in the light of this history, naturalism looks like a simple redescription of a long-standing theistic account of nature’s regularities. This points again to a curious dependence of naturalism on certain theological understandings of history and of nature.

Before setting out the structure of the book in relation to these historical contentions it is worth briefly clarifying exactly what ‘naturalism’ is taken to be for the purposes of the argument. One reason for focusing on naturalism is to avoid some of the misunderstandings and controversies that characterise discussions of the related themes of secularisation and disenchantment. Challenges to these latter conceptions have pointed to instances of desecularisation and re-enchantment, and it has been plausibly argued that enchantment never fully went away.Footnote ⁴ The case of naturalism is a little clearer, provided that we understand the term in a relatively straightforward way. For our main purposes, naturalism is simply the view that there are no supernatural entities or spiritual powers. Caltech theoretical physicist Sean Carroll describes it this way:

It is common to distinguish methodological naturalism from metaphysical naturalism. Most straightforwardly, in the sciences, naturalism is a methodological commitment that rules out making explanatory reference to supernatural entities. This is relatively uncontroversial even among theistic scientists.Footnote ⁶ However, this methodological stance often shades into the stronger position of a metaphysical naturalism which holds that the sciences, in reality, are incompatible with the proposition that there any supernatural entities. In practice, the methodological stance (pursue science as if supernatural entities did not exist) is often assumed to entail the truth of some metaphysical commitment (supernatural entities do not in fact exist) – this, typically because the explanatory success of the natural sciences is thought to vindicate the view that nothing exists except the material realities that science is capable of investigating.Footnote ⁷

Naturalism in this stronger metaphysical sense also reigns in the social sciences and humanities – and especially philosophy.Footnote ⁸ A century ago the American philosopher Roy Wood Sellars declared that ‘we are all naturalists now’.Footnote ⁹ This announcement may have been premature, but there is little doubt that most contemporary philosophers now identify themselves as naturalists in the sense of affirming that there are no supernatural entities. This is reflected in both the discipline of philosophy and the personal beliefs of its practitioners. The non-existence of God seems to be one of the very few things philosophers are able to agree on.Footnote ¹⁰ Related to this naturalistic orientation is a deference to the natural sciences and their methods. These are often held up as a model for philosophy and as the approach that offers the most secure path to reliable knowledge.Footnote ¹¹ Wilfrid Sellars, following in his father’s footsteps, put it concisely: ‘science is the measure of all things’.Footnote ¹² Philosopher Willard Quine, a strong advocate of naturalism, similarly recommended that philosophers accept ‘the fundamental conceptual scheme of science and common sense’. For Quine, it was the natural sciences, and not metaphysics, that most directly grappled with reality.Footnote ¹³ The deference to science that characterises analytic philosophy is not unrelated to the fact that for many philosophers theism was no longer an intellectually defensible position.

There are, however, different versions of philosophical naturalism that are important to distinguish from the straightforward denial of the existence of the supernatural. Another kind of philosophical naturalism focuses on the fact that human beings are natural creatures inhabiting a natural world. Things can get confusing here, because this recognition has spawned two rather different approaches which, following philosopher Huw Price, we can label ‘objective’ and ‘subjective’ naturalism.Footnote ¹⁴ Briefly, objective naturalists, in keeping with a privileging of scientific discourse, concern themselves with how phenomena such as consciousness might be related to neurons, or how human activities like making choices, having beliefs, and holding moral values might be understood in a world that on a standard scientific description consists only of fundamental particles interacting in space.

Subjective naturalists consider this a rather unpromising enterprise. They understand the problems identified by objective naturalists as essentially linguistic, arising out of the different ways in which the relevant terminologies are being deployed. The solution to philosophical problems, on this latter understanding is not, as in the project of objective naturalism, a matter of reconciling two different languages, or of reducing everyday language to scientific language, or indeed eliminating it if need be. Rather it is a matter of seeing how different vocabularies have come into existence and understanding the purposes they were originally intended to serve. In the first case, problems are solved by finding the best language with which to represent reality and by dispensing with imprecise expressions deemed to be inconsistent that (scientific) reality. In the second, problems are dissolved by showing how they emerge out of the misapplication of vocabularies that have been forced to migrate out of their original social or historical context. These two positions give rise to different conceptions of the task of philosophy. For objective naturalists the goal is a reductive, ahistorical conceptual analysis; for subjective naturalists it consists in something closer to anthropological, historical, and social analysis.Footnote ¹⁵ The difference partly turns on what counts as the ‘natural’ in naturalism. For objective naturalists, ‘nature’ is understood as the reality depicted by scientific language. For subjective naturalists, ‘nature’ is the biological and social matrix in which human activities including the sciences themselves are embedded. The approach of subjective naturalism, with its focus on linguistic communities, thus offers an explanation for why there is this difference in the first place.

While this book problematises aspects of contemporary naturalism understood as a rejection of supernaturalism, the approach adopted nonetheless has affinities with subjective naturalism as described above. To some degree, the historical considerations offered here are akin to the ‘therapeutic’ approach of the subjective naturalists. But to return to the definitional question, ‘naturalism’ as used throughout the book bears the straightforward sense of denial of supernatural and spiritual powers or entities.

The structure of this volume closely follows the logic of the case outlined above. Following a brief prologue (on which more below) the second chapter traces the fortunes of our modern understanding of ‘belief’ which is deeply informed by its original uses in a religious context. It begins with an account of faith/belief in early Christianity, showing how the primary meanings of the relevant expression related to trust rather than intellectual assent. In the medieval period, this social component of faith/belief was formalised in the conception of ‘implicit faith’, which enabled lay believers to affirm abstruse theological doctrines without the requirement of a full intellectual comprehension of what was being affirmed.

The third chapter describes how, during the sixteenth century, implicit faith became one of the chief targets of the Protestant reformers, who insisted that individuals take full responsibility for what they believed. True belief was thought to entail explicit knowledge of what was being assented to, along with some capacity to justify that belief in a way that did not simply defer to authority. Critiques of implicit faith represent the first articulation of what is now referred to as the ‘ethics of belief’ – the principle that we have an ethical duty to have evidence for the beliefs we hold. As a consequence of these critiques, belief came to be thought of more in terms of intellectual assent than affective trust. These early modern changes to the sociology and philosophy of religious belief contributed to the subsequent epistemological preoccupations of modern philosophy.

The chapter that follows reflects upon the implications of the new status of belief by reconsidering the history of traditional arguments for the existence of God. If disbelief in the supernatural had not been a genuinely live option before the appearance of modern secularity, and if there was little demand that beliefs be entertained only if supported by certain kinds of evidence, what was the point of articulating formal proofs of God’s existence? The thrust of this chapter is that the so-called classical proofs for God’s existence performed a very different function to the one that they were later to assume. They were more akin to spiritual exercises than logical arguments constructed from putatively neutral premises. Crucially, moreover, one of the central ‘proofs’ – that based on universal consensus – involved a simple appeal to the ubiquity and universality of religious belief. The demise of this argument in the early modern period signalled a major change in how belief in the supernatural came to be understood, indicating that the burden of proof was shifting from unbelievers to believers. At the same time there arose a new conception of natural theology, understood as an enterprise that could provide support for religious belief on rational grounds alone. The changing status of natural theology and proofs for God’s existence thus correlated directly with the appearance of a new notion of belief and the requirements for its justification.

Chapter 5 turns to the origins of our present understanding of the natural/supernatural divide, showing how the terminology of ‘the supernatural’ first emerged in the Middle Ages and gradually assumed its modern form between the seventeenth and nineteenth centuries. The attendant ‘-isms’ – naturalism and supernaturalism – arrive at the end of this period, during the 1800s. The original context for the naturalism/supernaturalism distinction was neither science nor philosophy, but the sphere of biblical criticism. From there it was imported into a scientific context. The nineteenth century also witnessed attempts to reconstruct the history of science with a view to arguing for a long-standing alliance between naturalism and science. A more accurate portrayal of the relevant history shows, to the contrary, that ‘science’ had been consistently aligned with theistic assumptions about the regularities of nature. These regularities were formalised as laws of nature in the seventeenth century, at which time they were understood as divinely authored imperatives to which nature necessarily conformed. In the nineteenth century, what had originally been understood as expressions of the divine will were simply redescribed in purely naturalistic terms as laws of nature by advocates of naturalism. Ironically, in this redescribed form, they were now claimed to represent evidence against theistic readings of nature.

The sixth chapter considers how the exceptionalism of Western naturalism was given legitimacy through an appeal to narratives of progress. The basic form of these narratives was derived from an original Protestant model that had divided history into two periods – one in which miracles had genuinely taken place followed by a second that was characterised by the absence of genuine miracle-workers. From the principle of the historical cessation of miracles it followed that medieval and contemporary Catholic miracles were fraudulent. Protestants also understood the Reformation as having ushered in an age of light after a period of medieval darkness. Subsequent eighteenth-century thinkers simply generalised and extended this argument, contending that the miracle reports from all historical periods were fraudulent. History could now be divided into an earlier period characterised by a naïve credulity in relation to miracle reports, followed by a more mature phase of history during which there was increasing recognition of the falsity of miracle reports. These same eighteenth-century thinkers also arrogated to themselves the mantle of enlightenment. The progressivist histories characteristic of the early social sciences and endorsed by advocates of scientific naturalism were doubly indebted to religious models since they also drew upon providential or eschatological notions of historical directionality. This raises the question of whether their progressivist philosophy of history is problematically dependent upon covert theistic assumptions.

The concluding chapter briefly considers questions about the possibility of religious experience in a secular age and offers some concise theoretical conclusions. These address how the theses outlined in the book relate to other ‘modernity stories’, including those of Charles Taylor, Brad Gregory, John Milbank, and Alasdair MacIntyre.

Immediately following this introduction is a prologue that offers a brief consideration of David Hume’s celebrated argument against believing miracle reports. The intention is not to add further to the oversupply of philosophical commentary on this argument, but rather to show how Hume’s stance nicely exemplifies several key issues relating to the emergence of modern naturalism as discussed in the book. Hume assumes the universal and unproblematic nature of such core conceptions as ‘supernatural’ and ‘laws of nature’. Lurking in the background of his logical argument, moreover, is a good deal of cultural condescension which is nonetheless central to making the argument work. Overstating the case somewhat, we might say his case against miracles can be read as a set of cultural prejudices and question-begging premises dressed up as a piece of philosophical reasoning. The prologue also makes reference to what I have called ‘Hume’s dilemma’. Hume relies upon the weight of the testimony to establish his case against believing miracle reports, but must also contend with the weight of testimony, across different times and cultures, to the existence of the supernatural. The dilemma is resolved by an appeal to historical progress (and arguably a dubious racial theory) that enables him to discount testimonies emanating from the past and/or other cultures. The real weight of the argument is thus borne by a set of historical assumptions that appear in the guise of non-essential afterthoughts. Hume’s ‘dilemma’ has not gone away and, if anything, is even more acute, since the traditions and beliefs of non-Western cultures are now more difficult to dismiss on the basis of dubious historical accounts of Western exceptionalism. To advert to philosophical terminology, there is a tension between the traditional ethics of belief on the one hand and the demands of epistemic justice on the other. The former is to do with an ethical imperative to have good reasons for what we believe; the latter with a responsibility not to dismiss the reported experiences of others, based upon their status.

A brief comment about the approach taken and the notes, which are extensive, is in order. The argument set out in this book covers a rather wide historical range and calls upon a number of disciplines – mostly history, but also philosophy, theology, and anthropology. The historical range is necessary, partly because it enables us to see just how peculiar naturalism is when viewed in historical perspective, and partly because during the nineteenth century naturalism was provided with its own foundation myths which encompass a similar historical span. The disciplinary mix simply reflects what answering the core questions requires, along with the fact that our disciplines do not map onto the division of knowledge as understood in the historical periods under discussion. This approach is admittedly fraught, especially given the high degree of scholarly specialisation that characterises modern academia. I though it worth the risk since the project seems important and demands an ambitious scope of enquiry. Even so, I am acutely conscious of having omitted significant historical episodes and compressed others (especially the medieval period) while not including disciplinary knowledge that would have been relevant. My optimistic assumption has been that these omissions would have been supportive of the argument anyway. Reviewers will doubtless correct me if I am wrong. Ideally, I would also like to have devoted more space to the contemporary implications of this analysis, especially for cross-cultural understanding. But my first priority was to articulate the argument and so this important discussion must be taken up by others or await a future publication project. This brings me to the notes, which may seem excessive. Following on from the observations above, they are there to signal my indebtedness to the scholarly labours of others who work in times and disciplines in which I am not an expert. The notes also provide additional evidence for claims summarily made in the main text. Finally, the broad scope of the text provided challenges not only for the author but will do so for the reader as well. Some of the notes, accordingly, will add nuance and, on occasion, provide guidance to some of the more detailed discussions best kept out of the text. Leaving these details to the notes makes for a more readable book, but I also hope that there will be those who enjoy the notes, too.