I'm certainly no authority, but in my recent skimming of Chicago critics, this notion that "a principle must be a principle of something else" stems from Aristotle's method. The active principle of a thing lies in its motive, its end or purpose for action or change. (With Aristotle one can always look at a thing from different perspectives, based on whether you're doing "science" - nature of a thing in itself - or "art" - its telos in relation to other things. I'm sure I'm garbling this, O philosophers, but anyway...).
The statement quoted below seems to align critical practice with my own sense (drawing on Mandelstam), anyway, of the poem as a combination of impulse and material, motive (gesture, genre) and language. & this is a different focus from that of the New Critics, & the mantras of langpo & related "theoretical" styles (text is prior to speech, text undermines authorial presence, etc).
Let's move on, class.