Another talk at the powers conference which grabbed my interest was by Jennifer McKitrick, on the metaphysical status of disposition-manifestations. The central debate is about which of two kinds of thing we should take the manifestation of a disposition to consist in. Are manifestations particular events, the ‘net results’ of the activation of dispositions, or are they instead partial contributions to the ‘net result’? McKitrick was defending the line that manifestations should be thought of as events; Mumford is the main author to have taken the opposing view.
A couple of examples are useful here. Consider the case of a boat pulled along a canal by two horses. Each horse exerts a force at an angle to the canal, but the direction in which the boat accelerates is along the canal. Each horse has the disposition to accelerate the boat, but what are the manifestations of these dispositions? Is the boat’s actual acceleration along the canal the manifestation of each disposition, or is each disposition manifested separately in some contribution to the boats actual acceleration? McKitrick was arguing for the former conclusion, which involves making the relation between particular disposition and particular manifestation many-one, rather than the one-one relation envisaged by Mumford.
One strand of McKitrick’s argument was epistemological. We cannot in general directly observe the individual contributions made by particular dispositions to the net result of a transition, but we want to say that we individuate dispositions by their manifestations. This leads to a tension; unless manifestations are events, either dispositions are individuated by something other than their manifestations, or we have problems individuating dispositions.
Another line of argument was metaphysical. Mumford gives us no clear general account of what the manifestation of a disposition consists in. In the case of the two horses towing a boat, it looks like he would say that the manifestations of the two individual dispositions to accelerate the boat are two individual ‘virtual’ acceleration vectors, which sum to the actual acceleration vector.
In discussion, Kit Fine emphasized some interesting properties of these virtual effects: only in combination with other virtual effects do they condense into a real, observable event. He was very inclined to recognise these virtual vectors as genuine (albeit peculiar) entities, admitted into our ontology for their explanatory power. If we’re prepared to be as ontologically generous as Fine, I think McKitrick’s arguments can be resisted. The manifestations of dispositions on this view are sui generis virtual effects; their postulation is justified in the same way as the postulation of unobservable theoretical entities like quarks.
A variant on this view would be to say that in certain scenarios, virtual effects can be directly observed. We certainly feel the push of the wind when we walk in a gale, even though we can resist this push and not fall over. It could be said that what we are feeling is a virtual effect, and hence that such effects can after all be observed; the line of thought is similar to the idea that we can observe singular causation directly. I think some delicate issues in philosophy of mind and action are likely to be raised here, so I’d like to try and skirt this debate by focussing on fundamental natural properties. I don’t think anyone is likely to say that we can observe directly the gravitational forces exerted on one proton by a distant pair of protons; but this kind of interaction is a paradigm of combination of virtual effects. If we have to accept virtual effects into our ontology at all, we will have to accept fundamental unobservable virtual effects as well as the more homely and potentially observable macroscopic virtual effects that feature in the common examples.
So it looks like the friend of virtual effects can postulate an inferential route to knowledge of them, perhaps combined with non-inferential knowledge of a macroscopic subset, defusing McKitrick’s epistemological objection. But a metaphysical strand of objection remains; perhaps these virtual effects are simply too strange to be admitted into our ontology. McKitrick pressed this line by challenging the audience to produce a clear account of what the virtual effects amount to, for example in the case of the boat pulled by the horses.
I thought I’d have a go at explaining virtual effects a little. A helpful example is the case of a train which is moving through a station. At the same time, I’m walking along the train with an equal and opposite velocity, so that I remain stationary relative to the platform.
McKitrick’s way of explaining this is that I have a disposition to change position* relative to the platform when I walk, but also a disposition to change position relative to the platform (in the opposite direction) when the train moves. These two dispositions cancel out, such that each of them is manifested in the same result; me remaining stationary.
On the virtual effect picture, there is a clear sense that can be given to the virtual effects. One virtual effect is my change of position relative to the train; the other virtual effect is the train’s change of position relative to the platform. The dispositions that both the train and I have are manifested directly in these virtual effects. In special relativity, change of position relative to an inertial reference frame is a genuine physical quantity, so in this case at least physical meaning can be assigned to virtual effects. The case obviously generalizes straightforwardly from macroscopic trains to microscopic particles.
However, to give an account of virtual effects applicable to the boat case, we have to complicate matters a bit. If we replace motion in the train case with acceleration, so that the train starts to pull out of the station just as I start to walk along it, then the virtual effects end up being accelerations relative to an accelerating frame. Arguably, in special relativity accelerations relative to accelerating frames are not genuine physical quantities, since accelerating frames are non-inertial.
Here however we can appeal to general relativity; an acceleration relative to an arbitrary frame of reference is a genuine physical quantity in general relativity. Here’s how I would then suggest explaining the boat case. In a frame of reference accelerating in the direction of one of the net forces applied by one of the horses, the boat really is accelerating in the direction of the force applied by the other horse. When we move to the correct non-inertial frame, we can see what was a virtual effect in another reference frame. Virtual effects in one reference frame are genuine physical effects in other frames.
McKitrick’s defence of manifestations as effects amounts to a pair of arguments against the virtual effect picture, and it looks like these arguments are inconclusive. We can get epistemological access to virtual effects through standard scientific inferential procedures, and we can identify them with genuine physical quantities by making use of varying frames of reference. The upshot is that both accounts of manifestations are still on the table. One traditionalist at the conference was overheard to remark that it looked like this was a thoroughly boring debate about terminology – that we have a stable conception of manifestations as effects, and another stable conception of manifestations as virtual effects, and that we use sometimes one conception and sometimes the other. I’m not sure whether this is right, but it’s food for thought.
* In combination, Jennifer said she didn’t like thinking of motion as a disposition to change position. But this is just the introductory case; skip to the acceleration case if you like.