28.1 Materialism
Material theories of the mind became popular in the 19th century when science began to describe nature as if it were a complex machine. In 1796, the French mathematician Pierre-Simon Laplace’s nebular hypothesis explained how the Solar System formed without the need for a creator (discussed in Book I). The atomic theory of matter was developed by John Dalton in 1808 (discussed in Chapter 6), and the British naturalist Charles Darwin’s theory of evolution by natural selection was published in 1859.
In the 1870s-1890s, the Italian neuroscientist Camillo Golgi and the Spanish neuroscientist Santiago Ramón y Cajal developed tissue-staining techniques that allowed them to map the path of nerve cells - known as neurons - in the brain.
Neurons are cells that can gather and transmit information along a neural pathway. The discovery of neurons led to the development of the neuron doctrine, first proposed by the German anatomist Heinrich Wilhelm Gottfried von Waldeyer-Hartz in 1891. The neuron doctrine states that neurons are the fundamental units of the nervous system and are responsible for all brain activity.
This lead to the idea that the mind and the brain are the same. The idea that particular states of the mind are identical to particular states of the brain is known as identity theory. Identity theory was first suggested by the British psychologist Ullin Place in 1956 and it was extended by the Australian philosopher Jack Smart three years later.
In 1965, the American philosopher Jerry Fodor suggested that the mind can be explained in functional terms. This means anything that performs the function of a mind is a mind, and so must be conscious.
Figure 28.1 |
Drawing of the cells found in the brains of chickens by Santiago Ramón y Cajal, c. 1905. |
Figure 28.2 |
Neurons in the brain of a macaque. |
28.1 Problem 1: Explaining qualia
Material theories of the mind must be able to solve the problems raised by Rene Descartes in 1641[1] (discussed in Chapter 26). Descartes showed that the existence of the external world can be doubted because we are only aware of qualia, and indistinguishable qualia can exist even when there’s no external object present.
In order to solve this problem, Bertrand Russell and the British philosopher Michael Lockwood both argued that qualia represent a conscious awareness, not of external objects, but of parts of our own mind.[2,3] Lockwood suggested that we can prove external objects exist if we accept the disclosure view, coupled with causal realism.
Causal realism is the view that we can at least derive the existence of external objects from the phenomenal qualities they invoke. George Berkeley argued that causal realism will inevitably lead to scepticism about the external world[4] (discussed in Chapter 27), and Lockwood uses the disclosure view to argue against this.
The disclosure view is a kind of naive realism. This is an extreme version of common sense realism, which holds that there’s a transparency between the external world and the phenomenal qualities we experience in consciousness. Lockwood agreed that this is false with respect to the external world, but followed Russell in arguing that there’s no reason to believe that we can’t have a transparent grasp of the conscious states of our own mind. In Mind, Brain & the Quantum: The Compound “I”, first published in 1989, Lockwood described the disclosure view as being analogous to,
a searchlight, sweeping around an inner landscape in part revealing qualities that were already part of the landscape.
This cannot be the complete story, however, because awareness itself is “realised as a neural activity of some kind”. This means that there is,
no reason to suppose that the intrinsic character of the brain state is, in general, unaffected by our becoming aware of it.[3]
Lockwood claimed that external objects must exist because we aren’t always conscious of what we perceive. If, for example, we focus on an object and then let our mind wander, qualia will drift in and out of our visual field without at any time ceasing to exist.
By 1940, the French philosopher Jean-Paul Sartre had already shown that the qualia we experience when we imagine are fundamentally different from the qualia invoked by external objects.[5] Sartre claimed that when we imagine something, it’s not the object that changes but our perception of it. When you look at an object, you perceive it, and when you imagine it, you form another type of consciousness. All consciousness is consciousness of something and, in the case of the imagination, an object that is absent presents itself.
28.3 Problem 2: Where is my mind?
If material theories of the mind can show that both qualia and external objects exist, then they will still have to show where qualia exist within the mind. Lockwood stated that,
it is not any neuronal firings per se that are registered in consciousness as phenomenal qualia, but something else that has an intimate causal connection with certain such firings.[3]
Identity theory states that qualia exist within the brain, in an analogous way to how lightning exists in the same space as electrical discharge. This analogy breaks down, however, because, as Austrian-British philosopher Ludwig Wittgenstein pointed out in 1958, we can’t determine the spatial location of mental events independently of the assumption that they must exist in the brain.[6]
Lockwood claimed that this problem can easily be resolved if we accept that the mind exists in time. This is because Albert Einstein’s theory of special relativity (discussed in Book I) shows that time and space are inseparable. This approach would not have been accepted by philosophers like Immanuel Kant, who argued that time is also a property of the mind.[7]
The argument that there’s no physical configuration to images was disputed in experiments conducted by the cognitive scientists Roger Shepard and Jacqueline Metzler in 1971.[8] Shepard and Metzler gave people two pictures of the same three-dimensional object from different angles and timed how long it took each person to mentally rotate the first image to that of the second. The time was found to be in direct proportion to the difference in angle between the pictures. This means that objects that would have taken longer to physically rotate took longer to mentally rotate.
28.4 Problem 3: The unity of consciousness
Descartes also claimed that the mind cannot exist as a physical substance because it is a unified whole, unlike external objects, which can always be divided into parts. This argument was disputed in experiments conducted by the American neurobiologists Roger Wolcott Sperry and Michael Gazzaniga in the 1960s and 1970s.[9,10]
Sperry and Gazzaniga knew that the brain is composed of two hemispheres, each containing two lobes. These are connected by the corpus callosum, which contains more than 200 million nerve fibres. In 1940, the American neurosurgeon William van Wagenen developed a form of surgery that splits the two hemispheres by severing this link.[11]
This procedure is known as a corpus callosotomy and in extreme cases, it can be performed to reduce epilepsy. People who have undergone a corpus callosotomy are sometimes referred to as split-brain patients. The fact that the brain can be divided led to the question of whether this was followed by a division in consciousness.
Figure 28.3 |
A diagram of a brain, showing the corpus callosum (red). |
The Australian neurophysiologist John Eccles suggested that consciousness is only contained within the left hemisphere of the brain, but this was refuted by evidence of consciousness within people whose left hemisphere had been completely destroyed.[12]
Lockwood stated that consciousness exists across both hemispheres as “simultaneous overlapping phenomenal perspectives”.[3] If this were true, then we would expect that after a corpus callosotomy a patient’s consciousness would either move to one hemisphere or continue in two separate streams that can no longer communicate.
Sperry and Gazzaniga tried to determine which view was correct by providing the two hemispheres of split-brain patients with separate information for the left and right visual fields. When asked what they can see with the right eye, split-brain patients always state the correct answer, but they will always deny seeing anything with their left eye.
The right eye sends information to the left side of the brain, and so it would appear that Eccles’ view is correct, consciousness only appears in the left hemisphere. This is an incorrect assumption, however, because the speech centre in the brain is also associated with the left hemisphere. If the right hemisphere did register the information, it would have no way to verbally communicate it.
The right hemisphere is associated with pictorial information. It allows us to recognise patterns and faces, transforms our vision into three dimensions, and is involved in musical recognition and ability. When split-brain patients were asked to draw what they saw, the right hemisphere could express the information.
Sperry concluded that,
both the left and the right hemisphere may be conscious simultaneously in different, even in mutually conflicting, mental experiences that run along in parallel.[10]
Experiments on split-brain patients show that the two hemispheres almost always try to cooperate, and it’s impossible to get them to compete, even in simple games. There are a few examples of split-brain patients whose separate hemispheres have expressed contradictory behaviour,[13] such the woman whose left hand tried to strangle her.[14] In these cases, however, there was far more intensive brain damage than just a split between hemispheres.
In 1973, the philosopher Roland Puccetti suggested that there’s a single mind present in both hemispheres even before the split.[15] This is illustrated with examples of divided attention, such as how people can talk and drive at the same time, despite being too distracted to be consciously aware of what their hands and feet are doing. Lockwood described how the two minds are integrated in most tasks because they have “a lifetime’s practice in mutual cooperation”.[3]
In 1971, the American philosopher Thomas Nagel argued that there’s no definite answer to how many minds we have.[16] In the case of split-brain patients, the hemispheres cannot communicate but they are not completely distinct either because there are properties that are experienced by both. Emotional phenomena, for example, are mainly associated with the diencephalons. This is located below the hemispheres, and so emotions are experienced equally by both minds.
Lockwood adopted the British philosopher Derek Parfit’s term ‘co-conscious’[17] to describe properties that are experienced by both hemispheres at the same time. Information that’s not co-conscious cannot be communicated between the separate hemispheres of split-brain patients.
It could be objected that there’s still room for Descartes’ sense of absolutes since a property is either fully co-conscious or it is not co-conscious at all, but Lockwood dismissed this by arguing that there’s no distinct point when a property stops being co-conscious. This is because the corpus callosum could be destroyed one nerve fibre at a time, and it’s not clear when certain aspects of the brain would cease to become fully co-conscious during this procedure.
Nagel suggested that there are other non-definite levels of consciousness. The boundary of our visual field, for example, does not appear to have a distinctive edge at the cutoff point of conscious awareness. If this were the case, then we might expect to view the world as if we were looking through a pair of binoculars.
28.5 Problem 4: Subjectivity
The final problem any material theory of the mind must overcome is caused by the fact that science is inherently objective, and so cannot explain the subjective nature of consciousness. This may be an impossible challenge. In the 17th century, Thomas Hobbes claimed that there are some things our mind is just not capable of comprehending[18] (discussed in Chapter 26).
In 1974, Nagel showed that we cannot imagine what it is like to be any creature that possesses a different type of brain to us. Nagel claimed,
if I try to imagine this, I am restricted to the resources of my own mind, and those resources are inadequate to the task.[19]
Nagel concluded that there are some things we can never truly comprehend, despite knowing that they are perfectly comprehensible in theory.
In 1989, the British philosopher Colin McGinn suggested that the human mind is incapable of comprehending itself entirely and that perhaps we cannot be expected to understand human consciousness any more than a dog can be expected to understand special relativity.[20]
However, we may not have to give up on explaining the subjective nature of consciousness. Quantum mechanics may require that our experiences are subjective[3] (discussed in 31).