Theory:

The poem 'A Slumber Did My Spirit Seal' by William Wordsworth was published in his most famous poetry collection named 'Lyrical Ballads'. The poem pertains to Romanticism, a literary genre that overtook England during the \(1800s\). The poem talks about the concept of death from a different perspective. Death is the most cruel thing that happens in the life of humans. People recognise the value of life only after having lost a person. The poem belongs to Wordsworth's 'Lucy Poems', which was a series of five poems that he dedicated to his beloved Lucy. The "Lucy poems" consist of "Strange fits of passion have I known", "She dwelt among the untrodden ways", "I travelled among unknown men", "Three years she grew in sun and shower", and "A slumber did my spirit seal". The poet does not reveal the identity of Lucy. There are speculations that Lucy might be a fantasy character created by the poet. Some believe that she could be based on the poet's sister Dorothy. The poet could also have used the character as a literary device to meditate upon his own thoughts. It also belongs to the category of Elegy Poems, which is a poem that laments the dead. The poet laments the unknown Lucy's death in the poem.
Reference:
https://en.wikipedia.org/wiki/The_Lucy_poems