Short Answer 
In John Keats’ imagery, “spirit ditties” symbolize ethereal, timeless songs that transcend reality and connect with the spirit, despite lacking actual sound. The urn, personified as a “foster-child” of Silence and Time, serves as a keeper of eternal moments, emphasizing the enduring impact of art and the preservation of beauty beyond the passage of time.
Step 1: Understanding “Spirit Ditties”
In John Keats’ imagery, the term “spirit ditties” refers to ethereal songs played by the figurines on the urn. These songs evoke a sense of timelessness and beauty, existing beyond the confines of reality. They symbolize the concept that music and art can transcend normal sensory experience and connect with the spirit.
Step 2: The Nature of the Songs
Keats elaborates that these songs are “ditties of no tone,” indicating that they do not have actual sound or melody in the physical world. Instead, they represent a harmony that exists purely in the realm of imagination and emotion. The void of sound does not diminish their emotional impact; rather, it enhances the mystical quality of the urn’s artistry.
Step 3: The Symbolic Role of the Urn
The urn is personified as a “foster-child” of “Silence” and “slow Time,” emphasizing its role as a keeper of eternal moments and stories. This adoption metaphor highlights how the urn has been imbued with meanings that defy the passage of time. The imagery of being nurtured by Silence and Time suggests a contemplative existence, where passed moments are preserved forever.