Roberto Gerhard described the problem of structure in the following manner:

My favourite simile is the experience of flying. Imagine yourself sitting next to the pilot of a glider aircraft. Your first problem is that of getting airborne. So it is, metaphorically speaking, with musical form. Your second problem is to keep airborne, how to take advantage of every condition that may confront you in the air, currents, countercurrents, air pockets and so forth; how to navigate your craft and set a course. Your third problem, when in sight of your destination, is (naturally) how to come down.

Believe me the hazards of the take off and the landing are conspicuous in musical form too. If you have been watching your pilot all the time, you will have realised how his skill, his reflexes, his compass-readings, his judgement of hazards and chance happenings combine instant by instant and result in choice, in doing this rather than that. In short you have been witnessing an activity which consists of a chain of decisions taken in full course of the operation. That, I submit, applies exactly to musical form. A chain of decisions, subserving an overall design and steering a course of action. Naturally they ought to be the right decisions every time, or things may go wrong. If you have to bale out or crash land the thing is not considered to be quite as successful.