The paper defends a naturalistic version of modal actualism according to which what is metaphysically possible is determined by dispositions found in the actual world. We argue that there is just one world – this one – and that all genuine possibilities are grounded in the dispositions exemplified in it. This is the case whether or not those dispositions are manifested. As long as the possibility is one that would obtain were the relevant disposition manifested, it is a genuine possibility. Furthermore, by starting from actual dispositional properties and branching out, we are able to countenance possibilities quite far removed from any state of affairs that happens to obtain, while still providing a natural and actual grounding of possibility. Stressing the importance of ontological considerations in any theory of possibility, it is argued that the account of possibility in terms of dispositional properties provides a more palatable ontology than those of its competitors. Coming at it from the other direction, the dispositional account of possibility also provides motivation for taking an ontology of dispositions more seriously.

Systematic metaphysics is defined by its task of solving metaphysical problems through the repeated application of a single, fundamental ontology. The dominant contemporary metaphysic is that of neo-Humeanism, built on a static ontology typified by its rejection of basic causal and modal features. This book offers and develops a radically distinct metaphysic, one that turns the status quo on its head. Starting with a foundational ontology of inherently causal properties known as ‘powers’, a metaphysic is developed that appeals to powers in explanations of causation, persistence, laws, and modality. Powers are properties that have their causal natures internal to them: they are responsible for the effects in the world. A unique account of powers is developed that understands this internal nature in terms of a blueprint of potential interaction types. After the presentation of the powers ontology, it is put to work in offering solutions to broad metaphysical puzzles, some of which take on different forms in light of the new tools that are available. The defence of the ontology comes from the virtues of metaphysic it can be used to develop. Particular attention is paid to the problems of causation and persistence, simultaneously solving them as it casts them in a new light. The resultant powers metaphysic is offered as a systematic alternative to neo-Humeanism.

The thought that diseases form natural kinds tends not to sit well with the essentialist treatment of natural kinds. The essentialist's candidates for the essences of diseases-etiological properties-rarely satisfy the essentialist's requirement that they be necessary and sufficient for membership within the kind. Consequently philosophers of medicine have tended to back away from treating diseases as natural kinds. However, this retreat was too hasty: there are good reasons for thinking that diseases form natural kinds. The problem lies with the essentialist treatment of natural kinds, and not with treating disease as forming natural kinds. A similar revolution has taken place within the species debate, where the notion of natural kind has been 'taken back' from the essentialist. Borrowing the revised treatment of natural kinds from the philosophers of species and modifying it slightly, I offer a proposed treatment of disease kinds in terms of homeostatic property clusters.

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