We believe that stones and lakes and mountains are not conscious. Neither are plants and trees. Animals (at least some species) and humans are conscious. Why the difference?
Is it a question of complexity? The human brain, it is claimed, is the most complex object in the universe. Brains, whether animal or human, are certainly very complicated collections of material parts indeed.
We are also taught in evolutionary theory that animals and humans, with their very complex bodies as well as brains, are very late arrivals in the history of the universe.
Putting these ideas together, do we have a plausible explanation of consciousness, why it is present in humans and animals and, apparently, not elsewhere?
The general trend in evolution is from the more simple to the more complex, from gases, stars and planets to plants, animals and humans. Consciousness comes on the scene at a particular point in the process. Furthermore, the more complex the organisms, roughly speaking, the broader the range of conscious states. Perhaps, worms can only have very limited experiences; human beings enjoy or suffer a very wide gamut of experiences. Worms know what it is like to be trodden on. So do humans. Humans know what it is like to add up sums, to listen to symphonies and to desire vengeance. Worms, as far as we know, do not.
But what has complexity to do with consciousness in any sense. Why should a collection of atoms and molecules in an enclosed bundle of cells have any conscious experience at all?
Complexity on its own does not seem enough. If scientists spent decades and billions of pounds constructing a computer or robot of the most intricate and complicated circuitry within their power, would there be the slightest of grounds for them to claim that it experienced conscious states?
Consciousness and Other Matters 