Causal closure

Physical causal closure is a metaphysical theory about the nature of causation in the physical realm with significant ramifications in the study of metaphysics and the mind. In a strongly stated version, the physical causal closure says that "all physical states have pure physical causes" — Jaegwon Kim, or that "physical effects have only physical causes" — Agustin Vincente, p. 150.

Those who accept the physical causal closure tend to think that all entities that exist are physical entities (physicalists), but not necessarily. As Karl Popper says, "The physicalist principle of closedness of the physical ... is of decisive importance and I take it as the characteristic principle of physicalism or materialism."

The physical causal closure has stronger and weaker formulations.

The stronger formulations of the physical causal closure assert that "No physical event has a cause outside the physical domain." — Jaegwon Kim. That is, the stronger formulations assert that for physical events, causes other than physical causes do not exist. (Physical events that are not causally determined may be said to have their objective chances of occurrence determined by physical causes.)

Weaker forms of the physical causal closure state that "Every physical event has a physical cause." — Barbara Montero, or that "Every physical effect (that is, caused event) has physical sufficient causes" — Agustin Vincente, (According to Vincente, a number of caveats have to be observed, among which is the postulate that "physical entities" are entities postulated by a true theory of physics, a theory of which we are ignorant today, and that such a true theory "will not include mental (or in general, dubious) concepts" (Note 5, p. 168).) or that "if we trace the causal ancestry of a physical event we need never go outside the physical domain." — Jaegwon Kim. Weaker forms of the physical causal closure are synonymous with the causal completeness, the notion that "Every physical effect that has a sufficient cause has a sufficient physical cause." That is, weaker forms of the physical causal closure allow that in addition to physical causes, there may be other kinds of causes for physical events.

The notion of reductionism supplements the physical causal closure with the claim that all events ultimately can be reduced to physical events. Under these circumstances, mental events are a subset of physical events and caused by them.

The physical causal closure is especially important when considering dualist theories of mind. If no physical event has a cause outside the physical realm, it would follow that non-physical mental events would be causally impotent in the physical world. However, as Kim has agreed, it seems intuitively problematic to strip mental events of their causal power. Only epiphenomenalists would agree that mental events do not have causal power, but epiphenomenalism is objectionable to many philosophers. One way of maintaining the causal powers of mental events is to assert token identity non-reductive physicalism — that mental properties supervene on neurological properties. That is, there can be no change in the mental without a corresponding change in the physical. Yet this implies that mental events can have two causes (physical and mental), a situation which apparently results in overdetermination (redundant causes), and denies the strong physical causal closure. Kim argues that if the strong physical causal closure argument is correct, the only way to maintain mental causation is to assert type identity reductive physicalism — that mental properties are neurological properties.

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