Zeman is not so much interested in solving the puzzle of what consciousness is as in detecting the preconditions for consciousness. He uses studies on mental disorders and drugs to ascertain when we are more or less aware of what we do. First and foremost, his experiments show how mundane consciousness is: a little more or less of this or that chemical makes a big difference on how you "feel".

Zeman also ponders on very simple but clearly relevant facts. Light is fundamental for life, and it is also the whole point of vision, and it is also a source of energy. By analyzing how visual input is processed, Zeman concludes that a) our perceptions are shaped by our past (we see what we expect to see) and that b) eye and brain continuously interact to make sense of what is being seen.

He interrupts his analysis for a brief recapitulation of the history of life on this planet and for a lengthy survey of modern theories of consciousness.

The most salient contribution of this book is the idea that consciousness depends, mostly, on events that take place within the brain. A lack of this or that chemical is enough to alter our personality. After all, there is consensus that consciousness is a product of neural activity, and neural activity is a material process that uses material elements, which, ultimately, are the (indirect) constituents of consciousness.

By the same token, he describes perception as the brain's reaction to being bombarded with energy picked up by the body's sensors. Without that external energy, there would be no visual or auditory or whatever processing. Whatever the brain does, it is initiated by an energy impulse coming from the outside. It is then processed according to the chemical structure of the brain, which, by definition, depends on the amount and kinds of chemicals in the brain.

Zeman is quite convinced that the thalamus is the central site of consciousness. During sleep, the thalamus interacts with the cortex in rhythmic bursts, while inhibiting all sensory inputs. When the body is awake, the thalamus works as an intermediary between the periphery and the cortex, shuttling back and forth sensory inputs and commands to move. The brainstem is the switch that turns the thalamus on and off.

This down-to-earth vision of consciousness concludes with an elegant and polite demolition of theories of consciousness (McGinn, Nagel, Searle, Chalmers, etc).