But if, at an interesting point of the conversation, an interloping voice intervened, Bach regarded it as an intruder and let his pupils understand that it could not be admitted. Notwithstanding his strictness on this point, Bach allowed his pupils considerable licence in other respects. In their use of certain intervals, as in their treatment of harmony and melody, he let them experiment within the limits of their ability, taking care to discountenance ugliness and to insist on their giving appropriate expression to the character of the composition. Beauty of expression, he postulated, was only attainable on a foundation of pure and accurate harmony. Having experimented in every form himself, he liked to see his pupils equally adventurous. Earlier teachers of composition, for instance, Berardi,(180) Buononcini,(181) and Fux,(182) did not allow such liberty. They were afraid to trust their pupils to encounter difficulties, and short-sightedly prevented them from learning how to overcome them. Bach’s system was wiser, for it took his pupils farther, since he did not limit their attention, as his predecessors did, to the harmonic structure, but extended it to the qualities that constitute good writing, namely, consistency of expression, variety of style, rhythm, and melody. Those who would acquaint themselves with Bach’s method of teaching composition will find it fully set forth in Kirnberger’s _Correct Art of Composition._(183) As long as his pupils were under his instruction Bach did not allow them to study any but his own works and the classics. The critical sense, which permits a man to distinguish good from bad, develops later than the aesthetic faculty and may be blunted and even destroyed by frequent contact with bad music. The best way to instruct youth is to accustom it early to consort with the best models. Time brings experience and an instructed judgment to confirm the pupil’s early attraction to works of true art. Under this admirable method of teaching all Bach’s pupils became distinguished musicians, some more so than others, according as they came early or late under his influence, and had opportunity and encouragement to perfect and apply the instruction they received from him. His two eldest sons, Wilhelm Friedemann and Carl Philipp Emmanuel, were his most distinguished pupils, not because he gave them better instruction than the rest, but because from their earliest youth they were brought up amid good music at home. Even before they began their lessons they knew what was good. On the other hand, others, before they became Bach’s pupils, either had heard no good music or their taste had been already vitiated by contact with bad. It at least attests the excellence of Bach’s method that even his pupils thus handicapped took high rank in their profession and distinguished themselves in one or other of its branches.(184)