The “pen” (Line 2) in “When I Have Fears that I Will Cease to Be” is a symbol for artistic creation. Books are just the vessels to hold all the speaker’s best ideas. It’s “in charact’ry” (Line 3) for them to do so. The pen gains supreme importance as the tool that brings “high piled books” (Line 3) into existence. The pen represents the moment in the creative process when inspiration strikes. Great writing happens when the writer’s ideas mature into clarity. Taken another way, the pen moves, brings thoughts into physical being, and makes those thoughts clear as the stark difference between shadow and light. The laudable accomplishment of creation is appropriately challenging. If the speaker wants to trace “shadows” (Line 8) with ink, they will do so alongside (or perhaps in spite of) the competing hand of chance and all its powers. It takes the skill to trace and the courage to pick up the pen.