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The Causal Closure of the Physical states that every physical event has a sufficient physical cause among prior physical events. If true, this principle constrains how non-physical properties (like mental properties) can causally affect a physical world, generating constraints on the mind-body problem.
You already understand causation — the relation that connects events such that one brings about another — and you've encountered mental causation: the question of how mental states like beliefs, desires, and intentions can cause physical actions. The causal closure of the physical is a claim about the completeness of the physical causal order: every physical event that has a cause at all has a sufficient physical cause among prior physical events. Put simply, physics never needs to reach outside of physics to explain any physical outcome.
This principle seems plausible given the success of the physical sciences, but it creates a serious problem for any view that takes mental states to be non-physical. If your decision to raise your arm (a mental event) causes your arm to rise (a physical event), and if the arm-rising already has a sufficient physical cause in nerve firings and muscular contractions, then what causal work is the mental decision doing? This is the exclusion problem: if a physical effect already has a sufficient physical cause, any additional mental cause appears redundant — epiphenomenal, present but causally idle, like the shadow of a moving object that neither helps nor hinders the motion.
The exclusion problem forces a choice among several positions. Epiphenomenalism accepts causal closure and concludes that mental states cause nothing — they are effects of physical processes but have no downstream causal power. This preserves closure but makes a counterintuitive claim: your pain does not cause you to wince; the pain and the wince are both effects of the same underlying neural events. Eliminativism goes further: if mental states have no causal role, they may simply be a folk-psychological fiction, eventually to be replaced by purely physical descriptions. Type identity theory avoids the problem by identifying mental properties with physical properties — your pain *just is* a pattern of neural activity. If the mental and physical are identical, there is no exclusion: the mental cause and the physical cause are one and the same.
Causal closure also pressures property dualism, the view that mental properties are distinct from physical properties even if mental events depend on physical events. If mental properties are distinct, how can they be causally relevant without violating closure? This is one reason some philosophers prefer to formulate the issue in terms of properties and causal relevance rather than events and causation. The depth of the exclusion problem — and the difficulty of every proposed solution — is why causal closure occupies a central place in the metaphysics of mind.
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