Final answer:
The completed line from Keats's 'Ode on a Grecian Urn' is 'Beauty is truth, truth beauty,' indicating a deep connection between beauty and eternal truth in Keats's perspective.
Step-by-step explanation:
The famous line from John Keats's Ode on a Grecian Urn that encapsulates Keats's philosophy is: "Beauty is truth, truth beauty". This line suggests a profound philosophical idea that the aesthetic experience of beauty is intertwined with the eternal truth. In the context of Keats's poem and the discussions by critics like Cleanth Brooks and T. S. Eliot, it highlights a paradox about the relationship between the beauty of art and the truth it conveys, reminiscent of ancient philosophical thought, such as Plato's ideas on transcendent beauty and truth.