I agree with Phil Ford that consideration of work-concept and performance brings up some interesting questions about the self/subjectivity and agency. In expanding the inquiry to composition (as an act and as a discipline), I think one needs to consider other regulative concepts at play, such as the 'composer-as-creator,' and the relationship of the autonomous work to tradition/heritage/past works. Down the road, I think that thinking about composition in these terms can impact our understanding of performance in general, not only in music.
Under the work-concept, we tend to think of those 'objects' we call works as resulting from human, generally the action of one person. This idea is embodied in copyright, in typesetting conventions of concert programs, and the organization of libraries. Even so, we all _know_ that the reality of compositional practice is substantially messier, and that composer's 'borrowing' or 'stealing' is so general as to be possibly fundamental to 'composition.' In part I'm thinking about Lutoslawski's wonderful statement about composers never hearing music, just hearing how to change music. In part I'm thinking of Brailau's great formulation that speaking of an individual composer of a work is never a good idea. But mostly I'm thinking about my own experience of discovering things in my works that are surprising, and the extent to which that process of discovery is a basic part of my compositional practice.
We use the idea of 'creation' as a shorthand for something that's a bunch more complex. Wether we want to think the composition of a work as the discovery or selection of a Platonic ideal that was already 'out there' (perhaps along the lines of Dodd's recent efforts to have creativity without creation, or Stravinsky's less extreme emphasis on the role of discernment in the compositional process) or as the result of the application of craft, or as emerging from a Promethean inspiration, we tend emphasize the individual's acts rather than context that was given in. Even the craft model, which carries strong implications of tradition and heritage is used mostly to capture the skills of a 'master' rather than the arc of past actors that led to that mastery.
I think that if we accept this shorthand as sufficient, we miss the chance to get at the very interesting question Ford is approaching here; which choices by or discernments of a composer are 'conscious,' 'significant,' 'signifying,' 'actorializing' or whatever term you happen to be using. The work-concept requires, or at least prefers, a work to be autonomous from other works; objects tend to be physically separable from one another, and if they are not, they are the same object. The problem is that we know that 'works-as-sound' tend to be extremely self similar (within a composer's output, and across different composer's outputs.) Either one is forced to considered every sound-event is the consequence of a choice (conscious or other wise) by a single composer, or consider the composer as part of a tradition of musiking of which this work in its particulars and similarities to other works as a particular instantiation or draft.
The professional and rhetorical frameworks of composition make the second view a difficult stance to take and maintain, and it has downsides. There is the danger of a fetishization of a tradition, and retreating to the uncritical repetition of past choices at the expense of individual insight. We also need to be careful in considering the expression of composers on these matters, since we are generally (and understandably) invested in a particular flavor of composer-concept, especially one which presents us as having agency.
But such a stance by composers (and scholars, I think) helps us come to grips with the power of the givenness of music, and to the profound role of adoption and re-adoption in our practice, to borrow a notion from Bernard Stiegler, developed more full in <Technics and Time but articulated elegantly in The Ister.
"[To be] human is essentially the process of the adoption of the past, and of technics, and it is the same. ... Technics is always new, we must always adopt it. We are fundamentally caught in a process of adoption."
Stiegler, Bernard, perf. The Ister. Dir. David Barison and Daniel Ross. Black Box Sound and Image, 2004.
I know that Chris will have more to say, so I'll leave this here for now...