When Alice steps through the looking-glass, she encounters a peculiar world where she meets animated chess pieces, characters from nursery rhymes, and talking animals. Everything there is inside out and upside down: so it is with consciousness.

Reflecting on the inception of consciousness, it is natural to suppose that there are just two alternatives. Either consciousness appeared in living beings suddenly, like a light switch turning on, or it appeared gradually, just as life did, through a range of borderline cases. For the former theory, consciousness is an on/off matter, but once it was there it became richer over time, like a beam of light becoming brighter and broader in its sweep. For the latter theory this is not the case. There are shades of gray. There is no one moment at which consciousness appeared.

Unfortunately, both alternatives face deep problems. The solution to these problems lies in the realization, strange as it may be, that a key element of consciousness itself was always here, as a fundamental feature of micro-reality. Varying conscious states were not, however: they appeared gradually. In Vagueness and the Evolution of Consciousness, Michael Tye explains in detail how this can be so. He also addresses questions about the location of consciousness in the brain, the causal efficacy of consciousness with respect to behaviour, and the extent of consciousness in the animal world.