It has struck me how and in what way the imagination can be applied to music and these two seem to quite well represent two different tendencies.
Cage concerned himself with forms and processes a priori that could be applied to any sonic material even though the goal was to liberate sound in the process. It was still a situation where the material would bend to these forms, regardless of how much or little control there was.
Varese on the other hand would accept forms as the result of working with the sound he choose. Here form bent to the material. (Historically the forms were a given and likewise neutral, but there was concern about 'how' it was going to be used, but imagination so expanded the possibilities they were no longer necessary.) In a sense Varese applied his imagination in a more traditional way for him the form was neutral as opposed to a means to an end. Scelsi appear to prefer applying imagination more in keeping with Varese. What happens happens because of what presents itself. Even a more melodically oriented composer as Sorabji likewise.
The problem of a priori applications lies in that it works outside of time, and we risk not experiencing the imagination in the moment shaping things according to an inner impulse.
Likewise the actual sound can become of secondary importance. The problem with the latter is the final post listening experience can be of little or no importance.
Perhaps the question is a Macro/Micro contrast in thinking about music.
Interesting Feldman fell in the middle of influences but seems to have sided more with Varese.
Anyway this is the concern of one persona at the outpost here. I am leaning these days away from a priori thinking.