The inmates of my cottage, all at rest, Have left me to that solitude, which suits Abstruser musings: save that at my side My cradled infant slumbers peacefully. (4-7)
Being alone is an important part of Coleridge's poem. If the other people in the cottage were up and about, his train of thought would probably be totally different. But since he's meditating on the extreme quietness of the natural world (and the town), this helps determine where the poem goes.
'Tis calm indeed! so calm, that it disturbs And vexes meditation with its strange And extreme silentness. (8-10)
You'd think it would help meditation—but, like people say in horror movies, it's too quiet. Coleridge doesn't have trouble coming up with some deep thoughts, but they do emerge in a way that seems a little disquieted and disjointed. The poem is an inspired ramble.
[…] the thin blue flame Lies on my low-burnt fire, and quivers not; Only that film, which fluttered on the grate,
Still flutters there, the sole unquiet thing. Methinks, its motion in this hush of nature Gives it dim sympathies with me who live, (13-18)
Since Coleridge is the only person in the house who is awake and meditating on his life, he's like the film of soot—since its fluttering mimics the movement of thoughts in the mind. There's a kind of isolated feeling to the image.