Explaining consciousness is one of the hardest problems in science. This should be surprising because we all know a lot about consciousness from our own experience. We could say that our own consciousness is the thing in the world that we know best. The philosopher Descartes thought that our knowledge of our own conscious thought was more secure than the rest of natural science. He thought that consciousness was unique, in that we know with absolute certainty that our own consciousness exists and we're the ultimate authority on it's character. Yet despite the seemingly excellent knowledge of our own consciousness, consciousness is one of the most puzzling things in the world. We don't know what it is about us as physical beings that makes us conscious. We don't know why we have consciousness and we don't know which creatures other than ourselves are conscious. Consciousness isn't just a scientific term, it's also a word that appears in everyday life. We might say, she wasn't conscious of the passing pedestrian, that he was knocked unconscious in the boxing ring. Or that our conscious experience of smelling a rose or hearing a symphony make life worth living. Consciousness is what philosophers call a folk concept. A concept that arises out of and is ingrained into our everyday interests. A scientific understanding of consciousness should approach our folk talk about consciousness with a great deal of care. What sort of things might folk mean when they talk about consciousness in their everyday life? One thing we might mean by consciousness is simply sentience. When we say that a creature's conscious all that we mean is that it acts in an intelligent way and it's responsive to its environment. We might for example say that the spider under the fridge is conscious of our presence. All we mean is that the spider's aware that we're here and has sensibly taken evasive action. A second and distinct meaning of consciousness is wakefulness. When we say someone is conscious, what we mean is that they are awake. They're not asleep or otherwise incapacitated. This sense of consciousness suggests that consciousness is a global state, a kind of switch that effects the whole of the mental life of the organism. A third thing we might mean by consciousness, is what the philosopher Ned Block has called 'access consciousness'. A thought is access conscious if it's broadcast widely in a creature's brain. And is poised to interact with a wide variety of the creature's other thoughts and to directly drive its behavior. Access conscious thoughts are usually the ones you can report if someone were to ask you, what are you thinking now? Remarkably, not all of our thoughts are access conscious. It's one of the most surprising and well confirmed findings of 20th Century psychology that the majority of our mental life is not access conscious. Our access conscious thoughts are only the tip of the iceberg in our mental life. A fourth thing we might mean by consciousness is phenomenal consciousness or qualia. To understand this, imagine taking a God's eye view of your mental life. At any given moment, there's a lot going on in your head. You have beliefs that Paris the capital of France. Desires to eat lunch soon. You make plans to go to the cinema tonight. And those plans result in motor action: turning the handle of your front door. You perceive that there's a computer in front of you, and you make perceptual discriminations between the screen and the keyboard of the computer. But in addition to all this activity, there's something else going on. Your mental life isn't just information processing, discriminations, forming plans, making judgements. It's also accompanied by subjective feelings. Imagine that someone were to place a piece of dark chocolate on your tongue. Now imagine instead that, that person were to put a breath mint on your tongue. You could of course tell the difference between these two environmental stimuli, you could discriminate between them. But we could build a machine to do that too. There's more going on in your case. It's not just that you can tell the difference between the two stimuli, it's that these two stimuli elicit different conscience feelings in you. It feels a certain way when you taste chocolate, and it feels a certain way when you taste a mint, and those two feelings are different. These conscious feelings, that accompany many episodes in our mental life, are what's meant by 'phenomenal consciousness' or 'qualia'. Things we might mean by consciousness include sentience, wakefulness, access consciousness, and phenomenal consciousness. For any of these different forms of consciousness, one might ask the questions with which we started. What is it about us as physical beings that make us conscious, why are we conscious and which creatures other than ourselves are conscious? Some of these questions turn out to be easier than others to answer. For example, we're making good progress at explaining what it is about us, as physical beings, that make us either awake or asleep. One set of questions, those concerning phenomenal consciousness, have turned out to be incredibly difficult to answer. These questions center around what's been called the hard problem of consciousness. The hard problem of consciousness is to explain how creatures such as ourselves have phenomenal consciousness. What is it about us as physical beings that produces phenomenal consciousness? Think of yourself from two different angles. From the subjective point of view, your own introspective take on your own mental life. You know that you've got phenomenal consciousness. You know that you've got conscious feelings, and that those feelings come in many different kinds. Philosophers use the expression 'what it's like' as a way of referring to conscious experiences that are occur when we do a particular activity. We might say, what it's like to stub one's toe. Or what it's like to eat a raw chili. In each case, what we mean is the conscious feeling that usually comes on when we do that activity. Reflecting on our conscious experience using what-it's-like talk, reveals that we already know a great deal about phenomenal consciousness. We know for example, that what it's like to taste chocolate is different from what it's like to taste mint. We know that what it's like to taste a clementine, is similar to what it's like to taste an orange. Phenomenal consciousness has a definite structure. And our conscious feelings bear relations of similarity and difference to each other. Reflecting on our conscious life from a subjective point of view, via introspection, is called 'phenomenology'. Now imagine viewing yourself as an object from the outside. From this point of view, it seems surprising that you have phenomenal consciousness at all. If we didn't know it already from our own experience, we would never have guessed it. Think about your brain as a physical object. Your brain is made up of over 100 billion neurons wired in a complex web. We know that your brain stores information, discriminates between stimuli, and controls your behavior, but we have no idea how your brain produces conscious feelings. We know that we have phenomenal consciousness, and that our phenomenal consciousness has a rich structure, but we have no idea how brain activity produces phenomenal consciousness. This is the hard problem of consciousness; explaining how brain activity produces conscious feelings. Why is the hard problem of consciousness so hard? One difficulty is that there's a gap between our two perspectives on consciousness. Phenomenology and brain science. Both seem to be legitimate sources of knowledge about our mental life. The difficulty is linking them together. Science has an impressive track record at unifying our knowledge. A common pattern in science is the unify by reductive explanation, by explaining high level phenomena in terms of low level phenomena. For example, in the kinetic theory of gases, high level phenomena involving the pressure and temperature of the gas, are explained in terms of low level laws and mechanisms involving the constituent molecules of the gas. But both pressure, temperature and the movement of the constituent molecules are all known from the third person point of view. The puzzle with phenomenal consciousness is to explain how our first person conscious feelings arise out of third person accessible brain activity. There's no precedent in science for this kind of reductive explanation. in fact, a number of philosophers, including Frank Jackson, David Chalmers and Thomas Nagel, have argued that science will never reductively explain phenomenal consciousness in terms of brain activity. We'll never solve the hard problem of consciousness. Let's look at Frank Jackson's argument for this claim. Frank Jackson's argument is based around a thought experiment. Imagine that a brilliant neuroscientist, Mary is born and grows up inside a black and white room. Mary's never seen colour but she's fascinated by how the human brain detects and processes colour. Inside her room, Mary's provided with encyclopaedias that describe the workings of the human brain. These encyclopaedias don't just contain current knowledge in neuroscience. They include every fact that there is to know about how the human brain works. From her encyclopaedias, Mary learns in exquisite detail how the human brain detects and processes colour information. One day, Mary's released from the room. When she goes outside, she spots a red rose and she experiences seeing colour for the first time. Jackson claims that, at this moment, Mary learns something new about human vision. Previously, Mary knew about how the human brain processes colour. Now Mary also learns about the subjective feelings that accompany seeing colour. The distinctive conscience feeling of seeing red. This conscience feeling is not something that Mary could have predicted based on her books alone. Mary needed to go outside the room and experience seeing colour herself to learn about this fact, about the human visual system. Solving the hard problem of consciousness requires showing how every aspect of phenomenal consciousness is determined by brain activity. Mary's predicament appears to show that we'll never achieve this. Mary knows all there is to know about how the human brain works, yet the facts about phenomenal consciousness still elude her. This means that even if we were lucky enough, like Mary, to have a completed neuroscience, we will would still be stuck with the hard problem of consciousness. We would still not know how conscious feelings arise from brain activity. Mary doesn't know this, so future neuroscientists wouldn't know it either. No matter how much neuroscience progresses, we would still be stuck with the hard problem of consciousness. Mary's predicament appears to show that we're stuck with our two different perspectives on conscious experience. The project of reducing phenomenology to brain science is doomed to failure.