Heraclitus asserted that no one can step into the same river twice. Zeno, his disciple, wanting to go further, asserted that no one can step into the same river even once. In trying to go further, Zeno and the Eleatics denied motion and set themselves back to what Heraclitus had abandoned.
The main point of Problema III is that, though the ethical calls for disclosure, Abraham could not speak. While it may not be "ethically defensible" for Abraham to conceal his undertaking, Abraham's relation to God brings him above the ethical. God's command to Abraham is unique to Abraham and is made only to Abraham, and so Abraham enters into a private relationship with God. As a result, this command isolates Abraham from the universal. Because the command is unique and private, it cannot be transmitted to other people in an understandable manner. How can Abraham explain to others that the ethical is his temptation: "I am suffering great anxiety because I am faced with the constant temptation to do what I, and everyone else, knows to be right"?
It may help to understand Abraham's paradox if we look at his story not from his perspective, but from someone else's. Suppose I were to meet Abraham on the street: if he were to tell me God had told him to kill his only son, I would think he was crazy. There is no proof of any kind that Abraham can produce to convince me that killing Isaac is indeed God's will. The command came privately to Abraham, and it cannot be shared or explained to anyone else. Further, if Abraham were to share his ordeal, it would no longer be a private ordeal that he shares with God. He would be making it public and would thus be creating a gulf between himself and God.
Therein lies the difficulty of Abraham's words to Isaac. In asking a question, Isaac calls for Abraham to speak, but Abraham cannot possibly tell the direct truth without breaking his covenant with God. While Abraham does not lie, he speaks in a way that cannot be understood. God will provide a lamb for the burnt offering only by virtue of the absurd, and so those words can only be understood by Abraham himself, who has made the leap of faith into the absurd. Abraham speaks the truth, but a truth that only he can understand. Irony and paradox both deal with contradiction, so it is only fitting that irony should be the only suitable manner of speech that can express the paradox.
The epilogue returns once more to the theme of the preface: everyone wants to move beyond faith, thinking that it is easily achieved. Hegel has a grand theory of history, according to which everything slowly progresses toward a final, utopic, synthesis. We build upon the knowledge and experiences of past generations until we arrive at the truth. For instance, Einstein's relativity could not have been discovered before the formulation of Maxwell's equations or Newton's laws. Einstein may have been brilliant, but he was still building upon past discoveries.
Johannes suggests that faith is not like science: we cannot pick up where the previous generation left off. Faith, like love, is a kind of passion, and we cannot pick up passion by proxy. The value of faith lies not in reflecting upon it disinterestedly, but in throwing oneself passionately into it. Faith must be experienced, not just intellectualized.