Short Answer 
Shelley’s essay “A Defence of Poetry” posits that poetry captures and immortalizes fleeting beauty and emotions. His poem “Mutability” reflects this theme by emphasizing the transient nature of life, reinforcing the idea that poetry serves as a reservoir for these ephemeral experiences.
Step 1: Understand Shelley’s Notion of Poetry
In his essay “A Defence of Poetry,” Shelley asserts that *poetry serves as a vessel* for capturing the ephemeral beauty and emotion found in life. He believes that poetry has the ability to *immortalize* these fleeting sensations, allowing them to be cherished long after they pass. This idea is foundational to understanding the purpose of his poetic works.
Step 2: Analyze the Themes in “Mutability”
In Shelley’s poem “Mutability,” he explores the *transient nature of life’s moments*. The poem illustrates how quickly beauty and emotions can disappear, emphasizing the *inevitability of change*. By focusing on the passing nature of these experiences, the poem mirrors his philosophical claims made in his essay about the role of poetry.
Step 3: Connect the Poem to Shelley’s Ideas
“Mutability” exemplifies the thesis presented in Shelley’s essay by *preserving these fleeting moments* through its verses. The poem encapsulates how poetry can act as a *repository of emotional experiences*, solidifying them against the backdrop of time. Ultimately, it serves to reinforce Shelley’s argument that poetry magnifies and holds onto the brief masterpieces of feeling and beauty within our lives.