The speaker is cautious about feeling sure of anything. In the opening lines, they admit they won't trust a thought or conclusion until it is properly fixed in their mind, as if certainty has to be earned rather than assumed. The word "dare" gives that caution a hint of nervousness, almost superstition.
The middle lines shift to a humble, physical scene: a rusted table that isn't pretty, books that aren't neat. Nothing here is polished, and the speaker doesn't apologize for it. This looks like a workspace where thinking actually happens, messy and imperfect.
The last lines suggest that a short stretch of time, "an hour or two," gets gathered up and brought to a head, as if the effort finally pays off in a moment of clarity. The mood is quiet, modest, and focused. Certainty here isn't grand. It comes from patient work in a plain, untidy place.