Interactionism or interactionist dualism is the theory in the philosophy of mind which holds that matter and mind are two distinct and independent substances that exert causal effects on one another. It is one type of dualism, traditionally a type of substance dualism though more recently also sometimes a form of property dualism.
Interactionism was propounded by the French rationalist philosopher René Descartes (1596-1650), and continues to be associated with him. Descartes posited that the body, being physical matter, was characterized by spatial extension but not by thought and feeling, while the mind, being a separate substance, had no spatial extension but could think and feel. Nevertheless, he maintained that the two interacted with one another, suggesting that this interaction occurred in the pineal gland of the brain.
In the 20th century, its most significant defenders have been the noted philosopher of science Karl Popper and the neurophysiologist John Carew Eccles. Popper in fact divided reality into three "worlds"--the physical, the mental, and objective knowledge (outside the mind)--all of which interact, and Eccles adopted this same "trialist" form of interactionism. Other notable recent philosophers to take an interactionist stance have been Richard Swinburne, John Foster, David Hodgson, and Wilfrid Sellars, in addition to the physicist Henry Stapp.
In his 1996 book The Conscious Mind, David Chalmers rejected interactionism, but in 2002 he reversed his position and listed it along with epiphenomenalism and what he calls "Type-F Monism" as a position worth examining. Rather than invoking two distinct substances, he defines interactionism as the view that "microphysics is not causally closed, and that phenomenal properties play a causal role in affecting the physical world." (See property dualism.) He argues the most plausible place for consciousness to impact physics is the collapse of the wave function in quantum mechanics.
One objection often posed to interactionism is the problem of causal interaction - how the two different substances the theory posits, the mental and the physical, can exert an impact on one another. Descartes' theory that interaction between the mind and the physical world occurred in the pineal gland was seen as inadequate by a number of philosophers in his era, who offered alternate views: Nicholas Malebranche suggested occasionalism, according to which mind and body appear to interact but are in fact moved separately by God, while Gottfried Leibniz argued in The Monadology that mind and body are in a pre-established harmony. On the other hand, Baruch Spinoza rejected Descartes' dualism and proposed that mind and matter were in fact properties of a single substance, thereby prefiguring the modern perspective of neutral monism.
Today the problem of causal interaction is frequently viewed as a conclusive argument against interactionism. On the other hand, it has been suggested that given many disciplines deal with things they do not entirely understand, dualists not entirely understanding the mechanism of mind-body interaction need not be seen as definitive refutation. The idea that causation necessarily depends on push-pull mechanisms (which would not be possible for a substance that did not occupy space) is also arguably based on obsolete conceptions of physics.
A more recent related objection is the argument from physics, which argues that a mental substance impacting the physical world would contradict principles of physics. In particular, if some external source of energy is responsible for the interactions, it would violate the law of conservation of energy. Two main responses to this have been to suggest the mind influences the distribution but not the quantity of energy in the brain and to deny that the brain is a causally closed system in which conservation of energy would apply.
Taking the argument a step further, it has been argued that because physics fully accounts for the causes of all physical movements, there can be no place for the a non-physical mind to play a role. The principle, in slightly different iterations, has variously been called causal closure, completeness of the physical, physical closure, and physical comprehensiveness. This has been the foremost argument against interactionism in contemporary philosophy.
Some philosophers have suggested the influence of the mind on the body could be reconciled with deterministic physical laws by proposing the mind's impacts instead take place at points of quantum indeterminacy. Karl Popper and John Eccles, as well as the physicist Henry Stapp, have theorized that such indeterminacy may apply at the macroscopic scale. (See quantum mind.) However, Max Tegmark has argued that classical and quantum calculations show that quantum decoherence effects do not play a role in brain activity. David Chalmers has noted (without necessarily endorsing) a second possibility within quantum mechanics, that consciousness' causal role is to collapse the wave function as per the Von Neumann-Wigner interpretation of quantum mechanics. He acknowledges this is at odds with the interpretations of quantum mechanics held by most physicists, but notes, "There is some irony in the fact that philosophers reject interactionism on largely physical grounds (it is incompatible with physical theory), while physicists reject an interactionist interpretation of quantum mechanics on largely philosophical grounds (it is dualistic). Taken conjointly, these reasons carry little force...".
There remains a literature in philosophy and science, albeit a much-contested one, that asserts evidence for emergence in various domains, which would undermine the principle of causal closure. (See emergentism.) Another option that has been suggested is that the interaction may involve dark energy, dark matter or some other currently unknown scientific process.
Another possible resolution is akin to parallelism--Eugene Mills holds that behavioral events are causally overdetermined, and can be explained by either physical or mental causes alone. An overdetermined event is fully accounted for by multiple causes at once. However, J. J. C. Smart and Paul Churchland have argued that if physical phenomena fully determine behavioral events, then by Occam's razor a non-physical mind is unnecessary. Andrew Melnyk argues that overdetermination would require an "intolerable coincidence."
While causal closure remains a key obstacle for interactionism, it is not relevant to all forms of dualism; epiphenomenalism and parallelism are unaffected as they do not posit that the mind affects the body.
Interactionism can be distinguished from competing dualist theories of causation, including epiphenomenalism (which admits causation, but views causation as unidirectional rather than bidirectional), and parallelism (which denies causation, while seeking to explain the semblance of causation by other means such as pre-established harmony or occasionalism).
In The Conscious Mind, David Chalmers argued that regardless of the mechanism by which the mental might impact the physical if interactionism were true, there was a deeper conceptual issue: the chosen mechanism could always be separated from its phenomenal component, leading to simply a new form of epiphenomenalism. Later, he suggested that while the causal component could be separated, interactionism was like "type-F monism" (Russellian monism, panpsychism, and panprotopsychism) in that it gave entities externally characterized by physical relationships the additional intrinsic feature of conscious properties.