Interventionism and Supervenience: A New Problem and Provisional Solution

Abstract

The causal exclusion argument suggests that mental causes are excluded in favour of the underlying physical causes that do all the causal work. Recently, a debate has emerged concerning the possibility of avoiding this conclusion by adopting Woodward's interventionist theory of causation. Both proponents and opponents of the interventionist solution crucially rely on the notion of supervenience when formulating their positions. In this article, we consider the relation between interventionism and supervenience in detail and argue that importing supervenience relations into the interventionist framework is deeply problematic. However, rather than reject interventionist solutions to exclusion wholesale, we wish to propose that the problem lies with the concept of supervenience. This would open the door for a moderate defence of the interventionist solution to the exclusion argument.

Notes

[1] Of course, it is possible to formulate causal exclusion problems without referring to supervenience. Nonetheless, supervenience is, as a matter of fact, a central premise used in mainstream formulations of causal exclusion (see, e.g. Kim 1998, 30). The appeal of supervenience in this regard is easy to understand, because it offers a means of postulating a relatively neutral (ontologically speaking), non-identity relation between different properties whose causal efficacy seem to be in tension with one another. Even when supervenience is not explicitly included in the arguments, it is a background assumption, which is enough to result in the problems we discuss in this paper. Thus, our main theses are also relevant for versions of the causal exclusion argument formulated without supervenience.

[2] This is the standard definition, but to be exact, sufficiency should be defined as follows: a set V is causally sufficient iff any common cause of two variables X and Y in V either belongs to V or has a cause or an effect that is also a common cause of X and Y that belongs to V (Baumgartner 2013, 9). This difference has no implications for our arguments.

[3] However, our points regarding the relation between supervenience and interventionism also hold if we adopt Baumgartner's definition of CMC, so nothing crucial turns on this.

[4] These considerations indicate a possibly deep tension underlying our respective treatments of causal exclusion within the interventionist framework and that of Baumgartner's, and have direct bearing on the points to implicit criteria that structure this discussion. By focusing on the metaphysical aspects of the issue, Baumgartner seems to assume that any commitments to the particular conditions that inform the means by which we represent or come to know the relations that we judge to be causally related only distract us from the true enterprise confronting us. We strongly disagree with this, and believe that the attitude underlying it constitutes an out of hand rejection of any interventionist treatment of causal exclusion. Space prohibits in-depth analysis of this point here, but we will elaborate on it in a follow-up article that is in preparation.

[5] If I is an intervention variable on C with respect to M, it follows from (IV*) that I can still fail to be an intervention variable on C with respect to P, if (1) it is a direct cause of P, (2) it is a cause of some other variable Z that is distinct from C and not on the causal path from C to P and is not related by supervenience to C, or (3) it is not statistically independent of a variable Z that causes P and that is on a causal path that does not go through C or any variable that is related to C by supervenience. Options (2) and (3) are possible only if the confounding variables are not included in the variable set V, since it is assumed that all variables in V that are not C or M or related to C or M by supervenience are held fixed. However, the supervenience relationship between M and P guarantees that even if we included all causes of M and P in V, and held all of them (except C and its supervenience base) fixed, there would still be a change in P whenever we intervene on C with respect to M. Option (1) also seems very implausible—it is difficult to see how I could be a cause of C, which causes the change in M, and at the same be a direct cause of the change in M's supervenience base. In any case, even if I fails to be an intervention variable for C with respect to P for this reason, this is not a problem for our argument: If I is direct cause of P and a contributing (via C) cause of M, the dependency between M and P is still explained by a common cause (in this case, I).

[6] This exhibits quite well the central problem we pointed out in the introduction: The tension between the largely metaphysical attitude involved in the traditional debate about causal exclusion, and supervenience more generally, and the largely epistemological attitude involved in much of the literature surrounding interventionism.

[7] The relevant class of interventions would be those interventions on the causal variable that are capable of making a change in the effect variable per the conditions of (IV*). This point here is simply to exclude interventions that change the causal variable but are not sufficient to bring about a change in the effect variable, as in the case of changing the position of the light switch in a way that does not turn on the light (Woodward 2003, 66–67). In other words, the relevant class of interventions should designate a contrast class of values that the causal variable can take in order to elicit its relevance to causing a change in the effect variable. This point bears directly to variables related by supervenience. Namely, with non-identical supervenient pairs of properties, there can be changes to the supervenience-base property that will not change the supervenient property. However, the reverse will not hold, i.e. any change to the supervening property will lead to a change in the base property. Hence, it is important to be clear that we are only talking here about those intervention-induced changes that preserves the dependency expressed by supervenience in the sense that a change implemented by an intervention will (a) cause a change in the effect variable and (b) exhibit covariant change in both the variables related by supervenience.

[8] Background assumptions or commitments may give further information for why one variable may be more desirable than the other for a particular explanation that is offered (a desire for explanatory parsimony, or in defense of a more unified explanation), but this seems to avoid the issue at hand. More importantly, this manoeuver can go in both directions, as other background assumptions or commitments may equally support the other variable.

[9] Eronen (2012) has briefly proposed this kind of solution, and Weslake (forthcoming) defends a similar approach in a sophisticated formal framework.

[10] Baumgartner (2013) also points out another potential problem for revised versions of interventionism: we can always include an intermediate variable P′ between M and P₂, and then another intermediate variable P″ between M and P′, and so on, so that M directly causes only the first physical event type outside its own supervenience base. However, this objection requires giving direct causes (as opposed to contributing causes) a special metaphysical status, which is something Woodward (2008b) explicitly denies. Even if we add intermediate variables, M remains a contributing cause for P₂.

[11] For instance, definitional, constitutional, and compositional relations between variables are all consistent with appearing as common cause structures from within the interventionist framework. For that matter, these relations are also all consistently representable with supervenience. Each of these relations nonetheless designate very distinct ways of relating two variables, and it is hence difficult to imagine that any serious researcher would be satisfied with two covarying variables simply being related by a common cause and moving on. This is an issue we will discuss in more detail in a follow-up article that is in preparation.