DESIGN, in Music, is defined by Rousseau to be the invention and the conduct of the subject, the disposition of every part, and the general order of the whole.
It is not sufficient to form beautiful airs and a legitimate harmony; all these must be connected by a principal subject, to which the various parts of the work relate, and by which they become one. Thus unity ought to prevail in the air, in the movement, in the character, in the harmony, and in the modulation. All these must indispensably relate to one common idea which unites them. The greatest difficulty is, to reconcile the observation of these precepts with an elegant variety, which, if not introduced, renders the whole piece irksome and monotonous. Without question, the musician, as well as the poet and the painter, may risk every thing in favour of this delightful variety; if, under the pretext of contrasting, he do not endeavour to cheat us with false appearances, and, instead of pieces justly and happily planned, present us with a musical minced meat, composed of little abortive fragments, and of characters so incompatible that the whole forms a heterogeneous assemblage.