The composition of music is no more an exact science than the painting of a picture. No rules can be laid down for it, no canons save those of beauty can be applied to it; and as invention, without which it cannot exist, may be said to be infinite, so there are no fixed bounds to its capabilities. Any treatise upon composition, therefore, can only consist of advice and criticism chiefly directed to what concerns taste and sense of proportion. To tell a student how to write music is an impossible absurdity. The only province of a teacher is to criticise it when written, or to make suggestions as to its form or length, or as to the instruments or voices for which it should be designed. He can thus keep impatience within bounds when invention is outpacing experience, and develop by sure, if sometimes necessarily slow, means the experience to equal the invention. For the rest his functions must be what those of this treatise must be, mainly to give hints as to what to avoid, leaving the constructive element to the pupil's own initiative.