 In On the Main Principles of the Fine Arts and Sciences, Mendelssohn sets out from the assumption that the human spirit has learned to imitate beauty, “the self-empowered mistress of all our sentiments,” in works of art. He then proceeds to ask what the arts have in common or, in other words, what beauty is. Batteux’s thesis that art consists in the imitation of nature merely defers the question or, better, requires its revision to the effect that we ask what the beauties of nature and of art have in common. After rejecting Hutcheson’s appeal to an aesthetic sense as a deus ex machina that forecloses inquiry, Mendelssohn contends that the common feature is a sensuously perfect representation (a contention that can be traced back to Sulzer and Baumgarten). “Sensuous” in this connection stands not simply for the fact that an object is sensed by the external senses, but also for the fact that an entire array of an object’s features is perceived all at once. Unlike the distinct representation of a triangle whereby the intellect distinguishes parts and aspects of the triangle from one another, a sensuous representation is clear but indistinct, that is to say, to have a sensuous representation is to perceive something without intellectually distinguishing its parts or aspects. But we can sensuously perceive an order or harmony to those aspects without intellectually distinguishing them. Beauty or the perfect sensuous representation — whether in nature or in art — accordingly pertains to forms, orders, harmonies, and “indeed, everything capable of being represented to the senses as a perfection” (Philosophical Writings, p. 172). On this definition of art, what is represented can be ugly or repugnant as long as the representation is sensuously perfect (though Mendelssohn does address, as discussed below, the mixed sentiment that accrues when the object represented is unpleasant). From these considerations, Mendelssohn derives two necessary conditions of fine art and letters: faithfulness of the representation (“imitation”) and, far more importantly, the artistry, even “genius,” involved. In this connection, he argues that, as far as individual beauties in nature are concerned, they are inferior to the beauties produced by arts. In On the Main Principles Mendelssohn also establishes the general distinction between fine arts and fine science along the lines of the difference between natural and artificial signs. Whereas the “fine arts” (beaux arts) — music, dance, painting, sculpture, and architecture — predominantly work with natural signs, the “fine sciences” (belles lettres) — poetry and rhetoric — typically employ artificial signs. The distinction is hardly ironclad, however, and Mendelssohn ends On the Main Principles with a discussion of ways the arts may borrow from one another (e.g., allegorical painting) or combine (e.g., opera).