Causal closure has stronger and weaker formulations.
The stronger formulations of causal closure assert that: "No physical event has a cause outside the physical domain." - Jaegwon Kim, or "Physical effects have only physical causes." - Agustin Vincente 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 theory state that "Every physical event has a physical cause." - Barbara Montero, or "Every physical effect (that is, caused event) has physical sufficient causes" - Agustin Vincente, or that "if we trace the causal ancestry of a physical event we need never go outside the physical domain." - Jaegwon Kim, The weaker form of causal closure is synonymous with causal completeness, the notion that "Every physical effect that has a sufficient cause has a sufficient physical cause." That is, the weaker forms allow that in addition to physical causes, there may be other kinds of causes for physical events.
The notion of reductionism supplements 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.
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 power of mental events is to assert token identity non-reductive physicalism - that mental properties supervene on neurological properties. That is, that 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 strong causal closure. Kim argues that if the strong 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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Of course, one can argue that not all events are "physical" events. The issue can then be raised as how, or even whether, non-physical events can affect physical events. According to the strong form of causal closure, that cannot happen.
One type of objection to this causal closure argument is that first-person subjective events defy the third-party stance of an unengaged observer that is the foundation of science. See the article Chinese room. An example where this conflict of views is under discussion is the field of neuroscience. Some argue that all mental states ultimately will be related to the interactions of neurons and synapses:
"...consciousness is a biological process that will eventually be explained in terms of molecular signaling pathways used by interacting populations of nerve cells.."—Eric R. Kandel, In Search of Memory: The Emergence of a New Science of Mind, p. 9
On the other hand, some argue a reconciliation is yet to be found between subjective and objective approaches:
"Epistemically, the mind is determined by mental states, which are accessible in First-Person Perspective. In contrast, the brain, as characterized by neuronal states, can be accessed in Third-Person Perspective...Either the First-Person Perspective, referring to mental states, is distinguished (and thus dissociated) from the Third-Person Perspective, which rather refers to neuronal states. Or the First-Person Perspective is reduced, subordinated, or eliminated in favour of the Third-Person Perspective...If the First-Person Perspective is reduced to the Third-Person Perspective, it should refer to neuronal states. This however is not the case..."—Goerg Northoff , The 'brain problem', pp. 2-3
"I argue that the bond between the mind and the brain is a deep mystery. Moreover, it is an ultimate mystery, a mystery that human intelligence will never unravel. Consciousness indubitably exists, and is connected to the brain in some intelligible way, but the nature of this connection necessarily eludes us."—Colin McGinn, The Mysterious Flame: Conscious Minds In A Material World, p. 5
- Barbara Montero (2003). "Chapter 8: Varieties of causal closure". In Sven Walter, Heinz-Dieter Heckmann, eds. Physicalism and Mental Causation: The Metaphysics of Mind and Action. Imprint Academic. pp. 173 ff. ISBN 0907845460.
- Jaegwon Kim (1993). Supervenience and Mind: Selected Philosophical Essays. Cambridge University Press. p. 280. ISBN 0521439965.
- Vicente, A. (2006). "On the Causal Completeness of Physics". International Studies in the Philosophy of Science 20: 149–171. doi:10.1080/02698590600814332.
- Sahotra Sarkar, Jessica Pfeifer (2006). Physicalism: The causal impact argument. "Physicalism". The Philosophy of Science: N-Z, Index. Taylor & Francis. p. 566. ISBN 041597710X.
- Max Velmans; Susan Schneider (15 April 2008). The Blackwell Companion to Consciousness. John Wiley & Sons. ISBN 978-0-470-75145-9. Retrieved 6 February 2013.
- Jaegwon Kim (1989). "The Myth of Non-Reductive Materialism". Proceedings and Addresses of the American Philosophical Association 63 (3): 31–47.
- This quote is from: Eric R. Kandel (2007). In Search of Memory: The Emergence of a New Science of Mind. WW Norton. p. 9. ISBN 0393329372. However, the same language can be found in dozens of sources. Some philosophers object to the unsupported statement of such conjectures, for example, observing that consciousness has yet to be shown to be a process at all, never mind a biological process. See Oswald Hanfling (2002). Wittgenstein and the Human Form of Life. Psychology Press. pp. 108–109. ISBN 0415256453.
- Georg Northoff (2004). "Chapter 1: The 'brain problem'". Philosophy of the Brain: The Brain Problem. John Benjamins Publishing. ISBN 9027251843.
- Colin McGinn (2000). The Mysterious Flame: Conscious Minds In A Material World. Basic Books. p. 5. ISBN 0465014232.