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Mental causation has been a hotly disputed topic in recent years, with reductive and non-reductive physicalists vying with each other and with dualists over how to accommodate, or else to challenge, two widely accepted metaphysical principles—the principle of the causal closure of the physical domain and the principle of causal non-overdetermination—which together appear to support reductive physicalism, despite the latter’s lack of intuitive appeal. Current debate about these matters appears to have reached something of an impasse, prompting the question of why this should be so. One possibility well worth exploring is that, while this debate makes extensive use of ontological vocabulary—by talking, for instance, of substances, events, states, properties, powers, and relations—relatively little attempt has been made within the debate itself to achieve either clarity or agreement about what, precisely, such terms should be taken to mean. Hence, the debate has become somewhat detached from broader developments in metaphysics and ontology, which have lately been proceeding apace, providing us with an increasingly rich and refined set of ontological categories upon which to draw, as well as a much deeper understanding of how they are related to one another. In preparing this volume, the editors invited leading metaphysicians and philosophers of mind to reflect afresh upon the problem of mental causation in the light of some of these recent developments, with a view to making new headway with one of the most challenging and seemingly intractable issues in contemporary philosophy.

If metaphysics is centrally concerned with charting the domain of the possible, the only coherent account of the ground of metaphysical possibility and of our capacity for modal knowledge is to be found in a version of essentialism: a version that I call serious essentialism, to distinguish it from certain other views which may superficially appear very similar to it but which, in fact, differ from it fundamentally in certain crucial respects. This version of essentialism eschews any appeal whatever to the notion of possible worlds in its account of the nature and ground of metaphysical possibility, for reasons that which I shall endeavour to explain and justify in this paper.

One currently popular methodology of metaphysics has it that ‘intuitions’ play an evidential role with respect to metaphysical claims. This chapter defends a realist methodology of metaphysics that implies that any rational being, simply in virtue of being rational, is necessarily capable of grasping the essences of at least some mind-independent entities. The notion of essence in play here is Aristotelian, whereby an entity’s essence is captured by an account of what that entity is, or what it is to be that entity. Concepts are just ways of thinking of things and, as such, may or may not be adequate to the real natures — the essences — of the things being thought of. Our ‘intuitions’, particularly as revealed by aptly constructed thought experiments, may cast interesting light on some of our concepts, but they are not and cannot be reliable guides to mind-independent truths of essence.

An individual, as this term will be understood here, is an entity to which the concepts of unity and identity fully and determinately apply. That is to say, an entity x is an individual just in case x determinately counts as one entity and x has a determinate identity. Many philosophers tacitly assume that all entities are individuals in the foregoing sense, and indeed that it is a necessary truth that they are. But this can certainly be disputed. It is, very arguably, both logically and metaphysically possible for there to be nonindividuals. The aim of this chapter is to clarify and explore both the notion of an individual and that of a nonindividual, to propose a typology of entities based on the individual/nonindividual distinction, and to illustrate the potential of these notions for application within and across the sciences.

Ontological dependence is a relation—or, more accurately, a family of relations—between entities or beings. For there are various ways in which one being may be said to depend upon one or more other beings, in a sense of “depend” that is distinctly metaphysical in character and that may be contrasted, thus, with various causal senses of this word. More specifically, a being may be said to depend, in such a sense, upon one or more other beings for its existence or for its identity. Some varieties of ontological dependence may be analyzed in modal terms—that is, in terms of distinctly metaphysical notions of possibility and necessity—while others seem to demand an analysis in terms of the notion of essence. The latter varieties of ontological dependence may accordingly be called species of essential dependence. Notions of ontological dependence are frequently called upon by metaphysicians in their proposed analyses of other metaphysically important notions, such as the notion of substance

It is argued that the dispute between philosophical naturalism and theism can, ultimately, only be rationally resolved in favor of theism, owing to certain internal inadequacies of philosophical naturalism that are commonly overlooked by both its friends and its foes. The criticisms of philosophical naturalism focus on certain questions concerning the ontological status of the objects of human reason and probe into the nature of human rationality and the conditions of its possibility. There is an implicit challenge to mainstream philosophical opinion concerning the relationship between human thought and reasoning and the sorts of facts about human brains that can be revealed by empirical neuroscience.

Are persons substances or modes? Two currently dominant views may be characterized as giving the following rival answers to this question. According to the first view, persons are just biological substances. According to the second, persons are psychological modes of substances which, as far as human beings are concerned, happen to be biological substances, but which could in principle be non-biological. There is, however, also a third possible answer, and this is that persons are psychological substances. Such a view is inevitably associated with the name of Descartes, and this helps to explain its current unpopularity, since substantial dualism of his sort is now widely rejected as ‘unscientific’. But one may, as I hope to show, espouse the view that persons are psychological substances without endorsing Cartesianism. This is because one may reject certain features of Descartes's conception of substance. Consequently, one may also espouse a version of substantial dualism which is distinctly non-Cartesian. One may hold that a person, being a psychological substance, is an entity distinct from the biological substance that is his or her body, and yet still be prepared to ascribe corporeal characteristics to this psychological substance. By this account, a human person is to be thought of neither as a non-corporeal mental substance, nor as the product of a mysterious ‘union’ between such a substance and a physical, biological substance. This is not to deny that the mind—body problem is a serious and difficult one, but it is to imply that there is a version of substantial dualism which does not involve regarding the ‘mind’ as a distinct substance in its own right.

Metaphysical realism is the view that most of the objects that populate the world exist independently of our thought and have their natures independently of how, if at all, we conceive of them. It is committed, in my opinion, to a robust form of essentialism. Many modern forms of anti-realism have their roots in a form of conceptualism, according to which all truths about essence knowable by us are ultimately grounded in our concepts, rather than in things 'in themselves'. My aim is to show that conceptualist anti-realism is an incoherent doctrine and how we can support metaphysical realism and robust essentialism, while still properly acknowledging the cognitive role of concepts in mediating our grasp of the nature of mind-independent reality.RésuméLe réalisme métaphysique est la conception suivant laquelle la plupart des objets qui peuplent le monde existent indépendamment de notre pensée et possèdent une nature indépendante de la manière dont nous pouvons éventuellement la concevoir. A mon sens cette position engage à admettre une forme robuste d'essentialisme. Beaucoup des formes modernes de l'anti-réalisme tirent leurs origines d'une forme de conceptualisme, suivant laquelle toutes les vérités que nous puissions connaître au sujet des essences sont en dernière analyse fondées sur nos concepts, plutôt que dans les choses « en elles-mêmes ». Mon but est de montrer que l'anti-réalisme conceptualiste est une doctrine incohérente, et comment nous pouvons soutenir le réalisme métaphysique et l'essentialisme robuste, tout en reconnaissant clairement le rôle cognitif des concepts en tant qu'intermédiaires dans notre appréhension de la nature de la réalité indépendante de l'esprit.