What is music?
In most times and places, this would not be a difficult question to answer. I doubt it would occur to a contemporary of Mozart, for example, to ask for a definition of "music". If they did ask, they would probably get an ostensive definition rather than a theoretical one: music is this, someone would say, pointing to the string quartet politely minuetting in the corner. The particular performing practice of eighteenth-century Austrian culture provided a unitary framework for music, and all was well.
Contemporary North American culture, by contrast, lacks any unifying worldview, artistic or otherwise, and so the performing conventions of "Western classical music" jostle for position with those of various vernacular musics from all over the world. We also live in a world accustomed to the extremes of postmodern art, in which the accepted boundaries of art are deliberately crossed. With so many competing examples of musical praxis, it seems difficult to come up with any definition of music that would include everything we want it to: Bach's B-Minor Mass? Nam June Paik's Solo for Violin? Easy listening? Inuit throat singing? And so the question becomes increasingly pressing: what is music?
I understand why this question seems imporant to so many people, but I never found it particularly interesting. I now believe it's the wrong question entirely, and that putting the matter in these terms does more harm than good.
Consider: are we trying to define the musical experience from the perspective of the creator, or the listener?
From the perspective of the listener, the significance of art lies in the fact that it provides access to an aesthetic experience. In the case of music, the listener's aesthetic experience is one where sounds become the intentional object of an imaginative perception in a sympathetic listener.* As we listen to sounds, we project upon them the musical qualities of pitch, rhythm, tonal orientation, and goal-directed movement which we value in our culture. These sounds can be produced by musical instruments, but they can equally be produced accidentally by phenomena in the natural world, or by machinery, just as a person can receive as much pleasure from a natural landscape as from a painting by Picasso. In this sense, literally any combination of sounds can become music; only the listener determines what is or is not significant. Cage's 4'33" is quite uncontroversial from this perspective: it is music because it causes us to hear the random noises of a concert hall as tones rather than as random noise.
(An anecdote. On my recent trip to the zoo, shortly after having viewed the enchiladas, I stopped to watch a group of birds. One individual, a pheasant of sorts, walked round the cage uttering forth a guttural croak at regular intervals; after a few minutes, a crane standing nearby added its distinctive squawk to the ensemble. For about a minute, they maintained this ostinato pattern with a precise rhythmic relationship. Finally, when no-one else joined in, the crane got bored and flew away. The significance of this experience was not in any musical expertise possessed by the zoo animals, but in my propensity to interpret their sounds in musical terms.)
On the other hand, one can equally look at music from the perspective of the creator: music is something created by a human intelligence (whether improvised or composed in advance, and whether one person is involved or many), and therefore ought to be held to the highest standards of craftmanship. If the work in question is a work of music, these standards demand that it should be written with skill, and that it should be successfully geared towards its purpose: to produce an aesthetic response in a receptive audience. This is the view of art endorsed by neo-Scholasticism. From this perspective, a work like 4'33'' or Paik's Solo is not art; it requires no skill to produce and thus often fails to produce any aesthetic reaction in its listeners.
People who take one perspective or the other will give diametrically opposed answers to the question of "What is music?". This is because they're really answering two different questions. Confusion then prevails. The person in your high-school music class who insisted that Cage's 4'33'' wasn't music was right: it doesn't display the purpose-driven craftsmanship we expect from successful compositions. Her sparring partner across the classroom, though, was also right: 4'33'' is certainly music, because it can create an aesthetic experience in a properly-disposed listener. The problem was not in either of these well-meaning people, but with their teacher for putting the question so crudely.
*I borrow this terminology from Roger Scruton's wonderful The Aesthetics of Music, shortly to be reviewed in these pages.