Natural x Artificial

"Natural signs are signs founded upon passive synthesis;
they are signs of the present, referring to the present in which they signify.
Artificial signs, by contrast, are those which refer to the past or the future as distinct dimensions of the present, dimensions on which the present might in turn depend.
Artificial signs imply active systheses - that is to say, the passage from spontaneous imagination to the active faculties of reflective representation, memory and intelligence."

Deleuze - Difference and Repetition (p. 99)