Endymion was the Romantic poet John Keats’s first big, ambitious project, a book-length telling of the Greek myth in which the goddess of the moon falls in love with a sleeping shepherd. As the 21-year-old Keats saw it, writing a long narrative poem on a classical subject was a test of his artistic mettle, a way to aim for the greatness of Milton or Chaucer. The poem’s very form suggests the scope of Keats's ambition here:

• Endymion is written (for the most part) in heroic couplets—paired rhyming lines of iambic pentameter.
• That means that each line uses five iambs, metrical feet with a da-DUM rhythm: "A thing | of beau- | ty is | a joy | for ever." (The extra unstressed syllable on the end there is known as a feminine ending, a softening effect that Keats often reaches for in Endymion.)
• This is the same form that Chaucer used in The Canterbury Tales and the form in which Keats would have encountered English translations of the Greek epics the Aeneid and the Odyssey. Grand company!

Endymion was thus both Keats's creative challenge to himself and his declaration to the world—a way of saying, "Hello, I'm John Keats, and I'm on the road to glory." This "Poetic Romance' (not a love story, but a fantastical quest tale, though a love story forms part of that tale) was made to be his making.

However, Keats's ambition to be, as he once wrote, "among the English poets after my death" was far from his only motive in writing this poem. Keats truly adored Greek myth, and he loved poetry with joy, awe, and fervor. Though Keats would look back on Endymion as an adolescent embarrassment, the poem nonetheless captured a lot of the qualities that would make Keats, Keats: sensuous delight, a longing for enchantment, an awed curiosity about the workings of art, and a deep faith in beauty.

The lines this guide examines form Keats's introduction to the poem proper, a preface that explains why and how the author is going to write the rest of the thing. A young man's work the poem may be, but the first lines ring with conviction—and have become some of the most famous verses in the English language.