  Categories provide a coarse grained description of the world. A fundamental
question is whether categories simply mirror an underlying structure of nature,
or instead come from the complex interactions of human beings among themselves
and with the environment. Here we address this question by modelling a
population of individuals who co-evolve their own system of symbols and
meanings by playing elementary language games. The central result is the
emergence of a hierarchical category structure made of two distinct levels: a
basic layer, responsible for fine discrimination of the environment, and a
shared linguistic layer that groups together perceptions to guarantee
communicative success. Remarkably, the number of linguistic categories turns
out to be finite and small, as observed in natural languages.
