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; and all these must indispensably relate to one common idea which unites them.