The philosopher David Chalmers has described two problems about consciousness. The first is called the easy problem. The easy problems are not easy at all. These are questions about how the brain works, how it transforms sensory input into actions, how perception works and – in general – how the brain operates as a physical system. The easy questions are not easy to answer, but at least there's not any conceptual difficulty thinking about how we might answer them.

The hard problem for David Chalmers is the problem of how and why any of this should give rise to any kind of conscious experience whatsoever. Where we stand now, there is no strong consensus about approaches to the hard problem: we don't really have a convincing answer, but we are nonetheless learning a great deal about how consciousness experiences depend on processes in the brain and the body.

In practice, what neuroscientists such as myself have been doing for quite a while is, instead of trying to directly answer the question of how physical stuff gives rise to consciousness, we look for correlations. We try to build explanatory bridges between what happens in the brain, in the body, and what happens in our conscious experience. We try to look for what have been called neural correlates of consciousness.

Your brain is perhaps the most complicated object in the universe. The human brain contains about 86 billion neurons and about a thousand times more connections. So, if you counted one connection every second, it would take you about three million years to finish counting. About three quarters of the neurons in the brain don't seem to be involved in consciousness at all: these are the neurons in your cerebellum, which is the little brain hanging off the back of your cortex. Three quarters of the neurons are there. People can be born without a cerebellum, or you can have damage to your cerebellum, and while you'll be affected in many ways, your conscious experience won't be much affected at all.

The other parts of the brain do seem to be involved in consciousness, but exactly the role they play is still the subject of a great deal of investigation. There's the brain stem, which is a region at the top of the spinal cord and below the main part of the brain. This part of the brain seems to be involved in enabling conscious experiences. So, if you have damage to the brain stem, you might lose consciousness altogether. This kind of damage can result in conditions like the vegetative state or coma. But what you're conscious of at any given time seems to depend on activity patterns in the cortex – the massively folded sheet of densely packed neurons that is the main feature of the human brain.

One way to think about consciousness is in terms of conscious level. Very roughly, this is how conscious you are at any moment. Right now, you're conscious and you're awake. So, wakefulness and consciousness go together, but they can come apart. When you fall asleep, you might lose consciousness to start with, but when you start dreaming, you are having conscious experiences even though you’re asleep. So, the mechanisms in the brain that underlie being conscious are not necessarily the same mechanisms that underlie being awake.

You can also go in the other direction. There are some conditions, like the vegetative state, where you are completely without consciousness, but the body still goes through the sleep-wake cycles. This is a case where you have wakefulness without awareness. This is a very challenging state for neurologists because it seems as though the person is cycling through states of wakefulness, yet there doesn't seem to be anybody at home. Actually, figuring out whether there is any residual consciousness in the vegetative state is one of the more important applications of understanding consciousness in medicine and in society.

One of the key things about conscious perception is that it's not a direct reflection of what's actually out there in the world. What we consciously experience is always a brain-based interpretation – or best guess – of the causes of the sensory signals that the brain receives. The light waves that hit our retinas at the back of the eye don't come with labels saying they’re from a coffee cup or a table. They don't even come with labels saying that they represent light. They're just electrical signals, and they're ambiguous about what's out there in the world.

Every act of perception involves the brain combining this ambiguous sensory information with its prior expectations about what the world is like. And that's what we see. What we consciously perceive is a best guess about the causes of sensory signals. This is an old way of thinking – it goes right back to Plato in his cave. We can also think of it in terms of Kant, who argued that we never have direct access to things as they actually are; they’re always hidden behind a sensory veil. Hermann von Helmholtz, in the 19th century, was perhaps the first person to describe perception as this act of unconscious inference, that the brain is always trying to make its best guess about what's out there, and that this is what we end up perceiving.

This separation between how things are and how things seem goes beyond what object is there or what colour it is; it extends also to the deep structure of perceptual experience. A good example of this is a phenomenon called change blindness. You might think that when we see something change, this immediately means that something in the world has changed. But change itself is part of perception. When we perceive something as changing, it means the brain has inferred that some change has happened, it doesn't mean that something has necessarily changed in the world. There are many demonstrations which show that if something in the world changes very slowly, we may not perceive it as changing at all. This is called change blindness.

Over the last few hundred years, views about how consciousness extends throughout the animal kingdom have varied greatly. Descartes, who back in the 17th century was famous for introducing the distinction between mind and body in the first place, was also famous – or infamous – for arguing that other animals were merely what he called bête machine or beast machines, creatures made of flesh and blood that didn't have the same kinds of conscious experiences that we humans do.

These days, it's more widely recognised that many of the mechanisms that underlie the conscious experience in humans are shared in the animal kingdom. Other mammals have pretty much the same thalamus and cortex – basic brain structures that we know are important for consciousness in humans. Once we go beyond mammals, it becomes quite difficult to know for sure, but there are certainly shared features of behaviour and of brain structures in birds, and in creatures like cephalopods, such as the octopus.