I believe the term modulation denotes in music the uninterrupted shift from one key to another: I do not know the term for change of rhythm without change of measure.
I am not ridiculing verbal mechanisms, dreams, or repressions as origins of poetry; all three of them and more besides may have a great deal to do with it.
A poem may be an instance of morality, of social conditions, of psychological history; it may instance all its qualities, but never one of them alone, nor any two or three; never less than all.
In a manner of speaking, the poem is its own knower, neither poet nor reader knowing anything that the poem says apart from the words of the poem.
