The open core of the causal exclusion problem: do intimate vertical relations between mental and physical events actually hold, and is non-reductive physicalism consistent with hardline horizontalism?

Non-reductive physicalism holds that mental events are neither identical to nor reducible to physical events, yet depend entirely on them, while physics is causally closed. Jaegwon Kim's causal exclusion argument threatens this picture: whenever a mental event and its physical base are distinct candidate causes of one physical effect, the mental cause seems to be either causally idle or to overdetermine the effect in the way two assassins' bullets overdetermine a death. Donaldson (2021) diagnosed the ensuing debate as a choice between two readings of what is at issue — whether some intimate 'vertical' relation (determinate/determinable, set/subset of causal powers, constitution, realization) actually holds between mental and physical events, versus whether some 'horizontal' theory of causation rules out the threatening overdetermination — argued that the vertical reading is the right one, and explicitly left both substantive questions open, together with the meta-question of how the exclusion problem should be individuated. This record formulates that unresolved core: the problem is to determine whether any candidate vertical relation holds in the intimacy-requiring sense that dissolves causal competition, or whether non-reductive physicalism must instead be reconciled with (or abandoned because of) 'hardline horizontalism', the denial of systematic overdetermination among supervenience-related causes. Post-2019 literature on both fronts remains contested, with no settlement.

Background

Non-reductive physicalism is the dominant metaphysics of mind: mental events and properties are not identical to physical ones, yet nothing mental varies without a corresponding physical variation (mental facts 'supervene' on physical facts), and the physical domain is causally closed — every physical effect that has a sufficient cause has a sufficient physical cause. The causal exclusion problem, pressed most influentially by Kim (1984), is the argument that these commitments jointly undermine mental causation. Suppose a mental event $M$ (an intention, say) is distinct from but supervenes on a physical event $P$ (its neural realizer), and both are candidates for causing a downstream physical effect $E$ (a bodily movement). Causal closure supplies $P$ as a sufficient cause of $E$; so either $M$ is idle, or $M$ and $P$ jointly overdetermine $E$ — each independently sufficient for the same effect, as when two assassins simultaneously and independently shoot one victim. Since systematic overdetermination of this 'firing squad' kind is implausible, the mental cause appears to be excluded.

Two families of responses structure the literature. The vertical strategy locates the solution in the vertical (synchronic, dependence) leg of the model: it posits a relation $R$ between $M$ and $P$ — the determinate/determinable relation (a specific way of being a more general property, as scarlet is of red; Yablo 1992), the subset view of realization (the causal powers of the mental instance — the causing capacities a property confers — form a subset of the powers of its physical realizer; Wilson 2009), material constitution, or realization/realizer relations — that explains the supervenience and is 'intimate enough' that $M$ and $P$ cannot be genuine causal competitors. Yablo's dictum that determinates do not contend with their determinables for causal influence is the paradigm: a pigeon trained to peck at all and only red triangles is caused to peck by the triangle's being red, not in competition with its being scarlet. The horizontal strategy instead proposes a theory of causation on which the threatened overdetermination cannot arise: for example, List and Menzies (2009) and Zhong (2014) deploy interventionist conditions (a factor counts as a cause only if suitably independent interventions on it would change the effect) or proportionality conditions (a cause must be described at a level of detail proportionate to its effect) under which supervening causes and their subvening bases cannot both count as causes of the same effect (Woodward 2015 develops the interventionist treatment; McDonnell 2017 presses objections to proportionality-based versions). Reductionist and eliminativist exits — identifying mental with physical causes, or denying mental causation — abandon non-reductive physicalism rather than defending it; Ney (2007) examines and criticizes the constitution-based variant of the vertical route, and Bennett (2003) frames the dialectic of 'breaking the analogy' between firing squads and non-reductive causation.

Donaldson (2021) argued that what is really at issue is the vertical question: horizontal theories can neither remove the firing-squad intuition (reclassifying one of two sufficient events as 'not the cause' does nothing to make systematic coincident sufficiency plausible) nor are they needed, since a sufficiently intimate vertical relation alone dissolves the competition. But he was explicit that this reframing settles nothing substantive: whether any candidate $R$ actually holds between mental and physical events is 'the further, significant issue' on which he is silent, and a residual party of 'hardline horizontalists' — those who refuse any systematic overdetermination among supervenience-related causes — may still find non-reductive physicalism unacceptable, a tension he flags for the outcome of debates about theories of causation that entail hardline horizontalism.

Problem Statement

Within the standard non-reductive model of mental–physical causation — a mental event $M$ and a physical event $P$, distinct, with $M$ supervening on $P$ (no mental difference without some underlying physical difference), both candidate causes of a physical effect $E$, under physical causal closure (every physical effect that has a sufficient cause has a sufficient physical cause) — determine each of the following three items.

1. (Vertical core) Whether any candidate vertical relation $R$ — the determinate/determinable relation (a specific property standing to a more general one as scarlet stands to red), the subset relation between causal powers (the causing capacities conferred by the mental instance forming a subset of those conferred by its physical realizer), material constitution (the relation between an object and the matter that constitutes it), a realization/realizer relation (the mental property being instantiated in virtue of its physical base), or another relation proposed to perform the same dialectical function of ensuring sufficient intimacy — actually holds between mental and physical events, and holds in the sense intimate enough to ensure that $M$ and $P$ do not causally compete, i.e., that assigning causal efficacy to $M$ involves no objectionable systematic overdetermination. Or whether every candidate is either not instantiated in the mental–physical case or insufficiently intimate, so that the vertical route to vindicating non-reductive mental causation fails.

2. (Horizontal core) Whether non-reductive physicalism is consistent with hardline horizontalism — the rejection of any systematic overdetermination involving causes that stand in supervenience relations to one another — and whether any theory of causation that entails hardline horizontalism (in the manner of proportionality-style accounts, on which causes must be described at a level of detail proportionate to their effects, or interventionist accounts, on which a factor is a cause only if suitably independent interventions on it would change the effect) is true or otherwise acceptable on independent grounds.

3. (Meta-question) How the exclusion problem itself should be individuated and adjudicated: whether it is the problem of breaking the firing-squad/non-reductive-causation analogy or the problem of demonstrating non-reductionism's consistency with hardline horizontalism, and how the firing-squad intuition is to be weighed against the theoretical costs of particular vertical relations or causal theories.

References

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