The mind-body problem is an ongoing issue that aims to understand the relationship between our body and our mind. Several philosophical viewpoints have been proposed to address this problem. One of them is dualism. The dualists believed that there are two types of substances: physical and non-physical. A substance can be conceptualised as an entity that is fundamental in nature and its existence is not dependent on any other entity. Descartes was one of the first proponents of Substance Dualism. He suggested that the mind and the body are two distinct substances. Body is a physical substance, made up of cells and has physical properties like mass. On the other hand, the mind is a non-physical substance as it is not made with cells, does not have mass, cannot be seen or touched, and cannot be located in space. Descartes believed that the mind can exist without the body.
Another viewpoint within dualism was put forward which states that there is one physical substance, but this substance possesses two distinct set of properties, namely physical and mental. This viewpoint proposes that the brain is a physical substance composed of neurons and other chemical substances with various physical properties like weight, but the brain also has some non-physical properties (the mental properties) such as consciousness and the ability to sense pain that cannot be reduced to physical properties. The mind and brain are therefore not separate physical entities but an inseparable entity with different properties. According to one type of Property Dualism, Epiphenomenalism, the mental properties of the brain are caused by the physical properties of the brain, but the physical properties of the brain cannot be caused by the mental properties. For example, when a person hurts himself/herself, the firing of c-fibers can result in the mental property of feeling pain.
Property dualism is often seen as an advancement of substance dualism and has several advantages when compared to substance dualism. By defining a single substance, it ascribes to physicalism and resolves the difficulties in explaining the location of the mind, a limitation of substance dualism. At the same time, it avoids the constraints imposed by physicalism that consciousness, thoughts, and emotions are simply the actions of the brain. Lahav and Shanks (1992) suggested that property dualist theory of consciousness is consistent with the neurobiological data and the contemporary theories of physics. According to these researchers, phenomenal properties cannot be reduced to microphysical properties that integrates the information in the brain. Further, they’ve inferred that these properties can integrate information using microscopic structures in the neuron without violating the principles of quantum mechanics.
Like substance dualism, property dualism has faced criticisms. For example, philosophers have questioned why we rule out the possibility of causal interactions between physical and non physical properties. Adding to this, property dualism does not allow the mental events to influence physical events when the interaction between physical and mental properties is unidirectional. For example, the cry after experiencing pain as a result of c-fibers caused by the nervous system in response to a stimulus cannot be interpreted as a physical property caused by a mental property. To overcome this limitation, the property dualists suggested another viewpoint, the interactionist property dualism which states that mental properties do causally influence the brain and the muscles (physical properties). For example, the experience of pain can result in a cry. Moreover, the irreducible nature of the mental properties has been questioned by the materialists. With its sophisticated brain imaging techniques, research in Neuroscience is able to explain our behaviour in terms of structural, chemical, and electrophysiological properties of the brain. For example, instead of considering the perception of an object as a mental property, the materialists can break it down into physical properties of the brain and the eye. The image of an object that falls on the fovea of the eye is carried to the inferior temporal cortex and the posterior parietal cortex for perceiving it. However, in the same paper, Goodale and Humphrey have mentioned that there is no evidence for controlling the stimuli and the underlying mechanisms for three visually modulated outputs, namely the pupillary light reflex, the synchronisation of circadian rhythms with the local light-dark cycle, and the visual control of posture. We might infer that all mental properties cannot be reduced to physical properties, thereby keeping the viewpoint of property dualism still alive today. Another criticism given by John Searle (1992) was that this viewpoint along with substance dualism and materialism consider physical and mental entities to be distinct and mutually exclusive. According to him, the physical particles in the universe are organised into systems, including biological systems. He said that, “Consciousness is thus an ordinary feature of certain biological systems, in the same way that photosynthesis, digestion, and lactation are ordinary features of biological systems.”
We can draw an analogy between property dualism and De Broglie Hypothesis in quantum mechanics which states that matter can behave both as particles and waves at the subatomic level. Here, matter is one substance with two properties. In line with the analogy and to defend Property dualisms against the criticisms mentioned above, David Chalmers suggested that it is not logically possible for the physical universe to be different if the microphysical facts are the same and that this logic cannot be applied to consciousness. He believed that the whole physical course of the universe could be exactly the same provided that one subtracts consciousness from it.
Therefore, we can see that in order to find an answer to the mind-body problem, philosophers have developed theories or viewpoints, each being unique and having some strengths and limitations. Property dualism served as a bridge between several theories, including substance dualism and materialism, protecting it from severe objections and sustaining it as a noteworthy viewpoint till date.