That psychophysical interaction occurs seems obvious. How it occurs seems inexplicable. It is a presupposition of common sense but prima facie inconsistent with science that mental events cause physical events and physical events cause mental events. If a commander's decision is a cause of an air strike, then a physical event has a mental cause. If eating overripe cheese causes vivid dreams, then a mental event has a physical cause. However, if the physical universe is a closed deterministic system, then any physical event is caused by distinct physical events sufficient for its occurrence. If physical causes are sufficient for physical effects, then nothing else is necessary, so no mental cause is necessary for any physical event to happen. It follows that mental causation is redundant in the physical universe.
The Third Law of Thermodynamics states that the quantity of energy in the universe is constant. If there were mental causation, extra energy would be introduced into the universe, so if the Third Law of Thermodynamics is true there is no mental causation.
It is just as scientifically inexplicable how mind, consciousness, or awareness could be produced by the brain. The brain is the most complex object known to exist. Nevertheless, for all its neurological complexity, the brain is only billions and billions of wholly physical atoms moving through empty space. It is hard to see how billions and billions of atoms could give rise to awareness: the reader's own awareness of this page, for example. Consciousness seems to be so radically qualitatively distinct from matter in motion it is unimaginable how it could arise out of it.
It is extremely difficult to specify exactly how it is possible for a neurological event to cause a mental event, or a mental event to cause a neurological event. If there exist both minds and bodies then the logically possible permutations of psychophysical interaction would seem to be these: Minds affect bodies but bodies do not affect minds; Bodies affect minds but minds do not affect bodies; Minds affect bodies and bodies affect minds; Minds do not affect bodies and bodies do not affect minds.
If the mental states of human beings bring about physical effects, then human beings are not wholly explicable in terms of scientific laws. One account of this inexplicability is that humans are spiritual beings. Humans as spiritual beings is explicable in turn if they are made in the image of God. It is then unsurprising if the finite mind-body relation is partly like the infinite God-world relation. If we could understand how the mental and the physical interact in causal relations, some insight might be gained regarding divine creation and divine intervention.
The pre-Socratic philosopher Anaxagoras (c. 500–c. 428 b.c.e.) famously claimed that "Mind … causes all things," but he neglected to explain how. Aristotle (384–322 B.C.E) claimed in the Nicomachean Ethics that choice is the efficient cause (sufficient condition) of action, so if choices are mental and actions are physical, there is mental causation. Nevertheless, Aristotle provided no account of how mental causation is possible. During the seventeenth century, the French philosopher and mathematician René Descartes argued for two-way psychophysical causal interaction between the immaterial soul and the human body, but he admitted to being incapable of explaining how this was possible.
One solution to the problem is to deny that there is causation in either direction, a view entailed by the psychophysical parallelism endorsed by Gottfried Wilhelm Leibniz (1646–1716), the German philosopher and mathematician, and his French partial contemporary Nicholas Male-branche. Leibniz argued that God has caused a "pre-established harmony" and thereby initiated the causal chain that results in mental events and the causal chain that results in physical events, which are correlated rather like the motions of two clocks, each of which tells the right time and therefore the same time. Malebranche agreed that God initiated both causal chains but claimed further that God intervened to determine the timing of each mental event and each physical event. Male-branche's version of psychophysical parallelism is sometimes called occasionalism for this reason. In the monist theory of Baruch Spinoza, the seventeenth-century Dutch-Jewish philosopher, thought and extension are two aspects of one substance and do not interact causally.
Much contemporary work on mental causation is also a reaction to Cartesianism. Descartes's mind-body dualism is rejected, but his recognition of psychophysical interaction is accepted. The attempt is then made to explain how mental causation is possible.
The most influential theory of mental causation of the last quarter of the twentieth century was anomalous monism, advocated by Donald Davidson (b. 1917), Professor of Philosophy at the University of California at Berkeley. Davidson argues in his 1970 paper "Mental Events" that mental events cause physical events because they are physical events. Davidson's motivation is to relieve the appearance of contradiction between three principles that he thinks are true: causal interaction, the nomological character of causality, and the anomalism of the mental.
According to the principle of causal interaction at least some mental events cause physical events and at least some physical events cause mental events. For example, if intentions, perceptions, and decisions are amongst the causes of the sinking of a battleship in a naval engagement then a physical event has at least some mental causes. If the perception of a physical object, say a battle-ship, causes beliefs then a mental state has at least some physical causes. On the principle of the nomological character of causality, if two events are causally related then they always fall under some strict deterministic law. On the principle of the anomalism of the mental there are no psychophysical laws, so there are no strictly deterministic laws relating mental events and physical events as causes and effects.
These three principles are prima facie mutually inconsistent. Seemingly, if mental events cause physical events or physical events cause mental events, then they are related by strict deterministic laws so it is then false that there are no psychophysical laws. On the other hand, it seems that if there are no psychophysical laws then either mental events do not cause physical events, and physical events do not cause mental events, or not every pair of events related as cause and effect falls under strict deterministic laws. Nevertheless, Davidson aims to reconcile the three principles by his anomalous monism.
Anomalous monism entails that mental events are identical with physical events. Every mental event is identical with some physical event, but not every physical event is identical with a mental event. Mental events are causes of physical events because they are physical events, and physical events cause physical events. Nevertheless, Davidson rejects the thesis that there exist psychophysical laws. According to nomological monism every mental event is a physical event, and there are psychophysical laws. Davidson accepts the first part of this statement but not the second. According to nomological dualism, no mental event is identical with any physical event, but there are nevertheless psychophysical laws because, for example, mental events are correlated with physical events in some close and invariable way that falls short of identity. Davidson rejects both parts of this. According to anomalous dualism (or Cartesian dualism) no mental event is identical with any physical event and there are no psychophysical laws. Davidson accepts the second part of this but not the first.
If there are no strictly deterministic psychophysical laws then there is no physical explanation of the mental. For example, it is not possible to predict someone's mental state given a complete knowledge of their physical state, or even a complete knowledge of their present and past physical states, or even a complete knowledge of the prior state of the physical universe. If there are no psychophysical laws then no mental events may be subsumed under strictly deterministic scientific generalizations.
If the mental is anomalous then room seems to be left for human freedom. If there is no strict deterministic law relating one's choices or decisions to physical events then they are not necessitated by physical events. Indeed, if one's choices and decisions may cause physical events then one's mental states are at least amongst the causes of one's own actions and, arguably, this is part of what it means for a person to have free will, to be an autonomous agent.
Davidson holds that the dependence of the mental on the physical is very close. The mental is supervenient on the physical. This means that if two mental events differ in some mental respect then they cannot only differ in that mental respect but must differ in some physical respect. However, if they differ in some physical respect it does not follow that they differ in any mental respect.
The doctrine that some property F supervenes on some property G is expressed by a cluster of views, because the supervenience relation admits of variants and degrees. First, if being F supervenes on being G, then if two objects—a, b —are indiscernible with regard to being F, then they are indiscernible with regard to being G. So, if the mental supervenes on the physical then it is not possible for two persons to be indiscernible with regard to some mental property without being indiscernible with regard to some physical property.
Second, if being F supervenes on being G, then if a is F and b is exactly like a in being F, then b is G. So, if two people share a mental property then they share a physical property. Third, if being F supervenes on being G, then a cannot change with respect to being F without changing with respect to being G. So a person cannot change in any mental respect without changing in some physical respect. It is unclear how much is entailed by "without changing with respect to being G ", but arguably if being F supervenes on being G, then a cannot cease to be F without ceasing to be G. In that case, a person cannot cease to posses a mental property without ceasing to possess a physical property. Also arguably if being F supervenes on being G, then a cannot begin to be F without beginning to be G. In that case a person cannot gain a mental property without gaining a physical property.
Weak supervenience is the doctrine that if in the actual world a is F, then in the actual world a is G. So if the mental is weakly supervenient on the physical, then if a person has a mental property in the actual world, then they have a physical property in the actual world. Strong supervenience is the doctrine that if in every possible world a is F, then in every possible world a is G. So if the mental is strongly supervenient on the physical, then if a person has a mental property in every possible world, then they have a physical property in every possible world.
Arguments for supervenience are harder to find than formulations. The supervenience of the mental on the physical is designed to capture the "intuition" that mental facts depend on physical facts but physical facts do not depend on mental facts. It also seems to promise dependence without reduction: Mental events are not nomologically reducible to physical events because there are no psychophysical laws. Mental events are not logically reducible to physical events because it is not true that any sentence or set of sentences about mental events can be translated into a sentence or set of sentences about physical events without loss of meaning.
Even though the mental supervenes on the physical there can be no hope of a reduction of psychology to neurology in the way in which, arguably, biology may be reduced to chemistry and chemistry reduced to physics.
Epiphenomenalism and psychophysical laws
It is objected to anomalous monism, notably by the Canadian philosopher Ted Honderich (b. 1933), that on this theory mental events do not cause physical events "in virtue of" being mental events but in virtue of being physical events. The mental properties of the mental events are not causally efficacious. Honderich points out that some pears on a weighing scale depress the scale in virtue of their physical property of being a certain weight, not in virtue of, say, being green. If this sort of objection is right then in anomalous monism the mental properties of mental events do no causal work. Mental causation has not been explained because nothing mental qua mental is causing anything physical.
Anomolous monism arguably collapses into a kind of epiphenomenalism: the theory that mental events are caused by physical events but physical events are not caused by mental events. In reply it might be urged that the sentence "Mental events cause physical events" is true according to anomalous monism because mental events cause physical events because they are identical with physical events that cause physical events.
It has been argued, notably by Honderich in his A Theory of Determinism (1988), that the neurological is sufficient for the mental: The occurrence of some neurological event is a sufficient condition for the occurrence of some psychological event and, as is logically entailed by this, the occurrence of that psychological event is a necessary condition for the occurrence of that neurological event. On this view there seems to be no reason in principle why psychophysical laws should not be discovered because true sentences about neurology would entail true sentences about psychological events. It ought then to be possible to predict the occurrence of psychological events from knowledge of neurological events.
It is empirically uncontroversial in the case of human beings (if not souls, computers, and deities) that the neurological is necessary for the mental. Neurological impairment leads to psychological impairment. If the neurological is necessary for the mental, then the mental is sufficient for the neurological. Arguably the dependency of the mental on the neurological is ultimately an empirical and contingent one. If computers, souls, or God have mentality but no neurology, then it is not a necessary truth that the mental depends on the neurological.
Karl Popper (1902–1993) has argued that the self-conscious mind is an evolutionary product of the brain that nevertheless acts causally upon it. However, if mental events are sufficient for neurological events and if the physical universe is a closed deterministic system, then neurological events are overdetermined. Some event is overdetermined if at least two conditions sufficient for its occurrence obtain. This would seem to make either mental causes or neurological causes redundant. If a neurological event is sufficient for the occurrence of a neurological event then no mental event is necessary for it. If a mental event is sufficient for a neurological event then no neurological event is necessary for it.
One solution, adopted by the contemporary English philosophers Tim Crane and D. H. Mellor for example, is to give up the assumption that the universe is a closed deterministic system. Crane and Mellor see no reason in principle why there should not exist psychophysical laws as scientifically respectable as physical laws.
See also Consciousness Studies; Descartes, RenÉ; Determinism; Experience, Religious: Cognitive and Neurophysiological Aspects; Functionalism; Mind-body Theories; Neurosciences; Supervenience
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