Arbitrary signs have nothing in common by nature with the designated subject matter but are assumed as signs by convention.

Mendelssohn concludes the essay on the “Main Principles of the Fine Arts and Sciences” with a brief but pregnant division of the arts. The basis of his division is a distinction between “natural” and “arbitrary” signs, which has precedents in Du Bos, in Leibniz and Wolff, and beyond all of them in St. Augustine. Signs are natural “if the combination of the subject matter signified is grounded in the very properties of what is designated,” as smoke is a natural product of fire or “The passions ar