At the same time that he [the poet] is trying to envisage the poem as a whole, he is trying to relate the individual items to the whole. He cannot assemble them in a merely arbitrary fashion; they must bear some relation to each other. So he develops his sense of the whole, the anticipation of the finished poem, as he works with the parts, and moves from one part to another. Then as the sense of the whole develops, it modifies the process by which the poet selects and relates the parts, the words, images, rhythms, local ideas, events, etc. …It is an infinitely complicated process of establishing interrelations.
Cleanth Brooks and Robert Penn Warren. Understanding Poetry (1938), p. 527.