The Symbolic Species : The Co-Evolution of Language and the Brain by Terrence W. Deacon
What separates humans from animals, Deacon writes, is our capacity for symbolic representation. Animals can easily learn to link a sound with an object or an effect with a cause. But symbolic thinking assumes the ability to associate things that might only rarely have a physical correlation; think of the word “unicorn,” for instance, or the idea of the future. Language is only the outward expression of this symbolic ability, which lays the foundation for everything from human laughter to our compulsive search for meaning.
It is this ability to do the symbolic thing that is the stuff that dreams and cyberspace is made of.