Consciousness is a topic long avoided by the scientific community, but it has troubled philosophers for hundreds of years. Each one of us knows that we are conscious but we cannot know what consciousness is like for anyone else. Given the private and subjective nature of human consciousness, it would seem almost impossible to determine whether another species is conscious. But although we can never get inside the head of another creature, science is increasingly developing techniques to understand consciousness.
Stephen Budiansky, the author of If a Lion Could Talk, says: "We ought to have an open mind about animal consciousness. But we also have to be sceptical, because we as humans have an almost compulsive tendency to interpret the world as a mirror of our own minds. When we see an animal behaving in an intelligent, flexible fashion we automatically assume that it must have a conscious mind much like ours.
"But it is possible for minds to display highly intelligent processes without these being conscious. Computers certainly do so; and even we perform many tasks that are done completely unconsciously."
A condition known as "blindsight" may help us understand what it might be like to be fully functioning and awake, but unconscious. "Graham" had an accident when he was eight years old which damaged part of the visual cortex, the section of his brain that processes sight. He is now blind on the right-hand side of his body and, because of the unusual nature of his syndrome, he has become a semi-professional research subject for scientists.
When he is forced to guess the location of objects placed on his blind side, their shape, direction of movement and even their colours, he is nearly always correct. "It's a very bizarre feeling to sit there and be told events are taking place on your right-hand side - as far as I am concerned, nothing at all happens," says Graham.
Graham is unconscious of his sense of sight. His brain has compensated for the damage by processing his vision through an older pathway in the midbrain, which allows his brain to "see" without his having a conscious image in his mind of what he sees. This may be the way animals operate - guessing accurately about what they see, but with no awareness of it. Evidence that this may be the case for some animals comes from brain studies of pigeons and tree shrews, which also process their sight along the same ancient pathway as Graham does.
The situation may be different for bigger-brained animals. In the Seventies Professor Nick Humphrey, of Cambridge University, was working with a monkey that had no visual cortex. Helen was, to all intents and purposes, completely blind.
Humphrey reports: "I had a hunch that there was something going on which might reveal a capacity for sight which even she wasn't aware of. I started working with her, and... a miracle happened. Within a few days, she was beginning to attend to what I was doing. Within a few years, you wouldn't have known that there was anything abnormal about her. Helen had somehow developed an ability to see without being conscious." The nearest analogy for it, he says, is ESP. "You know what's out there, but you don't know how you know it."
It seems that animals that process their vision in the same way as us - via the cortex (and are not brain-damaged) - may well be conscious. We have had a notoriously difficult time trying to define consciousness, but perhaps it may best be understood in two ways. The first is what philosophers refer to as qualia. This basically describes how sensations and experiences are perceived by an individual. If some animals do have this kind of consciousness, this is what they will be aware of - the unique experience of being alive, and how the world feels to a particular animal. A philosopher, Thomas Nagel, wrote a seminal paper in the Seventies entitled What is it Like to Be a Bat?. He concluded that we can never really understand what it is like to echolocate or fly in the dark, but what we do know is that there is something that it is like to be a bat.
The more sophisticated kinds of consciousness, such as self-awareness and awareness of other minds, may well be beyond the cognitive capacity of most animals. One of the ways of testing self-awareness is the so-called mirror test. Only children over the age of 18 months, great apes and, possibly, dolphins seem to be able to recognise themselves in the mirror. However, Dr Celia Hayes, a psychologist from University College London, disputes the claims made for this test. "When most people talk about self- awareness, they mean that the individual with self-awareness knows certain things about their body, and they know these things consciously - for example, that it's mortal, that your body contains a unique mind. You can pass the mirror test without knowing that you're mortal or that you're unique - all you have to be able to do is to spot certain links or connections between your body and the mirror image."
Most animals certainly seem to have a physical awareness of their own bodies - they know where they end and the world begins, and consequently can fly, run or jump without major catastrophes - but few, if any, animals have the kind of self-awareness that Hayes describes.
The final hallmark of consciousness is understanding the mind of another conscious being and, again, there is only experimental evidence to suggest that our closest living relative, the chimpanzee, may approach this level of conscious awareness.
Professor Sally Boysen, from the University of Ohio, tested chimpanzees' understanding of other chimps' mental states. The worst thing that can happen to a chimp in her lab is vaccination: they are terrified of needles. Boysen approached a cage in which one chimpanzee could see her, and another could not. When the chimpanzee that had not seen her was allowed out of the cage (apparently to be vaccinated), the chimp that had seen her started to bark, erect its hair and pull an expression of fear while looking at the chimp that was ignorant of the situation. If Sally approached the cage with food, the chimp that knew what was going on made none of these warning signals to the chimp in the inner enclosure.
Boysen says these results show that the chimpanzees understand when another chimp is ignorant of a potentially dangerous situation. "I've spent the last 24 years in the company of chimpanzees, and there's no question in my mind that they are sentient, conscious beings, who are ever processing what's happening around them. They're on an almost even par with us in terms of their abilities to reflect on themselves and others around them."
Not all animals may show the reflective self-awareness and awareness of other minds that perhaps mould our consciousness, and many may indeed possess little conscious awareness. However, a number of species could be conscious - we may not know what their conscious awareness is like, but there is something that it is like to be a bat, a bird or a beluga whale, if not a bacterium. When I asked Dr Miriam Rothschild what it was like to be a dog, she said: "I believe that dogs really see the world in a series of smells... and I often think that they have a rather poor life, because they live in a world where they only see everybody from the knee downwards, and with very nasty, smelly feet."
Sanjida O'Connell is the producer of `Are Animals Conscious?', to be shown on BBC 2, 6pm, 30 JanuaryReuse content