In Sketch of New Aesthetic of Music (published 1907), Ferruccio Busoni globally describes his ideas on the following subjects.

Music was born free
Busoni suggests that art-works consist of spirit, emotion, humanity, form, manner, and "the flavor of the epoch." While spirit, emotion, and humanity remain "unchanged in value through the years,", Busoni believes that the form, manner, and "flavor of the epoch are transient and are aging rapidly.".
When a composer creates a succesful musical work it has succesfully imitated nature and intepreted the voices of that time. At that point music cannot be bound to 'hallowed traditions, rules and principles'. "So what is the aim of music?" 'It was born free, and to win freedom is its destiny'.

Absolute music
Absolute music in Busoni's words is like 'program-music'. Absolute music 'lacks freedom because it is confined to preconceived symmetrical forms. "This sort of music ought rather to be called the ‘architectonic,’ or ‘symmetric,’ or ‘sectional,’ and derives from the circumstance that certain composers poured their spirit and their emotion into".
The original creators of the standard forms, used those forms as a loose guide for freely created music. Modern composers, however, are held back by the them because over time the forms have become restrictive. Busoni proposes compositions not defined by forms or programs, but created out of the 'natural development' of a motive (i.e. The composer doesn't know where the composition goes or ends, but he just follows the motive's development)

Notation and interpretation
Busoni is a proponent of improvisation. When a musician improvises he's trying to capture the music as he envisions it. Music composition is a form of improvisation but then on paper. Busoni uses the painter example: "notation is to improvisation as the portrait to the living model. It is for the interpreter to resolve the rigidity of the signs into primitive emotion."

Music of the future
Busoni envisions a music of the future, but then free from the restraints of the scale system used by classical composers. "we strictly divide ‘consonances from ‘dissonances’—in a sphere where no dissonances can possibly exist!". People are taught that there are 24 different keys, but according to Busoni, there are in fact only two: major and minor.
He admits the strenght of a system developed over 400 years, but asks composers why to limit their musical expression to this system.
Busoni sees a the future with instruments that will fully utilize all of the modal and scalar possibilities of the current twelve-note division of the octave, but it won’t stop there. He also predicts a further division of the octave into smaller degrees (see microtonality).

Music without dogmas
In Busoni's words: "Let us take thought, how music may be restored to its primitive, natural essence; let us free it from architectonic, acoustic and esthetic dogmas."

Busoni's book