Sometimes nature guards her secrets with the unbreakable grip of physical law. Sometimes the true nature of reality beckons from just beyond the horizon.

Greene contrasts two limits on human knowledge. In one case, “physical law” functions as a hard boundary: the universe may be structured so that certain facts are in principle inaccessible (for example, because of horizons, quantum limits, or constraints on measurement). In the other case, reality is portrayed as tantalizingly close—“just beyond the horizon”—suggesting that some mysteries are not forbidden but merely not yet reached by current theory, technology, or conceptual frameworks. The pairing captures a central theme in modern physics: progress comes from pushing against what seems unknowable, while also recognizing that nature may contain genuine, law-imposed ceilings on what can be observed or explained.