John Donaldson “Vertical versus Horizontal: What is really at issue in the exclusion problem”—forthcoming in Synthese

By sheer luck, this followed on nicely from the Devlin Brown paper from yesterday.  Brown distinguishes between “horizontal” and “vertical” responses to the exclusion problem. Vertical responses look at the relation between the physical and mental causes. They try to show that the two are ontologically related in such a way that they their co-existence as causes is not really problematic—because they are not fully distinct causes. “Vertical” because that is how the relation between mental and physical causes is usually depicted in toy causal diagrams (as in the example below).  Horizontal responses, by contrast, look at the causal relation itself, generally depicted horizontally, or diagonally in the mental → physical case.


Donaldson’s argument is that vertical strategies are the only ones that can salvage non-reductionist physicalism. If vertical strategies can succeed, horizontal ones are unnecessary. On the other hand, a horizontal strategy—however successful its account of causation—cannot be enough, because it cannot address the coincidence aspect of (systematic) overdetermination by mental and physical causes. Why should a certain physical cause always be accompanied by a mental cause, whenever it does its work?  (Donaldson notes that this problem arises even if, as with Zhong (2014), you avoid overdetermination by not counting one of the properties as a cause. They're still always there!)

I agree with Donaldson’s distinction, and with his claim that a horizontal strategy cannot succeed alone. As I said yesterday, I think the key to avoiding objectionable overdetermination is that the two causes are not fully distinct; and the ontological link between them will also explain their co-appearance.  But it seems to me that a vertical strategy alone is not sufficient, either.  A suitably compatibilist account of causation is needed too.

Consider the reductive physicalist's response to the exclusion argument. They avoid overdetermination by identifying the (token) mental and physical causes.  This is the ultimate vertical strategy; there is no coincidence problem for identicals.  But it’s reductive physicalism.  Non-reductive physicalism needs the two causes to be ontologically indistinct, but it also needs them to still count as two causes. And for that we need to explain how their causal contributions can be distinct.  If the causal relation between the two causes and the effect is the same, then non-reductive physicalism looks like epiphenomenalism: the mental cause is not contributing anything.

What’s needed, then, is an account of causation on which ontologically connected mental and physical properties can both count as making separate causal contributions. In other words, a “horizontal” approach to the exclusion problem. 

Donaldson spends some time discussing the “proportionality” horizontal approach, adopted by both Zhong and List and Menzies (2009). I agree that this won’t do it—it avoids overdetermination by ruling out either the physical or the mental cause as a cause, where I want both to count. But I think a (related) approach of contrastive interventionism can do the job. There are interventions on the mental cause that change the outcome, and also interventions on the physical cause that change the outcome, and the two sets of interventions aren’t identical.  More on that next, I think.