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What Did Mary Know?

Marina Gerner on a thought experiment about consciousness.

Imagine a girl called Mary. She is a brilliant neuroscientist and a world expert on colour vision. But because she grew up entirely in a black and white room, she has never actually seen any colours. Many black and white books and TV programmes have taught her all there is to know about colour vision. Mary knows facts like the structure of our eyes and the exact wavelengths of light that stimulate our retinas when we look at a light blue sky.

One day, Mary escapes her monochrome room, and as she walks through the grey city streets, she sees a red apple for the first time.


What changes upon Mary’s encounter with the red apple? Has Mary learnt anything new about the colour red upon seeing the colour for the first time? Since Mary already knew everything about the physics and biology of colour perception, she must surely have known all there is to know about the colour red beforehand. Or is it possible that some facts escape physical explanations? (‘Physical’ in this sense refers to all the realms of physical science, including chemistry, biology, neuroscience, etc.). If Mary has learnt something new, then we can conclude that scientific explanations cannot capture all there is to know, argues Professor Frank Jackson, who thought up this scenario in ‘Epiphenomenal Qualia’, in The Philosophical Quarterly (1982). The story of Mary is known as the ‘knowledge argument’ and it has become one of the most prominent thought experiments in the philosophy of mind.

You might say, “Hang on a minute, how was it possible that Mary grew up in a black and white room in the first place?” Never mind the first place. Some philosophers have put forth that she wore special goggles. But this issue need not concern us, because philosophical thought experiments depend on logical coherence rather than practical feasibility. Philosophers devise such narratives to think through an imagined situation, so as to learn something about the way we understand things. Thought experiments require no Bunsen burners or test tubes; they are laboratories of the mind. In thought experiments, time travel is logically possible, but no philosophy professor is expected to travel back in time to prove their point.

Reinvigorating The Debate

The reason Professor Jackson devised the thought experiment involving Mary was to challenge the physicalist school of thought. In philosophy of mind debates, proponents of physicalism argue that what really matters is physical matter. For them, consciousness is all about the brain; or more specifically, it is identical to the brain. Physicalists have formulated ‘identity theories’ that equate human consciousness with the human brain. Long before contemporary debates on the mind-body issue, physicalists were directly opposed by dualist philosophers. Nowadays there are not many straightforward dualists left, with the notable exception of David Chalmers – kudos to him. Descartes would have been proud of him, because the dualist position goes right back to Descartes’ idea that the body is a different entity from the soul (mind). Descartes argued that they are two different substances. In-between the physicalists and the dualists you will find proponents of supervenience theories. They argue that consciousness emerges from (a.k.a. fancy word ‘supervenes’) brain activity but is not reducible to it. For the sake of this article I am going to focus on the opposing perspectives of physicalists and anti-physicalists, rather than on the more nuanced supervenience theories. (If you are interested in supervenience, look up Sydney Shoemaker’s work.)

Jackson’s thought-experiment about Mary challenges physicalism in the following way. Physicalists claim that physical science can fully explain consciousness. However, when Mary sees the red of the apple, says Jackson, she learns something new, despite having previously learnt all the physical facts about colour vision. Thus, argues Jackson, Mary has come to know a non-physical fact; so proving that not all knowledge is physical.

Mary as bat
What is it like to imagine yourself to be a bat?

Jackson was not the first to challenge physicalism over the question of consciousness. Anyone who engages with the mind-body problem [ie, How do the mind and the brain relate?] will discover that consciousness is almost always the pea on which the philosopher princess rests. Consciousness is what keeps philosophers awake at night. In his famous article ‘What Is It Like To Be A Bat?’ (Philosophical Review, 4, 1974) the philosopher Thomas Nagel wrote: “Without consciousness the mind-body problem would be much less interesting. With consciousness it seems hopeless” – a great admission by one of the key figures in the philosophy of mind.

Nagel argued that it is impossible for human beings to know what it would be like to be a bat. Certainly, bats have an idea of what it feels like to be a bat on a daily basis. However, because we have no sense equivalent to bats’ sonar, we cannot begin to imagine what it is like to use that sense. Yes, we can do our best to imagine ourselves hanging upside down in a pitch-black cave, but even then we can only imagine this from a human perspective, rather than from the perspective of a bat. With this argument, Nagel was one of the first to reinvigorate the debate in the second half of the twentieth century on how consciousness may be explained, or rather, whether consciousness can be explained at all.

Are Colours Physical?

When Nagel wrote that we don’t have any idea of how to share the mental state of a bat, he was challenging the physicalists’ perspective that everything can be captured from a scientific, objective point of view. Nagel argued that some facts can instead only be captured from subjective points of view. Even if we objectively know how the bat’s sonar system works and enables the bat to avoid collisions, other questions remain unanswered such as ‘What is it like to perceive the walls of a cave using a bat’s sonar system?’ This argument about the ‘what it is like’ subjective aspect of being a bat is different from Jackson’s argument, because Jackson says we cannot explain our own ‘what it is like’ sensations, let alone those of another person, or a bat. While Nagel argues that the trouble with bats is that they are too unlike us, Jackson thinks that this is hardly an objection to physicalism, because physicalism makes no special claims about the extrapolative powers of human beings. Never mind that we can’t understand or explain what it’s like to be a bat – we don’t possess that kind of knowledge about ourselves either. According to Jackson, we can’t explain our own qualitative sensations: what it is like to be happy and sad, or what it is like to hear a saxophonist play at night. (All these sensory aspects of experience will from now on be called ‘phenomenal states’.)

In 1974 Nagel thought about bats and so made his case arguing that physical knowledge does not explain phenomenal states. In 1982, almost a decade later, Joseph Levine coined the expression ‘the explanatory gap’ to express the problem faced by any attempt to explain consciousness in physical terms. Without wishing to reject physicalism altogether, Levine put forward the idea that there is a gap in our ability to explain the connection between phenomenal states and the properties of our brains. We have no satisfactory understanding of why brain processes produce the taste of chocolate, or the visual sensation of seeing blue. If you hear a philosopher wonder “How can colour perception arise from the soggy grey matter of our brains?” you will recognize that they’re acknowledging the ‘explanatory gap’. Generally, we lack any explanation of how a phenomenal state is identical with a physical state of the brain.

According to Levine, the difficulty of explaining consciousness is unique. He says that we do not struggle to explain why water is H2O, or why heat is molecular kinetic energy, in the same way that we struggle to explain why brain states are phenomenal states.

Another decade later, in 1995, Chalmers proposed a distinction between the ‘easy’ and the ‘hard’ problems of consciousness in his book The Conscious Mind. Mental properties divide into phenomenal ones and psychological ones, argued Chalmers, and the latter are far easier to explain than the former because they don’t involve any deep metaphysical enigmas. Easier aspects to explain include our ability to discriminate between different things, or to categorize them, or to remember them. These easier aspects can be explained through physical accounts. According to Chalmers, the easy problem of consciousness is accounting for cognitive ‘abilities and functions’, and in order to explain them, one only needs to specify the mechanism that can perform the function. Meanwhile, the hard problem of consciousness is to explain how it is that we experience phenomenal states at all: to explain why there is a certain feel to the phenomenal state of pain, for example. So even though we might be able to explain that the body needs pain for a particular function (i.e. as a warning system) we cannot explain how brain processing gives rise to a rich inner life.

The Physicalists Respond

Nagel’s arguments kicked off the ‘what it is like’ subjectivity debate; Jackson’s story about Mary has challenged physicalism in relation to sensations; Chalmer’s ‘hard and easy problem’ and Levine’s ‘explanatory gap’ arguments try to highlight what it is that we cannot explain. Now let us look at the ways in which physicalists have responded to the challenges that have been mounted against them. Let’s consider some of the reactions Mary has received, and then proceed to a recent response to the knowledge argument called the ‘phenomenal concept strategy’ (which is not as brain-racking as it sounds.)

To begin with, some philosophers have simply doubted that Jackson’s argument is coherent. Mary doesn’t learn anything new, just because she would in fact know that the apple is red once she saw it, argues C.L. Hardin. Based on her complete physical knowledge of colour vision, Mary would see the red apple and joyfully exclaim, “Oh, so this is red!” If we were to show Mary a blue banana instead, she would not be fooled; she would know that it had the wrong colour, argues Daniel Dennett, in what he coined the ‘blue banana trick’. Hats off to Dennett for coming up with a fun name for his argument, but both Hardin’s and Dennett’s arguments can be countered by saying that the experience of seeing red goes beyond the ability to recognize red.

Another philosopher, Owen Flanagan, plunges straight into human biology and argues that the phenomenal state of seeing red is a physical event, and that an individual seeing red undergoes ‘red-channel activation’. In order to be able to undergo red-channel activation, a causal interchange between the person and an external red object is needed. In other words, the individual needs to be hooked up with a red thing so that experience of seeing red can be triggered. So phenomenal states of seeing red are physical events, because they require red-channel activation. According to Flanagan, what Mary goes through is a physical event. Two other arguments – ‘the experience argument’ and ‘the ability argument’ – also build on the idea of a first-person perspective being a physical event.

The experience argument probably echoes the first doubt that comes to your mind in relation to Jackson’s thought experiment, dear reader. Namely, the question whether one can know all there is to know by taking lessons and reading books. David Lewis argues that you can’t learn certain things by being told about the experience, however thorough your lessons may be. Some facts you can only learn by experience, although they may nevertheless be physical facts.

The experience argument can be extended into the ability argument. Both Lewis and Laurence Nemirow claim that when Mary sees red after her escape, her sensation of what it is like just means her acquiring certain practical abilities. According to Nemirow, knowing what an experience is like is the same as knowing how to imagine having that experience. According to Lewis, knowing what an experience is like is possessing the ability to recognize it, the ability to imagine it, and the ability to predict one’s behaviour. Lewis eloquently explains that knowing what it is like is not knowing that – it is knowing how.

Flanagan, meanwhile, distinguishes between ‘linguistic physics’ and ‘complete physics’ to say that there is no reason to think that just because Mary is a leading expert on colour vision she can express phenomenal states in the vocabulary of physics. Mary being an expert on colour vision does not entail that she or anybody else could express the phenomenal state of seeing red in physicalist terms, or even more basically, in words, but phenomenal states are nevertheless physical states. However, Jackson responds that physical knowledge must be complete knowledge, which means that it not only encapsulate the physical world, but that it should also be able to express and explain all the facts about the physical world.

To recap, Jackson’s purpose in conceiving the knowledge argument was to show that there are non-physical facts or properties. His argument is that although Mary knows all the physical facts about seeing red, she still learns a new fact when leaving her room and seeing something red. Jackson therefore concludes that there are facts that evade the physicalist theory. According to Jackson, if physicalism was true, Mary must have had complete knowledge about seeing red even in her monochrome room. But she did not have complete knowledge, he argues, because physicalism cannot explain phenomenal states as it explains physical facts.

The Phenomenal Concept Strategy

The most common responses philosophers of the physicalist persuasion give to the knowledge argument are based on the ‘new knowledge, old fact argument’. For instance, contemporary physicalists such as David Papineau have formulated something called the ‘phenomenal concept strategy’ to counter anti-physicalist arguments. Papineau denies Jackson’s distinction between physical and phenomenal facts or properties. Instead, he asserts that the knowledge argument itself provides “an excellent way of establishing the existence of distinctive phenomenal concepts” (Thinking About Consciousness, 2002). Papineau claims that the difference between phenomenal and material concepts is a difference at the level of sense, not reference. In other words, phenomenal and material concepts refer to the same thing through different means. So proponents of the phenomenal concept strategy argue that while our intuition for some sort of dualism is correct, this intuition is due not to the nature of phenomenal states, but rather to the concepts we use to refer to them, in contrast to the concepts we use to refer to our brain states.

What exactly is a ‘phenomenal concept’? Well, it is somewhat akin to a picture. Think of mental representations of the sort that can occur in thought. We do not have to think of concepts in linguistic terms – which means concepts do not have to be expressible in scientific language.

Papineau argues that we hold phenomenal concepts about phenomenal states we have experienced. For instance, we hold a phenomenal concept of the taste of chocolate or of the sound of a drum. When Mary sees a red apple, argues Papineau, it activates the relevant neural region in her brain. This activation can be compared to a reusable stamp. Following her original experience of red, Mary’s brain has acquired an ‘original’ stamp from which to make future ‘moulds’, and after its original activation this stamp can only be re-activated by the relevant experience. So Mary can consequently imagine and introspectively classify experiences of red.

Papineau holds that while humans require original external experiences of phenomena, one could conceive of creatures born with introspective imaginative abilities, who do not need any specific experiences to set up those stamps. In those creatures the moulds necessary for seeing colours, and the dispositions to use them, would be hard-wired. A creature like this would be able to imagine seeing something red without ever having seen something red. However, humans are not like this. Papineau gives the example that although we might be capable of imagining seeing a red circle even though we have never actually seen one before, purely by combining our previous experience of seeing red with our previous experience of seeing a circle, we cannot imagine the colour red without having first actually seen something red.

According to physicalists, phenomenal concepts have inimitable features that make them distinct from all other concepts. What makes them special is the uniquely direct relation they seem to provide for a person to their own mental state. When we see red, we seem to be acquainted with the sensation through the concept. For this reason, some philosophers describe phenomenal concepts as ‘recognitional’, ‘demonstrative’ or ‘quotational’ concepts.

Phenomenal concepts offer this special intimacy (why not drop the word ‘intimacy’ into a highly abstract discussion on knowledge?) because they refer to their referent directly. More precisely, phenomenal concepts are made out of instances of the phenomenal states to which they refer. So the phenomenal concept of seeing red is a vision of red in our imagination. This means that phenomenal states are actually deployed while we conceive phenomenal concepts. Papineau illustrates this feature particular to phenomenal concepts as opposed to physical concepts with the example of an ache: we can think of an ache in material terms by envisioning brain activities, facial grimaces, or the flinching of body parts; or we can think of the ache in terms of what it would feel like for us to be in a state of pain – what it would feel like for us to experience that ache. The first concept is purely functional or physical, whereas the second concept refers to the pain phenomenally. It offers, to use Hume’s words, a ‘faint copy’ of the pain. Both concepts refer to the same ache; the difference lies in the way the state is conceptualized.

This is a key argument physicalists use to say that actually, consciousness is something physical after all. Again, they say that our intuition for dualism is correct, but the dualism is actually only at the conceptual level, not on a metaphysical level. But there are potential problems with this idea. Would a phenomenal concept necessarily be a faint copy of a pain for, say, a doctor who deals with pain every day? And can such concepts really capture all there is to a phenomenal experience? If we could explain the sensation of seeing a red apple in scientific terms, maybe we could also explain the feeling of love with reference to the concept of a heat wave, as one proponent of physicalism has suggested. But I doubt that.

What We (Don’t) Know

Consciousness is likely to always be somewhat mysterious. This may be an unduly pessimistic view of our capacity to articulate a truly comprehensive picture of the world and our place in it, admits Jackson:

“But suppose we discovered living on the bottom of the deepest oceans a sort of sea slug which manifested intelligence. Perhaps survival in the conditions required rational powers. Despite their intelligence, these sea slugs have only a very restricted conception of the world by comparison with ours, the explanation for this being the nature of their immediate environment. Nevertheless they have developed sciences which work surprisingly well in these restricted terms. They also have philosophers, called slugists. Some call themselves tough-minded slugists, others confess to being soft-minded slugists. The tough-minded slugists hold that the restricted terms (or ones pretty like them which may be introduced as their sciences progress) suffice in principle to describe everything without remainder. These tough-minded slugists admit in moments of weakness to a feeling that their theory leaves something out. They resist this feeling and their opponents, the soft-minded slugists, by pointing out – absolutely correctly – that no slugist has ever succeeded in spelling out how this mysterious residue fits into the highly successful view that their sciences have and are developing of how their world works. Our sea slugs don’t exist, but they might. And there might also exist super beings which stand to us as we stand to the sea slugs. We cannot adopt the perspective of these super beings, because we are not them, but the possibility of such a perspective is, I think, an antidote to excessive optimism [concerning scientific explanations].”
From ‘Epiphenomenal Qualia’ in The Philosophical Quarterly (1982)

© Marina Gerner 2013

Marina Gerner is a PhD student at the London School of Economics, as well as a culture journalist and feature writer.

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