The notion of dispositional modality, discussed by Stephen Mumford and Rani Lill Anjum in Getting Causes from Powers, put me in mind of another analysis of the interplay between causation and modality: that of Gregg Rosenberg in A Place For Consciousness (2004).
Recall (see prior post here) that Mumford and Anjum analyze causation in terms of dispositions, or powers. These powers tend toward (dispose toward) their manifestations -- they do not necessitate them. Necessity is not the modality of causation. In addition, it is argued that dispositional modality is distinct from standard philosophical notions of possibility (logical or metaphysical possibility). Dispositional modality (dispositionality for short) does not involve “pure” possibility, since only certain manifestations are possible. In Chapter 8 of their book, the authors say: “Dispositionality…can be understood as a sort of selection function…that picks out a limited number of outcomes from all those that are merely possible.” Also: “The idea of a selection function is simply one that identifies a subset from a realm of possibilities. (p.189)”
In his book, Gregg Rosenberg introduced a model of causation which featured a notion which seems related to the idea of the selection function: this was a constraining function on the space of possibilities. Rosenberg, unhappy with both the Humean perspective on causation, as well as the theories of causal responsibility or causal production on offer, endeavored in chapter 9 of his book to strip down the notion of real causation to a bare minimum. This led him to the following notion of “causal significance”: “The causal significance of a thing is the constraint its existence adds to the space of possible ways the world could be…Causal significance shows causation to be an operator on a space of possibility. (p.150 emphasis original)” And: “It is a theory designed to understand how constraints propagate, so it explains how the actual world comes to be just a sliver of what could have been. (p.152)”
I say they seem like related notions, but constraint could be viewed as the negative image of selection. Selection picks out a few possibilities, while constraint rules out all of the others.
I find causal constraint to be a beguiling idea. Tentatively, it would seem to leave “pure” possibility in place as the fundamental metaphysical notion, in constrast with Mumford and Anjum’s argument for the irreducibility of dispositional modality.