There’s a whole lot of imagining and speculating in this poem. For example, from lines 30-37, our speaker imagines the thoughts his neighbor might think if he questions the necessity of his old stone wall. The speaker’s reality and that of his neighbor are very different, and these contrasting versions of reality form the backbone of the juicy debate which takes place in the world of this poem: old vs. new, tradition vs. innovation, isolation vs. community.
If there exists no wall to plague him, our speaker wouldn’t be nearly as imaginative as he is.
Tradition stunts the neighbor’s ability to imagine or accept other versions of reality.