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 causal closure principle (CCP) says that "Physical effects have only physical causes." - Agustin Vincente, p. 150 
Those who accept the causal closure thesis 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."
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,. 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. (According to Vincente, a number of caveats have to 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, 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.
The validity of causal closure has long been debated. In modern times, it has been pointed out that science is based on removing the subject from investigations, and by seeking objectivity, creating a susceptibility to the subject-object problem. This outsider status for the observer, a third-person perspective, is said by some philosophers to have automatically severed science from the ability to examine subjective issues like consciousness and free will. A different attack upon causal closure discussed by Hodgson is to claim science itself does not support causal closure. Some philosophers have criticized the argument for causal closure by supporting teleology and mental-to-physical causation via a soul.
There seem prima facie to be irreducible purpose-based (or teleological explanations) of some natural phenomena. For instance, the movement of a writer's fingers on the keyboard and a reader's eyes across the screen is irreducibly explained in reference to the goal of writing an intelligible sentence or of learning about the causal closure arguments, respectively. On the face of it, an exclusively non-teleological (descriptive) account of the neurological and biological features of hand movement and eye movement misses the point. To say, "I am moving my fingers because my brain signals are triggering muscle motion in my arms" is true, but does not exhaustively explain all the causes. Both are causes. In Aristotelian terms, a biological account explains the material cause, while the purpose-based account explains the final cause.
The causal closure thesis challenges this account. It attempts to reduce all teleological final (and formal) causes to efficient causes. Goetz and Taliaferro urge that this challenge is unjustified, partly because it would imply that the real cause of arguing for causal closure is neurobiological activity in the brain, not (as we know it is) the purpose-based attempt to understand the world and explain it to others.
One way of clarifying the causal closure argument is to add a premise specifying that there are no irreducible teleological causes. This, of course, is the attempted conclusion of the argument. Putting the conclusion as one of the premises of the argument is formally valid, but fallacious (begging the question). It is no more sound than the argument that, "I believe in the Bible because it is the written word of God through his prophets. Obviously, God would not lie to his prophets. After all, the Bible says so."
Another way of clarifying the causal closure argument is to specify that: "Every physical effect [that has a physical cause] has a physical causes." But this is a tautology, and therefore true but trivial, like the statement "Every angel that has wings is an angel that has wings" or "Every white male who is bald is a bald white male."
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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
- Vicente, A. (2006). "On the Causal Completeness of Physics". International Studies in the Philosophy of Science 20: 149–171. doi:10.1080/02698590600814332.
- Popper and Eccles, Karl (1977). The Self and its Brain. New York: Springer. p. 51. ISBN 0415058988.
- 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.
- 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. doi:10.2307/3130081.
- Benjamin Libet, Anthony Freeman, Keith Sutherland (2000). "Editors' introduction: The volitional brain". The Volitional Brain: Towards a Neuroscience of Free Will. Academic. pp. ix–xxii. ISBN 9780907845119.
- Erwin Schrödinger (1956). "Chapter 3: The principle of objectivation". Mind and Matter: The Tarner lectures. p. 118.
By this I mean the thing that is so frequently called the 'hypothesis of the real world' around us. I maintain that it amounts to certain simplifications which we adopt in order to master the infinitely intricate problem of nature
- > FT Hong (2005). Vladimir B. Bajić, Tin Wee Tan, eds, ed. Information Processing and Living Systems. Imperial College Press. p. 388. ISBN 9781860946882.
The origination of free will is an illusion from the third-person perspective. However, it is a reality from the first-person perspective...
- > Thomas Nagel (2012). "Chapter 4: Cognition". Mind and Cosmos: Why the Materialist Neo-Darwinian Conception of Nature is Almost Certainly False. Oxford University Press. p. 71. ISBN 9780199919758.
[Higher-level cognitive capacities] cannot be understood through physical science alone, and..their existence cannot be explained by a version of evolutionary theory that is physically reductive.
- U Mohrhoff (2000). "The physics of interactionism". In Benjamin Libet, Anthony Freeman, Keith Sutherland, eds. The Volitional Brain: Towards a Neuroscience of Free Will. Academic. p. 166. ISBN 9780907845119.
But the laws of physics presuppose causal closure...Hence it follows that the behaviour of matter in the presence of a causally efficacious non-material mind cannot be fully governed by those laws.
- David Hodgson (2012). "Chapter 7: Science and determinism". Rationality + Consciousness = Free Will. Oxford University Press. ISBN 9780199845309. Hodgson relies upon the free will theorem1,2 of scientists John Conway and Simon Kochen based upon the role of the observer in quantum mechanics, which supports the view that "belief in determinism may thus come to be seen as notably unscientific." (p. 121)
- Stewart Goetz, Charles Taliaferro (2008). "Strict naturalism, purposeful explanation, and freedom". Naturalism (Intervensions) (Paperback ed.). Eerdmans. p. 26. ISBN 0802807682.
- Falcon, Andrea. "Aristotle on Causality". Stanford Encyclopedia of Philosophy. Stanford. Retrieved 2014-03-10.
- 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.