Theory of the mind as a non-physical thing. According to dualism, although the brain is a physical substance, the mind (or soul) is a non-physical substance, sharing no properties with any physical substance. Minds, on the dualists’ hypothesis, can be aware and have experiences, whereas physical substances such as the liver or the brain, cannot. Histori-cally, Plato and Descartes gave substance dualism its most systematic treatment and defence.
Among the problems facing dualism, the most intractable has been the nature of the interaction between the mind and brain. Specifically, the problem is this: how can something with no physical properties whatever causally interact with physical stuff; how can it influence the brain or be influenced by it? Descartes recognized the problem, but hoped an explanation might lie in an interaction between the soul and very fine materials in the PINEAL GLAND, an organ located on the underside of the brain. This proposal was unsatisfactory, however, because even very fine material is still material. No recent proposal has fared any better. With the development of modern science, the interaction problem has become acute, especially because of direct conflict with current physics and the law of conservation of mass-energy. Modern biology also presents dilemmas for dualism. Humans, like other primates, are an evolved species, and brain EVOLUTION appears to hold the key to increased behavioral complexity in non-human animals. In all likelihood, the human brain evolved from earlier hominid brains, and it is known to share much of its organization and chemistry with non-human brains. This suggests that humans’ mental capacities to think, feel and so forth have precursors in non-human brains. NEUROSCIENCE, the study of the brain, has failed to reveal any focal structure in the brain that could be the seat of CONSCIOUSNESS, and the pineal gland can be destroyed without any effect on consciousness.
Property dualism tries to avoid some of these problems by dropping the notion of a non-physical substance whilst adopting the hypothesis that the human brain, unlike other physical systems, has the capacity to generate non-physical properties such as feeling PAIN. On this view, mental states emerge from the physical brain, but are not themselves physical. In what sense are they ‘non-physical properties’? Adherents say it means the properties cannot be explained in terms of the physical properties of the brain; either they cannot be explained at all or their explanation will require an autonomous science of the mind or perhaps a new physics.
More recent arguments supporting dualism mainly depend on introspective observations concerning how very different are physical properties such as the activity of a neuron, and mental properties such as feeling pain. Like Descartes, some are also convinced that the creative nature of speech (see SPEECH PERCEPTION) and LANGUAGE comprehension implies that mere mechanical devices such as the brain cannot account for language use. Related to this, some philosophers feel that meaningfulness of words and sentences in thought, is evidence that meaning cannot be a physical property.
Several additional reasons motivate dualism, one deriving from the idea of free will, and one from the hope of immortality. Some philosophers consider uncaused choice to be a defining characteristic of humans. Here is the argument: (1) if choice is caused by a mechanism such as the brain, then it is not free; (2) humans do have free will. Therefore: choice and decision-making are independent of physical activity of the brain. Physicalists typically counter this argument by denying the first premise. They suggest that voluntary actions are those that are caused by the agent’s desires, reasons and beliefs, and that uncaused choice would be like a random or ‘out-of-the-blue’ actions. Randomly occurring events would not be the outcome of what the agent really wanted, and hence the agent could not be held responsible for the action. According to this hypothesis, some kinds of causal antecedents of choice involve responsibility, while other conditions, such as having an epileptic seizure or schizophrenic hallucinations, can excuse an agent. Understanding the nature of the causal antecedents, and which do and which do not excuse an agent, is considered to be the joint task of COGNITIVE NEUROSCIENCE and jurisprudence. Belief in an immortal soul that survives the death of the physical body is sometimes a basis for adopting some version of dualism. This belief is a central tenet of many religions, and the apparent conflict between science and religion may be considered resolvable by presuming each to have its autonomous domain of subject matter. Others reject the independence tactic, viewing the conflict to be entirely real, and preferring to try to resolve it, one way or the other.
