Plato claimed that human souls move between two worlds: a world of immaterial ideas, and a world of material, physical appearances. When a person is born, a soul temporarily moves from the world of ideas to the physical world, and nestles in the newly-born human body. This soul stays there until the physical body expires, at which point it returns to its non-physical home. Not only human souls, but the essence of each and every physical object, resides in a separate world of ideas. How else can we explain that we have certain ideas in our minds – abstract ideas like justice or beauty, concepts of geometry, ideas of a supreme being; ideas which we cannot have acquired from perception of the physical world? We only perceive particular instances of these abstract ideas, never the ideas themselves. Surely, we must have been born with these innate ideas, and accessing them is simply a matter of remembering them …
René Descartes ( † 1650 ) is the father of modern-day dualism in Western philosophy. In reply to his own question “What can I know for certain, without any doubt?” he postulated a dualist answer.
Mind and matter are fundamentally different. The physical world is made up of substances and properties that are very different from those that make up the mental world. Matter is extended – located in space and time – and governed by a closed system of physical causal laws. We only find spatial dimensions in the physical world. Mind is not extended – an idea does not weigh anything, and doesn’t take up space or time – and is governed by logical laws of rationality.
One problem for the dualist, sounds like this. If mind and matter really are so fundamentally different, if they really are defined by fundamentally different properties, how are they able to interact? How does mind causally affect matter, and vice versa? How can a thought to move my arm, cause my arm to move? If mind is not matter, then what is it? Some speak of souls, others of energy, consciousness and emergence. But what do they all mean?
These questions, this inability to grasp mind and consciousness, has driven philosophers to three possible conclusions.
Yes, there is, claim some. Explaining the human experience in purely physical terms leaves something out – that which makes us uniquely human. For some this may be art, or morality, for others religion. Think of it this way. Science may reduce music to a sequential succession of audio waves in various degrees of length, pitch and volume, that trigger the human auditory system, which in turn causes neural pathways to fire and interact with other neurons and thereby create new or reinforce existing neural pathways and stimulate the exchange of dopamine, serotonin and whatnot, which then causes the heartbeat to increase, the pupils to enlarge and the mouth to salivate. But science has hereby not fully explained what humans experience when they hear music. Even though robots may be able to write and hear music, can their experience of writing and hearing music ever be the same as the rich, self-conscious, intellectually challenging and satisfying, emotionally evocative human music experience? One can easily apply this same question to seeing colours, tasting food and of course, love. Can these human experiences be scientifically reduced or programmed in physical, causal phenomena?
No there isn’t, others claim. Thinking that there is an experience separate from physical processes, is an illusion, a flawed attempt at explaining something we don’t (yet) understand. The history of human knowledge is defined by revealing these illusions, and replacing them by scientific explanations. Thunder is not created by a thunder god, but rather by a meteorological process – a process we now understand and can explain in the framework of the laws of nature. Mental illness is not a punishment from God, but an imbalance in neural mechanics. Falling in love is not a hit by Cupido’s arrow, but a well-orchestrated event in the chain of the species’ evolution and continuation. We should accept that our old explanations of the world were wrong, and that they have no place in our growing understanding of the physical world, including the physical workings of the human mind. Human behaviour is not a result of elusive, mental unicorns, but rather a physical reaction in a chain of physical events.