It is sometimes natural, at least for philosophers, to claim that some facts obtain in virtue of some other facts obtaining. The fact that someone in this room is typing right now, for instance, obtains in virtue of the fact that I am typing right now. This is not the same as saying that the proposition that I am typing entails that someone is typing. After all, the proposition that I am typing entails that 1 + 1 = 2, and every other necessary proposition, but it misses the point of the ‘in virtue of’ relation to claim that 1 + 1 = 2 in virtue of the fact that I am typing. Nor is it right to identify the ‘in virtue of’ relation as a straightforwardly causal relation, although some find it natural to say ‘someone in this room is typing because I am typing’. To use ‘because’ in this way is not to imply that my typing causes someone in this room to be typing, as fire causes smoke, or a virus causes flu. We are not dealing with the kind of causal relation that interests natural scientists. Rather, the relation is of a peculiar interest to metaphysicians. It is often called the relation of metaphysical grounding. The fact that I am typing grounds the fact that someone in this room is typing. This is not to say that all supporters of the idea of grounding commit to an ontology of facts. One can avoid saying things like ‘fact X obtains in virtue of fact Y’, and thus seemingly committing to the existence of facts, by instead saying things like ‘Y because X’ – again using ‘because’ in a special metaphysical sense.

Many questions of metaphysical interest can be expressed as questions about grounding. Indeed, grounding may well be the concept that links together and renders intelligible a number of difficult philosophical questions. ‘Is a whole no more than sum of its parts?’ can be interpreted as: ‘Is the existence of the whole grounded in the existence of the parts?’, or (using the special ‘because’) ‘Does the whole exist only because the parts exist?’. Similarly, ‘Does the wrongness of an action consist in its arising from a bad intention’ can be rendered: ‘Is the action wrong only because it arises from a bad intention?’. ‘Does the meaning of a proposition consist of its use’ can be taken as: ‘Does a proposition have a certain meaning only because it is used by all competent speakers of the language in a certain way?’. And so on. For a non-philosopher, this is probably no advertisement for the usefulness of the notion of grounding. For a philosopher, however, it may be.

Metaphysical Grounding, edited by Fabrice Correia and Benjamin Schnieder, collects together a series of articles developing and debating the notion of grounding. Beyond this, the topics of the articles vary quite widely. Some, such as those by Kit Fine, Jonathan Schaffer, Jonathan Lowe, and Kathrin Koslicki presuppose a commitment to grounding talk, and defend specific views about the logic and semantics of grounding claims. Others, including those by Chris Daly and Paul Audi, debate whether the notion of grounding is acceptable at all – whether, that is, it is intelligible, useful, etc. J. Robert G. Williams, David Liggins, and Stephen Barker discuss ideas that may be seen as rivals to the idea of grounding – the idea of truth-making, for instance. Michael Della Rocca contributes the sole historical article, interpreting and drawing out the implications of some of Leibniz's and Spinoza's key arguments in terms of grounding. Detailed accounts of the contents of these chapters can be found in the introduction to the volume as well as in other reviews of it.

The chapters vary widely in their level of presentation. Some, like Lowe's, are difficult, involving complex and sometimes confusing ideas and arguments. Others, such as Daly's, Della Rocca's, Koslicki's, and Azzouni's are recondite, sprawling in their references to various debates and discussions in recent philosophical literature. Fine's, Williams', Liggins', and Barker's seem to me both recondite and difficult. Audi's and Schaffer's, by contrast, are self-contained and straightforward; comprehending them requires only attentive reading of their arguments, although a properly critical reading may require a good deal more than this.

The volume includes an introduction that anybody interested in grounding should find useful. It begins with a historical discussion of what appear to be grounding claims made by Plato, Leibniz, and Bolzano. Although Aristotle seems to be a key figure worth discussing in this context, the authors beg off saying much about his ideas, claiming to lack ‘the space – and, frankly speaking, the confidence’ to do so (2). I am not sure these excuses justify the omission. Not only was Aristotle a key theorist of grounding, Aristotelian ideas were at the centre of discussions of grounding in medieval philosophy (something the introduction does not mention). The common Scholastic idea of the ordo naturae closely resembles the idea of a metaphysical structure of reality that today's philosophers define in terms of relations of grounding and dependence (see, for example, page 105 of Audi's contribution). Moreover, the connection between grounding and explanation formed a central topic in Renaissance and Early Modern philosophy, much of which forms the basis of contemporary thought, as Della Rocca's chapter hints.

Following this historical excursus, the introduction lays out various strategies for formulating grounding claims before going on to discuss the relation between grounding and other philosophical concepts: essentiality, modality, explanation, existential dependence, truth-making, and reduction. It ends by raising and replying to various sceptical challenges to the notion of grounding. These challenges come in the form of the following questions: 1) Is grounding talk intelligible at all? 2) Does the concept of grounding confuse various different phenomena with each other? 3) Do grounding relations really exist? 4) If they do, can we know about them? Somewhat disappointingly, neither the introduction nor the chapters discuss challenges 2–3. Daly's article raises challenge 1 in some detail, and Audi argues for an affirmative answer to it. Also, Daly (86) and Audi (116–17) hint at an argument for giving an affirmative answer to 4, by suggesting that grounding claims might be indirectly subject to empirical confirmation. Thus, perhaps, we can know about grounding relations indirectly through observation. But these hints are never developed into a full response. The volume would, I believe, have been more useful to students of grounding if it had given a little more attention to these sceptical challenges.

It would also have done well to address another challenge. Are grounding relations are really mind-independent, as grounding theorists suppose them to be? Some defenders of the notion of grounding (e.g. Fine and Audi – 38, 119–20) believe that grounding should be understood in terms of explanation; to say that X grounds Y is to say that X explains Y. But to whom does it explain it? As Correia and Schnieder point out in the introduction:

This is something the grounding theorist would not want. Grounding relations are, after all, meant to exist independently of anyone's cognitive predicament. A claim about grounding is meant to be about the metaphysical structure of reality in itself, not about how things are represented from within one cognitive predicament. For this reason Correia and Schnieder believe it advisable ‘to separate the objective notion of grounding, which belongs to the field of metaphysics, from an epistemically loaded notion of explanation’ (24). What is required, then, is a non-epistemically loaded notion of explanation. It is, however, far from obvious that there is any such notion. Audi proposes that ‘it is not hard to imagine perfectly ordinary people implicitly grasping that one [fact] explains another’ (116). His example is as follows: ‘“Don't do that!” “Why ever not?” “It's wrong!” “Why is it wrong?” “It's unfair!”’. He is right that one can imagine ordinary people seeing how an act's being unfair explains its being wrong. But is this true independently of a particular cognitive predicament? Somebody, in a different predicament, might feel that wrongness explains unfairness, at least in some cases. Suppose I ask you at what point a taxation scheme becomes so progressive that it is unfair. Despairing of finding any strict utilitarian rule to determine the case, you reply that progressive taxation becomes unfair when people are required to contribute more than it is right to ask of them. The wrongness of the required contribution explains the unfairness of the scheme. In other words, wrongness explains unfairness. Which way the order of explanation runs between wrongness and unfairness is a subjective matter, depending on one's cognitive predicament.

This is another reason it would have been worthwhile to discuss Aristotle. In the Physics and in other works, Aristotle proposed that good explanations use what is better known to explain what is worse known. But he distinguished between things that are better known by us and those that are ‘better known by nature’ (γνωριμωτέρων φύσει).Footnote ¹ It is unclear whether this ascribes a faculty of knowing to nature, or whether Aristotle meant that it is in the nature of certain things to be better known than others, regardless of who knows them. At any rate, the idea seems to be one of an objective order of explanation, independent of any cognitive predicament, in terms of which grounding can be understood. Yet there was considerable discussion after Aristotle of whether such an objective order of explanation can really be identified, especially since claims that some things are better known ‘by nature’ than others are almost always subject to dispute.

If there is no such objective order, then grounding relations seem to be mind-relative, given the closeness of the notion of grounding to that of explanation. This is not only a problem for grounding; it applies also to truth-making, ontological dependence, ‘requirements on reality’, and other cognate notions to grounding discussed in this volume. It is, I believe, the most important issue with which the promoters of these various notions must grapple. Little grappling with it occurs in this volume, to its detriment. Nevertheless, the rich, varied, and fascinating collection of perspectives it presents provides ample material to stimulate serious thought about this and other equally important issues in contemporary metaphysics. With luck we shall one day catch up with Aristotle.