Most poems have a controlling metaphor; that is, the poet has written about something, someone, or some concept that he/she is comparing to something or someone else. This tension between the two parts being compared is the controlling metaphor.
The difficulty comes about with some poems because metaphors may take one of four forms, depending on whether the literal or figurative terms are repectively named or just implied. Here are the four variations of metaphor:
1. Both the terms, literal and figurative are named. [this is one already mentioned: "Juliet is the sun."]
2. The literal term is named, but the figurative one is implied. [snake is mentioned, implying evil]
3. The literal term is implied and the figurative one is named. [Dickinson's "a narrow fellow in the grass," implying a snake]
4. Both the terms, literal and figurative are implied. (Of course, this is the hardest to identify. [Wallace Stevens's "Sunday Morning" has this type of metaphor: His first line begins, "Complacencies of the peignoir" in a poem in which a woman meditates on questions of deth, mutabillity and permanence. The woman is drawn to these questions in the first stanza.]
To use Robert Frost's short poem, "Fire and Ice," as an example, the reader knows that Frost compares both Fire and Ice to hatred even though the literal word is implied, especially when he agrees in great understatement--"Ice will also suffice"--that both Fire and Ice can equally destroy the world.