A Dispositional Theory of Possibility Andrea Borghini † Neil E Williams †† Abstract The paper defends a naturalistic version of modal actualism according to which what is meta- physically 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 impor- tance 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. Introduction They say that the world is full of possibilities.You can do this or that, this could happen or that could happen – it is all familiar talk – but what does it mean to say that the glasses could have broken, or that they might break, when most of the time they do not? These are the problems that arise when we start to talk of possibilities. In what follows, we advance a theory of what must be the case for these things to be possible. In other words, we are concerned with the ontology of possibilities: what some theorists might call the ‘truthmakers’ for our modal talk, when we talk of what is metaphysically possible. 1 What we aim to provide is an account of metaphysical possibility. 2 What we argue is that possibilities are grounded by the † Department of Philosophy, College of the Holy Cross, 1 College Street, 01610 Worces- ter MA, USA; Email: aborghin@holycross.edu †† Department of Philosophy, University at Buffalo, 135 Park Hall, University at Buffalo, Buffalo, NY 14260, USA; Email: new@buffalo.edu 1 What we mean by ‘truthmakers’ is what Armstrong describes as ‘the ontological ground of truths’ (Armstrong 1989b, 9). That being said, we are only interested in the ontological ground of truths, and only contingent modal truths at that. We have nothing to say about statements, the truth-making relation (whatever it is, if anything at all), truth, or any other semantic matters. 2 As our chosen truthmakers are dispositional properties, some might complain that this is an account of nomic necessity, being too restricted for metaphysical necessity. However, as the reader will see in parts 2 and 3, the account of possibility is far greater than what is typically thought of as nomic possibility – making the present account more like one of super-nomic dialectica Vol. 62, N° 1 (2008), pp. 21–41 DOI: 10.1111/j.1746-8361.2007.01130.x © 2007 The Authors. Journal compilation © 2007 Editorial Board of dialectica Published by Blackwell Publishing Ltd., 9600 Garsington Road, Oxford, OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA