1. Problem 1: Explaining qualia ↑
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, British mathematician Bertrand Russell and British philosopher Michael Lockwood both argued that qualia represent a conscious awareness, not of external objects, but of parts of our own mind[3a].
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. Irish philosopher George Berkeley argued that causal realism will inevitably lead to scepticism about the external world, 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 is no reason to believe we cannot 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"[3b].
Lockwood claimed that external objects must exist because we are not 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, French philosopher Jean-Paul Sartre had already shown that the qualia we experience when we imagine are fundamentally different to the qualia invoked by external objects.
Sartre claimed that when we imagine something, it is 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.
2. Problem 2: Where is my mind? ↑
If material theories of the mind are able to 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"[3c].
Identity theory states that qualia exist within the brain, in an analogous way to how lightening 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 cannot determine the spatial location of mental events independently of the assumption that they must exist in the brain.
Lockwood claimed that this problem can easily be resolved if we accept that the mind exists in time. This is because German-Swiss-American physicist Albert Einstein's theory of special relativity shows that time and space are inseparable. This approach would not have been accepted by philosophers like German philosopher Immanuel Kant who argued that time is also a property of the mind.
The argument that there's no physical configuration to images was disputed in experiments conducted by cognitive scientists Roger Shepard and Jacqueline Metzler in 1971.
Shepard and Metzler gave people two pictures of the same three-dimensional object from different angles and timed how long it took for 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.
3. 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 American neurobiologists Roger Wolcott Sperry and Michael Gazzaniga in the 1960s and 1970s[10a].
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, American neurosurgeon William van Wagenen developed a form of surgery that splits the two hemispheres by severing this link.
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.
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.
Lockwood stated that consciousness exists across both hemispheres as "simultaneous overlapping phenomenal perspectives"[3d]. 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 field. When asked what they can see with the right eye, split-brain patients always know 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 for 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, and 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"[10b].
Experiments on split-brain patients show that the two hemispheres almost always try to co-operate, 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, such the woman whose left hand tried to strangle her. In these cases however, there was far more intensive brain damage than just a split between hemispheres, and Lockwood suggested that the part of the brain that initiates co-operation must also have been damaged.
If part of the brain exists to establish co-operation between two split hemispheres, then it would make sense to assume that it has a use even when the corpus callosum is intact.
In 1973, philosopher Roland Puccetti suggested that there's a single mind present in both hemispheres, even before the split. 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 co-operation"[3e]. Perhaps Puccetti's claim could be proven if we had an example of someone whose 'co-operation system' has been destroyed but who retains an intact corpus callosum, if Puccetti is correct then we might expect to see signs of a split consciousness.
In 1971, fellow American philosopher Thomas Nagel argued that there is no definite answer to how many minds we have. 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 British philosopher Derek Parfit's term 'co-conscious' to describe properties that are experienced by both hemispheres at the same time. Information that is 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 claim by arguing that there is 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 is clearly ambiguous as to 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 cut-off 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.
Non-distinctive boundaries also appear when we are focusing on one sensory experience, or spatial region, and are far less aware of what is happening in the background.
4. Problem 4: Subjectivity ↑
In 1974, Nagel showed that we cannot image what it is like to be any creature that possesses a different type of brain to us. Nagel claimed that:
"if I try to imagine this, I am restricted to the resources of my own mind, and those resources are inadequate to the task".
Nagel concluded that there are some things we can never truly comprehend, despite knowing that they are perfectly comprehendible in theory.
In 1989, British philosopher Colin McGinn suggested that the human mind is incapable of comprehending itself entirely. He stated that:
"just as a dog cannot be expected to solve the problems about space and time and the speed of light that it took a brain like Einstein's to solve, so maybe the human species cannot be expected to understand how the universe contains a mind and matter in combination".
It's certainly true that we are limited by the capacity of our brains but we may not yet have to give up on explaining the subjective nature of consciousness. Quantum mechanics may require that our experiences are subjective[3f].