Robert Frost's poem "Mending Wall" is a poem that questions traditions. One neighbor is in favor of having a wall between their property simply because it's traditional, whereas the other neighbor sees the needlessness of the wall and how it only serves to separate society.
The first point of view is expressed by the speaker's neighbor who says, not once but twice, "Good fences make good neighbors." In other words, in his view, the only thing in this world that makes a good neighbor is something separating us from our neighbors. Since neighbors offer companionship, he is arguing that the only nice companions in this world are the inanimate objects called fences. Yet, the speaker also notes that the neighbor sees no real reason for having the wall between them except that it is traditional; his father before him had a wall, so he'll have one too.
In contrast, the second point of view is expressed when the speaker notes the needless separation between them that the wall creates, as well as the needless work. The speaker depicts the needless separation when he notes that on certain days during the spring, the two neighbors meet to repair the wall together and "set the wall between us once again." He further depicts the separation the wall creates when he notes, "We keep the wall between us as we go." The speaker also notes the frivolity of the wall by pointing out that the wall is only separating pine trees from an apple orchard, and his apples aren't likely to cross over and eat his neighbor's pine cones. The speaker points out that a wall makes sense when it keeps cows secured on the property, but there are no cows to secure.
By the end of the poem, the reader agrees with the speaker in saying that his neighbor "moves in darkness" that is not caused by just the shade of the woods or other trees. To be in darkness can be seen as another way of saying that someone is unenlightened. In other words, the speaker is asserting that his neighbor is unenlightened by being stuck in the past and holding onto ideas that made sense only in the past.