The speaker starts by questioning how much anyone can truly know, and each line chips away a little more. First knowledge looks shaky, then belief, then even the link between what we believe and what is real. The poem's flat, almost logical tone makes this feel like someone thinking aloud, working through a problem step by step. Even the definition of knowledge versus belief ends with a shrug: in practice, the speaker admits, they probably couldn't prove anything.
Then the argument turns practical. If our picture of reality is incomplete, our knowledge imperfect, and the results of our actions beyond our control, why bother trying to do right? The mood is tired and uneasy, close to despair. The final line cuts through it with a sudden, blunt answer: "every moment we must choose or die." Doubt doesn't excuse us from deciding. Not choosing is its own kind of death, so the speaker lands on a stark, urgent conclusion.