In the philosophy of mind, mind–body dualism denotes either that mental phenomena are non-physical, or that the mind and body are distinct and separable. Thus, it encompasses a set of views about the relationship between mind and matter, as well as between subject and object, and is contrasted with other positions, such as physicalism and enactivism, in the mind–body problem. Aristotle shared Plato's view of multiple souls and further elaborated a hierarchical arrangement, corresponding to the distinctive functions of plants, animals, and humans: a nutritive soul of growth and metabolism that all three share; a perceptive soul of pain, pleasure, and desire that only humans and other animals share; and the faculty of reason that is unique to humans only. In this view, a soul is the hylomorphic form of a viable organism, wherein each level of the hierarchy formally supervenes upon the substance of the preceding level. For Aristotle, the first two souls, based on the body, perish when the living organism dies, whereas there remains an immortal and perpetual intellective part of mind. For Plato, however, the soul was not dependent on the physical body; he believed in metempsychosis, the migration of the soul to a new physical body. It has been considered a form of reductionism by some philosophers, since it enables the tendency to ignore very big groups of variables by its assumed association with the mind or the body, and not for its real value when it comes to explaining or predicting a studied phenomenon. Dualism is closely associated with the thought of René Descartes (1641), who holds that the mind is a nonphysical—and therefore, non-spatial—substance. Descartes clearly identified the mind with consciousness and self-awareness and distinguished this from the physical brain as the seat of intelligence. Hence, he was the first documented Western philosopher to formulate the mind–body problem in the form in which it exists today. However, the theory of substance dualism has many advocates in contemporary philosophy such as Richard Swinburne, William Hasker, J. P. Moreland, E. J. Lowe, Charles Taliaferro, Seyyed Jaaber Mousavirad, and John Foster. Dualism is contrasted with various kinds of monism. Substance dualism is contrasted with all forms of materialism, but property dualism may be considered a form of non-reductive physicalism.
Types Ontological dualism makes dual commitments about the nature of existence as it relates to mind and matter, and can be divided into three different types:
Substance dualism asserts that mind and matter are fundamentally distinct kinds of foundations. Property dualism suggests that the ontological distinction lies in the differences between properties of mind and matter (as in emergentism). Predicate dualism claims the irreducibility of mental predicates to physical predicates.
Substance or Cartesian dualism Substance dualism asserts that mind and matter are fundamentally distinct kinds of foundations. There are different types of substance dualism. Most substance dualists hold the view that the mind and body are capable of causally affecting each other, known as interactionism. Notable defenders of substance dualism include John Foster, Stewart Goetz, Richard Swinburne and Charles Taliaferro. Cartesian dualism, most famously defended by René Descartes, argues that there are two kinds of substances: mental and physical. Descartes states that the mental can exist outside of the body, and the body cannot think. Substance dualism is important historically for having given rise to much thought regarding the famous mind–body problem. It is compatible with theologies which claim that immortal souls occupy an independent realm of existence distinct from that of the physical world. Cartesians tend to equate the soul to the mind. The Copernican Revolution and the scientific discoveries of the 17th century reinforced the belief that the scientific method was the unique way of knowledge. Bodies were seen as biological organisms to be studied in their constituent parts (materialism) by means of anatomy, physiology, biochemistry and physics (reductionism). Mind–body dualism remained the biomedical paradigm and model for the following three centuries.
Emergent dualism Emergent dualism is a type of substance dualism that has been defended by William Hasker and Dean Zimmerman. Emergent dualism asserts that mental substances come into existence when physical systems such as the brain reach a sufficient level of complexity. Hasker defines emergent dualism as:
Human persons are not identical to any physical body, but consist of a physical body and a non-physical substantial soul, and (b) the human soul is naturally emergent from and dependent on the structure and function of a living human brain and nervous system. Hasker has argued that emergent dualism is consistent with neuroscientific discoveries showing the dependence of mind on brain. He likens the individual mind to a magnetic field in its qualitative difference from the physical properties that generate it and also in its ability to act on the brain that generates it. Consciousness is said to arise when the brain reaches a certain threshold level of organizational complexity and when properly organized gives rise to the soul.
Thomistic dualism
Thomistic dualism is a type of dualism derived from the views of Thomas Aquinas. Edward Feser has written that:
Aristotelians and Thomists (those philosophers whose views are derived from St.Thomas Aquinas) sometimes suggest that their hylomorphic position is no more a version of dualism than it is of materialism. But though their view is not a Cartesian form of dualism, it is clear from a consideration of how the human soul differs from the souls of plants and animals (at least on the Thomistic variation of hylomorphism) that the view does amount to a kind of dualism: Thomistic dualism or hylomorphic dualism, as it has variously been called. Thomistic substance dualism has been defended by J. P. Moreland and Scott B. Rae. Thomistic substance dualism distinguishes itself from Cartesian substance dualism by denying that the body and soul are different substances. Instead, a person is composed of only one substance, the soul, while the body is considered an ensouled physical structure. J. P. Moreland has commented:
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