Dispositional Modality as Restricted Possibility

I’ve been interested in the metaphysics of dispositional properties (or powers), and I’ve ordered Getting Causes from Powers, a new book from Stephen Mumford and Rani Lill Anjum. I look forward to reading this later in the fall, but it the meantime I have read a couple of related papers by the authors (see an earlier post here).

In “Dispositional Modality” Anjum and Mumford argue that the modal value of dispositions is distinct from necessity and possibility: it is described as “sui generis” and “irreducible”. What I thought was interesting, though, is that the authors themselves include a nice account of dispositional modality in terms of restricted possibility, which seemed to me had the flavor of a reduction.

I’ll pass over the first several sections of the paper, which covered some familiar ground: the fundamental disagreement with Hume about powers; the failure of the semantic reduction of dispositional ascriptions to conditionals; and the fact that dispositions, by their nature, clearly do not necessitate their manifestations (they are disposed toward, or tend toward their manifestations).

So, dispositional modality is distinct from necessity, but what about possibility? In section 5, Anjum and Mumford argue that dispositional modality is also distinct from possibility, in the sense that it is different from “pure” possibility. By pure possibility, the authors mean the broadest sort of logical or metaphysical possibility. A glass vase is disposed to shatter when dropped. One might suppose it is logically possible that the vase will turn to jelly when dropped, but it is clearly not disposed to do so – at least in our world.

The authors say we might think of dispositional modality as a subclass or restricted version of possibility. But this is already a familiar idea. Modal theorists talk about various nested classes of possibility: logical possibility (typically the broadest notion), various formulations of metaphysical possibility, and nomological possibility -- the subclass which holds the laws of nature fixed (some philosophers would identify one or more of these). Anjum and Mumford see dispositions as giving rise to “natural” possibility: “The reason some things are naturally possible is because there are dispositions for them.”

The authors don’t elaborate greatly on this point, but it seems as if natural possibility is the set of all the possibilities inherent in the dispositions contained in the state of the natural world at a given time. Building on this idea: when it comes to individual natural objects or systems, in saying they possess dispositional properties (or powers), we might equally well say they possess a certain restricted bundle of possibilities. Then we might turn to a discussion of how causation is explained in terms of the “actualization” of some of these possibilities (compare the “manifestation” of a disposition), perhaps depending on how a system interacts with other systems which similarly bear possibilities.

If one can retell the story of dispositions with restricted sets of possibilities, might this be a reductive analysis? It’s not completely clear to me, because one might say the introduction of restricted sets of possibility of the type needed is an irreducible extension of our notions of modality. And perhaps the idea of an object bearing a set of possibilities, rather than bearing properties, isn’t coherent (but maybe the terminology can be worked out). In any case, this paper’s comparison of dispositional modality to the idea of restricted possibility was very thought-provoking.