I am inspired to revisit this topic because of some thoughts Beardsley expresses on artistic form in his book on aesthetics. As is typical of philosophers he manages to ask more questions than he answers! One of the first that comes up (on p. 165 in Aesthetics) is the problem that the very mention of the word "form" implies a distinction with "content" as they seem to go together. If we are talking about a bottle of water it is easy to see what is the form and what the content. It is also fairly easy if we are talking about, say, Shakespearean sonnets. The form is the sonnet ("The form consists of fourteen lines structured as three quatrains and a couplet.") and the content is the imagery and themes expressed in the poem. But when we talk about the musical version, the sonata, the distinction is far less obvious and the relationship between form and content a complex one.
Theorists of a certain persuasion state that the form of a sonata is tripartite: exposition, development and recapitulation. The exposition presents one or more themes, the development develops these themes through a variety of harmonic excursions and thematic fragmentation and the recapitulation returns to the original themes. The characteristic harmonic structure of sonata form for much of music history was that while the exposition presented different themes in different keys, the recapitulation restates the themes in keys other than the tonic, in the tonic. This gives the form the necessary element of finality. Well, that sounds all very nice. But as soon as you sit down to examine some actual sonatas, you are likely to discover that only the dullest of composers follows that format very strictly. Nearly every time Haydn sat down to write a sonata form movement he did it in a different way. Often he uses only one theme. Mozart, on the other hand, broke the "rules" in a different way: he tends to have a bunch of themes. Beethoven reinvented sonata form in almost every piece and Schubert recreated it again in a different way.
In music, form and content are so interrelated that the one creates the other. To pick two examples from Beethoven, the opening movement of the Moonlight Sonata is so unlike any other first movement in a piano sonata that it has almost nothing to do with sonata form:
The form of that movement is basically harmonic and the unity comes from the texture. Contrast that with the first movement of the Symphony No. 5 by Beethoven where the unity comes from the tight focus on that opening four-note motif:
In what sense are these two movements in the same form? I could present examples almost indefinitely!
I think the fundamental truth of form in music composition is that it is as much a unique creation with each piece as is the content--in the best music, the two are really inseparable. In a symphony by Sibelius, the meter, the tempo, the harmony and the themes go together in constantly transforming ways to create the structure and the direction of the musical movement. Like all good form, it is rather magical! Let's listen to the Symphony No. 6 by Sibelius (he described it as "the scent of the first snow" in a letter) that is a pretty good example. The performers are Esa-Pekka Salonen and the Swedish Radio Symphony Orchestra: