What is a principal purpose of using the word "shadow" in the poem?

The use of the word "shadow" in the poem serves to highlight the contrast with the meaning of "substance." In literature, shadows often represent the ephemeral, elusive, or insubstantial aspects of existence, which can stand in stark contrast to the idea of substantive reality. By emphasizing this dichotomy, the poet engages the reader in deeper contemplation about what is real versus what is merely an impression, evoking a sense of deeper thematic significance. This symbolic usage invites the reader to reflect on the nature of existence and how perceptions can differ from reality.

While other concepts such as mystery, the passing of time, and setting imagery each play important roles in poetry, the primary focus here is on the interplay between the tangible and intangible, where "shadow" becomes a vehicle for exploring the essence of what is not visible or concrete.