  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 are, by their very nature, connected with certain movements in our limbs as well as with certain sounds and gestures” (“Main Principles”; Philosophical Writings, p. 177). Signs are arbitrary that “by their very nature have nothing in common with the designated subject matter, but have nevertheless been arbitrarily assumed as signs for it,” such as the “articulated sounds of all languages, the letters, the hieroglyphic signs of the ancients, and some allegorical images” (“Main Principles”; Philosophical Writings, pp. 177–8). The arbitrary signs could also be called conventional. Mendelssohn’s chief distinction is then between those arts that convey their content by artificial signs, namely poetry and rhetoric, and those arts that employ natural signs, which convey both reference to content and the expression of feeling through natural signs and do “not presuppose anything arbitrary in order to be understood,” namely painting, sculpture, music, dance, and even architecture insofar as it conveys any meaning and expression. In fact, Mendelssohn distinguishes between the fine arts and the beautiful sciences, or between beaux arts and belles lettres, on this basis: the fine arts employ natural signs, and the beautiful sciences or belles lettres employ arbitrary or conventional signs (“Main Principles”; Philosophical Writings, pp. 178–9). Among the belles lettres, poetry and rhetoric are distinguished by the fact that “The main, ultimate purpose of poetry is to please by means of a sensibly perfect discourse, while that of rhetoric is to persuade by means of a sensibly perfect discourse.” Mendelssohn does not explain why the fact that poetry and rhetoric employ artificial rather than natural signs entitles them to be called sciences rather than arts; perhaps what he has in mind is that since the meanings of arbitrary signs can be codified, there is more room for precision in the interpretation of poetry and rhetoric than there is in the various fine arts (with the exception of their allegorical or iconographical aspects, which as Mendelssohn has suggested are more like arbitrary than natural signs). In the case of rhetoric, moreover, there was a long tradition going back to antiquity of formulating rules for how persuasion can be achieved, and perhaps this made it seem like more of a science than an art to Mendelssohn.