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THE nature of consciousness is notoriously hard to crack and there are so many speculative ideas about it that it is hard to keep track of them. Another book on the subject may seem one too many, but Christof Koch鈥檚 deep synthesis of a profound theory of consciousness and the latest neuroscience is an exception.

At last, we have a theory of consciousness that makes clear and testable predictions, many wholly unexpected. It says even the cleverest computers can never be conscious, while many more species of animal than we thought have sparks of consciousness.

More exotically, it implies that it may be possible to experience the great void 鈥� the deep meditative state that mystics seek 鈥� and for lovers to meld minds. But perhaps most important of all, the theory suggests that consciousness has a function and thus why it evolved.

Koch, who runs the Allen Institute for Brain Science in Seattle, spent decades working with the late Nobel laureate Francis Crick searching for the parts of the brain closely involved in generating consciousness. Koch鈥檚 knowledge of neuroscience complements the idea at the heart of The Feeling of Life Itself.

That idea, writes Koch, 鈥渋s the singular intellectual creation of Giulio Tononi, a brilliant, sometimes cryptic, polyglot and polymath renaissance scholar鈥�.

Tononi is a professor at the University of Wisconsin with whom Koch now works closely. He devised the integrated information theory of consciousness, which drew public attention for its 鈥渃onsciousness meter鈥�, a method of searching for awareness in people thought to be in vegetative states. But this is the first book to try to explain the theory to a general audience.

Tononi鈥檚 approach doesn鈥檛 start in the brain or in philosophy, as you might expect, but by asking a simple question: what is conscious experience like?

Its essence can be captured in five properties, Koch believes. First and foremost, consciousness is 鈥渋ntrinsic鈥�, a private experience. I know that I am conscious and have experiences but can鈥檛 observe your consciousness or be certain that you feel anything. Consciousness exists for itself.

Experience is also 鈥渟tructured鈥�, containing many different things. For example, someone sits in front of you on the bus, traffic passes by, your neighbour鈥檚 headphones leak bass and so on.

And each conscious experience is 鈥渋nformative鈥�, it differs from every other one. It is 鈥渋ntegrated鈥� into one whole picture, and it is 鈥渄efinite鈥� in that you only have one conscious experience at a time, which can鈥檛 be reduced into parts without losing something.

The five properties need some thought but Koch explains them well and lets you attempt to negate them to prove that they 鈥渃annot be doubted鈥�. Let鈥檚 agree and follow Koch with a leap from these five properties of experience to five matching principles that must be obeyed for a physical system to generate consciousness.

This is Tononi鈥檚 great achievement. He proposes five principles (intrinsic existence, composition, information, integration and exclusion) that make it possible to see whether any set of mechanisms, be they neurons or electronic networks, can generate experience. If a system doesn鈥檛 produce intrinsic experience, it isn鈥檛 conscious, if the whole can be reduced to parts, it isn鈥檛 conscious, and so on.

鈥淎t last we have a theory of consciousness that makes clear, testable predictions, many wholly unexpected鈥�

Tononi鈥檚 big leap is to go from experience to a defining physical theory of consciousness, without dabbling with the brain. That is truly radical.

Koch provides a metaphor to help show how the theory deals with 鈥渋ntrinsic existence鈥�, the private nature of consciousness. He goes back 2400 years to Plato鈥檚 dialogue with the 鈥渟tranger from Elea鈥�, who argues that for something to be said to exist, it must affect something else, or be affected by something else. But consciousness seems to be an exception. We feel sure it exists but can give no evidence of it 鈥渃ausing鈥� anything, nor can anyone else observe it.

The way out is to say that consciousness must have an internal cause-and-effect structure that gives it causal power over itself. This may sound odd, but 鈥渃ausal power鈥� can be precisely defined mathematically and makes sense of an old intuition from neuroscience: that sustained feedback in the brain, called recurrent or re-entry processing, is essential for consciousness.

And, as Koch explains, it fits perfectly with his search for consciousness in the brain. Circuits in a zone of the posterior cerebral cortex, a part of the brain that research shows is essential for consciousness, contain elements that can interact tightly with one another and have high causal power. Brain areas like the cerebellum have more neurons than the cortex but don鈥檛 have this structure and aren鈥檛 able to generate consciousness.

By now, I hope you are both intrigued and puzzled, because then I will have conveyed how I felt reading Koch. The theory is profound and you need time to think it through. But its promise that it can examine any system to see if it generates consciousness leads to surprising predictions.

Can computers be conscious? Absolutely not. Their circuitry is of the wrong type; they have no intrinsic causal power and form no whole. Bad news for those seeking digital immortality by uploading their brains.

Are animals conscious? The answer isn鈥檛 a straightforward yes or no, for some animals may have a tiny glow of experience, while others like humans have a brighter light. Honeybees, for example, have brain areas that heavily reconnect to themselves. So being a bee may well feel like something.

Then there is the more exotic prediction for lovers who dream of merging their consciousnesses. If we could completely connect one brain to another, the theory says merging is possible. But beware: you wouldn鈥檛 feel like one part of a dual consciousness but would be lost in a single, strange, new mind.

Tononi鈥檚 idea also explains how meditation might lead to a profound sense of the void. You might think quietening nervous activity would simply lead to unconsciousness. But Tononi鈥檚 theory shows a key difference between the absence of presence and the presence of absence. One leads to unconsciousness, the other to 鈥減ure consciousness鈥� without sensation or memory.

For me, the icing on the cake is a function for consciousness. This part of Tonini鈥檚 theory builds on research using artificial, non-biological creatures dubbed 鈥渁nimats鈥�. In simulations where they learned to navigate mazes and the most successful passed on their characteristics to new generations, very skilled animats evolved. Their tiny brains were the ones that best integrated information. This shows that, in essence, organisms with the internal cause-and-effect structure most efficiently pack into their brains distinctions about the world around them. The resulting survival advantages may explain why consciousness evolved.

Koch鈥檚 mind-stretching book provides a rich feast, leaving me with a desire to understand more about this often difficult theory.

That said, there is a sense in which the theory is also simple, for it extends familiar physics to the inner view. 鈥淭extbook physics deals with the interaction of objects with each other, dictated by extrinsic causal powers. My and your experiences are the way brains with irreducible intrinsic causal powers feel like from the inside,鈥� Koch writes. 鈥淐ausal powers of two different kinds is the only sort of stuff needed to explain everything in the universe. These constitute ultimate reality.鈥� Now there鈥檚 a grand statement.