Final answer:
John Keats's first great poem is "Ode to Psyche," but "Ode on a Grecian Urn" is also a notable work that delves into the themes of art, beauty, and truth.
Step-by-step explanation:
The first great poem by John Keats, which cemented his reputation as a master poet, is generally considered to be "Ode to Psyche". However, it is essential to note that his work "Ode on a Grecian Urn" is also widely recognized for its profound beauty and depth. In "Ode on a Grecian Urn", Keats explores the relationship between beauty, art, and truth, particularly through the controversial last lines of the poem, "Beauty is truth, truth beauty." Critics such as T.S. Eliot have debated the meaning of these lines, while others like Cleanth Brooks have defended the unity and coherence of the poem around its central paradox. Ultimately, Keats' odes are pivotal compositions in English literature, capturing complex emotions and philosophical contemplations about aesthetics and the human experience.