But is the aesthetic value of a duet really equal to the sum of the values of its parts played separately? No such thing. The query of one instrument may indeed be in itself a beautiful phrase, independently of the answer given by the other; but as seen in relation to that answer it acquires a totally different emphasis, a meaning which we never suspected. The accompaniment part, or even the solo part, played by itself, is simply not the same thing that it is when played in its proper relation to the other. It is this relation between the two that constitutes the duet. The performers are not doing two different things, which combine as if by magic to make a harmonious whole; they are cooperating to produce one and the same thing, a thing not in any sense divisible into parts; for the "thing" itself is only a relation, an interchange, a balance between the elements which at first we mistook for its parts. The notes played by the piano are not the same notes as those played by the violin; and if the duet was a merely physical fact, we could divide it into these two elements. But the duet is an aesthetic, not a physical whole. It consists not of atmospheric disturbances, which could be divided, but of a harmony between sounds, and a harmony cannot be divided into the sounds between which it subsists.
Robin George Collingwood, Religion and Philosophy, p. 113.