Short Answer 
The theme of immortality in John Keats’ “Ode on a Grecian Urn” emphasizes how art preserves emotions and moments beyond time. Keats illustrates that while audible melodies bring joy, the deeper, unheard melodies evoke everlasting feelings, showcasing the eternal nature of beauty and love captured in art.
Step 1: Understanding Immortal Art
The theme of immortality in art is prominent in John Keats’ “Ode on a Grecian Urn.” This theme suggests that art transcends time, preserving moments and emotions forever. Keats uses vivid imagery to illustrate how elements like music, nature, and love captured in art can outlive their creators and continue to resonate with audiences.
Step 2: Melodies and Their Sweetness
Keats highlights the idea of melodies in his poem, stating, “Heard melodies are sweet.” This line signifies that while audible melodies bring joy, the unheard melodies (those that dwell in the mind) are even more delightful and eternal. This notion emphasizes that art evokes feelings that are everlasting, making them immortal beyond the physical experience of sound.
Step 3: The Eternal Nature of Beauty and Love
In the poem, Keats mentions, “Fair youth, beneath the trees, thou canst not leave thy song.” This conveys that the beauty of the youth and surrounding nature will remain unchanged, showcasing how art captures timeless beauty. Moreover, he states, “She cannot fade, though thou hast not thy bliss.” This indicates that the emotions and beauty represented in art will endure, regardless of the transient nature of human experiences.