Aristotle's Poetics identifies form as one of the four causes of the artwork — the formal cause that makes a thing what it is rather than something else — and this Aristotelian concept of form underwrites the entire tradition of formalist poetics that treats the poem's structure as philosophically inseparable from its meaning; Plato's theory of Forms — eternal, perfect archetypes of which particular things are imperfect copies — has influenced poets who see the concrete world as the manifestation of ideal forms, from Spenser's Fowre Hymnes to Stevens's "forms of the sky"; A poet choosing to write a sonnet rather than free verse is making a philosophical decision about form — accepting that the constraints of the form will shape not merely the expression but the thought itself, that form is not a container but a constitutive principle; In Hegel's aesthetics, the history of art is a history of the relationship between form and content — in symbolic art (Egyptian), form exceeds content; in classical art (Greek), form and content achieve perfect unity; in romantic art (modern), content exceeds form; Adorno's claim that form is sedimented content — that the conventions and techniques of artistic form contain the traces of historical experience — makes formalist and historicist approaches to poetry inseparable; Charles Olson's "Projective Verse" manifesto argues for organic, breath-based form against inherited metrical form, making the choice of form a philosophical claim about the relationship between consciousness, body, and world