However, there is a difference between an urn and a nightingale in that the nightingale is not an eternal entity. Furthermore, in creating any aspect of the nightingale immortal during the poem the narrator separates any union that he can have with the nightingale. The nightingale's song within the poem is connected to the art of music in a way that the urn in "Ode on a Grecian Urn" is connected to the art of sculpture. As such, the nightingale would represent an enchanting presence and, unlike the urn, is directly connected to nature. As natural music, the song is for beauty and lacks a message of truth. Keats follows Coleridge's belief, as found in "The Nightingale", in separating from the world by losing himself in the bird's song. Although Keats favours a female nightingale over Coleridge's masculine bird, both reject the traditional depiction of the nightingale as related to the tragedy of Philomela. Their songbird is a happy nightingale that lacks the melancholic feel of previous poetic depictions. The bird is only a voice within the poem, but it is a voice that compels the narrator to join with in and forget the sorrows of the world. However, there is tension in that the narrator holds Keats's guilt regarding the death of Tom Keats, his brother. The song's conclusion represents the result of trying to escape into the realm of fancy. Like Percy Bysshe Shelley's "To a Skylark", Keats's narrator listens to a bird song, but listening to the song within "Ode to a Nightingale" is almost painful and similar to death. The narrator seeks to be with the nightingale and abandons his sense of vision in order to embrace the sound in an attempt to share in the darkness with the bird. As the poem ends, the trance caused by the nightingale is broken and the narrator is left wondering if it was a real vision or just a dream. The poem's reliance on the process of sleeping is common to Keats's poems, and "Ode to a Nightingale" shares many of the same themes as Keats' "Sleep and Poetry" and "Eve of St. Agnes". This further separates the image of the nightingale's song from its closest comparative image, the urn as represented in "Ode on a Grecian Urn". The nightingale is distant and mysterious, and even disappears at the end of the poem.