Powers (or dispositional properties) are the centerpiece of George Molnar’s causal realist metaphysical model. The following question arises: are powers the only sort of property, or are there others?
Molnar considers whether powers (with the features he ascribes to them) could be the star players in an ontological monism (called pan-dispositionalism). It would seem that adopting pan-dispositionalism, along with the natural assumption that manifestations of dispositions constitute changes in the properties of objects, leads to a regress. Powers only manifest as changes to other powers. But is this a vicious regress? Molnar considers a couple of arguments that it is indeed vicious. If objects are things which take up space, then powers need to be joined by space occupying properties in order to constitute objects. But a look at particle physics seems to show that elementary constituents of nature do not have volume or occupy space in the way common sense implies. So this doesn’t seem to be a convincing objection – it’s not clear that objects need “space-occupying” non-power properties. A second objection says that objects need to have non-power properties which are qualities. But are there mind-independent physical qualities? What are they? Candidates such as size, shape, color are all phenomenological, rather than fundamentally physical, according to Molnar.
Molnar doesn’t see that the regress objections are fatal, but nevertheless concludes pan-dispositionalism is unlikely to be true for a posteriori reasons. Since powers are intrinsic to objects (in his theory), he considers the reality that they are “portable”. They are not necessarily altered if I move the object somewhere else. He sees the need for non-powers which are responsible for what scientists call symmetry operations (hence “S-properties”). These are essentially positional properties (positions in space-time) with one exception – if parts of a complex object have identical powers, and they exchange roles (say swapping the two hydrogen atoms in a water molecule), the powers of the complex object are not affected. This is a property of numerical identity.
So, Molnar does end up with a property dualism. There are powers, which are intrinsic dispositions of objects; and there are non-powers, which are extrinsic and basically have to do with placement of objects in space-time.
Certainly, as property dualisms go, this is a pretty bare bones version compared to what the term “dualism” usually connotes. On the other hand, while Molnar is presenting a pretty ambitious metaphysical system, he is not trying to explain the mind, which is what traditionally motivates dualistic theories.
One critique I have centers on Molnar’s reliance throughout on his interpretations of physics. Powers are deemed intrinsic because, for instance, charge seems like an irreducible property of an electron. He says qualitative properties like size and shape don’t exist, because they do not feature in physics. He argues positional properties make sense because of symmetry operations which can be conducted on physical systems. A mostly unstated but crucial assumption underlying all of this is a somewhat old-fashioned view that physics supports the conception of objects moving around in a static space-time container. So a theory of powers which is motivated largely by a priori analysis of causation has its details shaped by these a posteriori inputs from physics. There are a couple of obvious concerns: first that his interpretations of physical theories may be incorrect, and second, that the theories he’s interpreting are provisional and may be superseded.
We don’t have a final theory of physics yet, but we can make an educated guess that the conception of objects moving in a space-time container (already extremely distorted in quantum field theory) will not survive. In General Relativity, there is a dynamic interaction between space-time and matter fields, and this seems to compromise the separation of properties into powers and positional properties (doesn’t a dynamic space-time need powers, too?). Further, in some quantum gravity research programs (my favorite ones), both space-time and matter fields are seen as emerging from a more fundamental basis (specifically a causal network of quantum mechanical interactions).
Still, the idea that positional properties are needed seems right to me. The nature of quantum mechanical propensities appears to depend on the relation between quantum systems. In a theory featuring a causal network of quantum mechanical interactions, causation would depend both on propensities (powers) and position in the network. One thing I think we could drop altogether is the idea of an object. What we think of as objects would be complex patterns of causal events in this way of thinking.