Modal Problems with the Theory of Powers

This is the last of four posts prompted by my reading of George Molnar’s Powers: A Study in Metaphysics.

While in previous posts I quibbled with some of the ontological details of Molnar’s theory, I think there are bigger problems which ultimately undermine his approach. These problems center on modal issues. On the one hand, the positing of necessary connections between powers and manifestations to explain causality is too easily rejected by the adherents to a Humean view. On the other hand, from the perspective of a reader sympathetic to causal realism, the theory has the wrong modal structure to support an asymmetric causality which matches our intuition. At the end of the post, I argue that the recasting of powers as propensities can lead to a superior theory.


The theory of powers is motivated by a desire for causal realism. The alternative Humean view is that causality has no mind-independent reality: the world has regularities, but not causation. Now, the nature of a power is only revealed by its manifestation. For this reason, Molnar explains that it is a feature of the theory that the connection between a power and its manifestation is a necessary one. Hume, at least in the usual modern interpretation, denied the existence of necessary connections between distinct things (recall a power is distinct from its manifestation, since it can exist in the absence of the manifestation). While Molnar argues that Hume’s dictum can be rejected, as it ultimately leads to an overly corrosive skepticism, many philosophers would apply the dictum in this case. It’s especially easy to see this for those who would take a four-dimensional block perspective on the universe as a model. How would the events in the block universe alter if we removed these necessary causal connections? The answer is not at all. So, the ontologically sparser view is deemed superior.

But of course, the block universe model implies there is no mind-independent asymmetric causal flow in nature. If the theory of powers can help us explain the kind of “real” causation which matches our intuitions, shouldn’t we endorse it?

Well, by my reading, Molnar’s theory of powers doesn’t achieve this goal. The asymmetry of causality means the past is fixed while the future is open (please note I’m not committing to a global evolution in conflict with relativity). Molnar doesn’t have the right modal structure for this asymmetry. For a given causal past (which is determined), we need to explain what happens next (which is undetermined). To recap the theory, there are necessary connections between powers and manifestations; also, recall that powers actually exist, and do so even when not manifested. So how does this explain the fact that manifestations sometimes occur and sometimes don’t? How can a necessary connection give us a possible manifestation?

Molnar explains that the causal effects we normally think about are actually complex conjunctions of power properties and some non-power properties. This can explain the apparent existence of indeterminism in our perception. But at the mind-independent fundamental ontological level, there doesn’t seem to be any basis for indeterminism in the theory.

I think the solution would be to sacrifice the actuality of powers and turn them into propensities. I’m defining a propensity to be a power which is a possibility. A manifestation event is the actualization of such a possibility. Molnar considers this idea very briefly and rejects it in just a couple of paragraphs. He evidently takes it as a given that something which is “merely” possible cannot enter into causality.

The ontological cost of admitting causally relevant possibilia seems high to many. We need to replace the traditional ontological division between the “actual” and the “merely possible” with a modal realist vision which admits a role for possibilities which are “real” enough to enter into causal relations and be actualized. The specific combination of power/propensities and/or other kinds of properties needed to achieve an actualization event can be worked out in more than one way. Gregg Rosenberg’s theory is a recent one which features this kind of modal realism. In his model of causation, two types of properties (effective and receptive) are needed for actualization. Jennifer McKitrick recently reviewed Rosenberg’s book and offered detailed comments on his model of causation. While her criticisms were often perceptive and thought-provoking, she also had (in my opinion) a surprisingly hard time grappling with this sort of modal realism.

I think if you really want a satisfying treatment of causality, you need to have this modal dimension. The nice thing about it is that it neatly matches the best (IMO) interpretation of quantum mechanics!

P.S.
For a critique of the theory of powers which approaches from a somewhat different angle (but also is skeptical about the modal nature ascribed to powers by Molnar as well as McKitrick), see the paper “What Do Powers Do When They Are Not Manifested?” by Stathis Psillos, available on his website. Thanks to Antonio for alerting me to this paper.