Art and Illusion:

If we assume … that natural signs can simply be copied from nature, the history of art represents a complete puzzle. It has become increasingly clear since the late nineteenth century that primitive art and child art use a language of symbols rather than “natural signs” … All art originates in the human mind, in our reactions to the world rather than in the visible world itself, and it is precisely because all art is “conceptual” that all representations are recognizable by their style.

by E.H. Gombrich, 1960