Science has been trying to ignore the problem of consciousness for a long time.

• In the 1600s, René Descartes declared that nothing is more obvious and undeniable than the fact that we are conscious. Your consciousness can't be a fantasy. At the same time, your consciousness does not obey any of the usual rules of science. It doesn't seem to be physical. It can't be observed or really described. Descartes concluded that it had been bequeathed to us by God.
• This Cartesian dualism remained the assumption into the 18th century. But it was unacceptable to the secular scientist that took the position that only physical things exist.
• As late as 1989, the British psychologist Stuart Sutherland declared that it is impossible to specify what consciousness is, what it does, or why it evolved.
• In 1990 Francis Crick and Christof Kock mentioned in a paper that most of the work in both cognitive and neurosciences makes no reference to consciousness because most don't know of a useful way of approaching the problem.

We could tell simple mechanical stories about the machinery of the eye, but it doesn't begin to explain the breathtaking experience of depth and clarity.

Consciousness can't just be made of ordinary physical atoms. Philosopher David Chalmers argues that it must be an additional ingredient in nature.

Most may think of consciousness as something above our physical being - as if your mind were driving your body. But to accept this as a scientific principle would discount the laws of physics.

Everything we know about the universe tells us that reality consists of physical atoms and their component particles. If non-physical mental stuff did exist, how can it cause physical things to happen?

Professor Daniel Dennett, the high-profile atheist, argues that consciousness is an illusion, that the normal functioning of the brain just makes it look as if there is something non-physical going on.

His opponents think he simply denies the existence of something everyone knows: that of their inner experience of sights, smells, emotions, and rest.

Patricia Churchland, a self-described neurophilosopher, thinks neuroscience will eventually show that consciousness is just brain states.

Several ideas are surfacing: "global workspace theory," "ego tunnels," "microtubules," even speculation that quantum theory may provide an answer. But some thinkers raise the possibility that they should not assume that they can solve every big philosophical puzzle that arises.

In the last few years, several scientists and philosophers are looking at panpsychism, the idea that everything in the universe might be conscious, or potentially conscious, or conscious when putting into particular configurations.

A narrower view is that anything could be conscious, providing that the information it contains is sufficiently interconnected and organized. The human brain fits the description, so do the brains of animals, though their consciousness may be different. But that principle may also apply to the internet or a smartphone.

The "integrated information theory" has been tested. A device has been designed to measure how integrated the brain's neural circuits are. When people fall into a deep sleep, the device demonstrates that their brain integration declines too.

In theory, you can take any device, measure the complexity of the information contained in it, and then work out whether or not it was conscious.