So far, I have interpreted O’Brien’s formulation of the content causation problem (“How can mental phenomena be causally efficacious of behaviour in virtue of their representational contents if these contents are not determined by intrinsic properties of the brain?” O’Brien this collection, p. 2) as something along the lines of “How does mental causation in virtue of representational content work, if not in the way we already know it sometimes works?” In so doing, one already engages in an interesting discussion about content causation. But closer examination reveals that this understanding of the problem is an oversimplification. The content causation problem is supposed to be much more severe. It is not a problem about finding alternative explanations to the ones we already have, but about the consistency of all available explanations. A better understanding of the problem will help us to evaluate whether O’Brien’s suggestions, which are doubtlessly interesting, are really motivated by the problem at hand.
The three theses, that (i) the causal efficacy of mental phenomena is grounded in their representational contents, that (ii) these “are not determined by the intrinsic properties of the brain” (O’Brien this collection, p. 2), and that (iii) the brain’s causal efficacy for behaviour is grounded only in its intrinsic properties, supposedly “form an inconsistent triad” (ibid., p. 2). Yet, strictly speaking, they are not inconsistent: why not say that what we do in theses (i) to (iii) is gathering information about (human) behaviour—our object of enquiry—but on two levels of description? On both levels, we attempt, metaphorically speaking, to travel back along the causal chain that brings behaviour about. On the one level, we then discover that intrinsic properties of the brain are responsible for it to cause behaviour (ibid., p. 2, thesis 3). On another level, we trace behaviour back to mental phenomena, which owe their capacity to cause it to their representational contents (ibid., p. 2, thesis 1). Why not assume that these two levels both provide us with (true) formulations of what is happening, but which—since they depend on two conceptual frameworks that are not intertranslatable—must be regarded as nomologically incommensurable? If so, they could never both be part of a unified causal theory (see Davidson 1970). This picture seems perfectly intelligible at first. But on both levels, we talk about causes of (presumably the same) behaviour.
Within a physicalist framework, we take behaviour to fall, in the end, under the description of a physical event. As such, it is subject to the principle of causal closure: if it is caused, it has a sufficient physical cause (Kim 1989, p. 43). Hence, we should pay close attention to the fact that “our best neuroscience informs us that the changes to musculature that constitute our behavioural responses are wholly determined by the intrinsic properties of the brain to which they are causally connected” (O’Brien this collection, p. 2). Thus it is not only assumed that the brain can be causally efficacious of behaviour in virtue of its intrinsic properties, but also that whenever behaviour is caused, it is always caused in the brain and in virtue of the brain’s intrinsic properties. Yet mental phenomena, which are of a non-physical kind, are also mentioned in (i) as a cause of behaviour. But with brain states already providing a sufficient cause for behaviour, what role in causation can they possibly play (Kim 1989, pp. 43–44)? As long as we cannot answer question, we are forced to reject the possibility that behaviour is over-determined, or, in other words, we are forced to accept both mental phenomena and brain states as two separate causes of behaviour. So causally efficacious mental phenomena should be reducible to the physical cause already provided by the states of the brain, or we must conclude that they are not a cause at all. In the latter case, this would mean that we would have to deny “that our beliefs and desires, and our perceptions and thoughts ultimately have a causal impact on our behaviour” (O’Brien this collection, p. 1) and we would have to accuse every discipline accepting this tenet—O’Brien names folk psychology and the computational theory of mind (ibid., p. 2)—of operating with a faulty ontology, pointing out causes that do not really exist.[5]
Now we have made explicit a metaphysical constraint that was left implicit in the formulation of the content causation problem: mental properties and all their capacities, e.g., their capacity to cause behaviour, must be reducible to properties of the physical brain. Knowing this, we see where the supposed inconsistency comes from: we traditionally think of representational content not as determined by the brain’s intrinsic properties, but rather as determined by what it is about (O’Brien this collection, p. 2–3). But if the content causation problem dares us to integrate these two things, namely the description of mental phenomena as causing behaviour in virtue of their representational contents and our theory of the same behaviour being caused by processes in the brain in virtue of the brain’s intrinsic properties, then we might conclude with O’Brien:
A solution to the content causation problem requires something that prima facie appears impossible: an explanation of the relational character of mental content that invokes only the intrinsic properties of the brain. (this collection, p. 3)
This is what turns the content causation problem as formulated by O’Brien from an interesting question into an urgent philosophical problem. The apparent impossibility of a solution relies on the idea of a sharp distinction between relational and intrinsic properties. If our best shot at understanding whatever we describe as the “relational character” of representational content is to understand it as a relational property,[6] then its irreducibility to intrinsic properties of the brain is built into it—and so is the insolvability of the content causation problem, given the metaphysical constraint just mentioned. Nevertheless, O’Brien aims to provide a solution.