Almost everything in this poem is an instruction. The speaker tells us to picture an apple, to look at a lecturer, to arrange facts, to trace history. Each command shows a way we organize the world with lines: diagrams, chalk gestures in the air, slanted charts, curves of statistics, even the old picture of humans rising "up from the ape." Even the apple, with its green side and red side, ends up joined by a line.
The speaker never says outright how they feel, but the tone gives them away. The short, bossy sentences and the dry "how else?" sound slightly amused, maybe a little uneasy. The last two lines turn the poem toward a quiet warning: line is "inescapable," and we can't even question whether it's really there. The poem suggests that the line may be a habit of our minds rather than a fact about the world. The mood is cool and analytical, with a faint chill underneath, like being shown the bars of a cage we never noticed.