This fragment of a quote from Robert A. Heinlein's "The Moon is a Harsh Mistress" touches on a profound aspect of human nature: our contrasting responses to known and unknown threats. Known dangers, however perilous, often come with a certain level of predictability and control. Humans can prepare for, plan against, and sometimes even accept these dangers because they understand the risks involved. This understanding can empower courage and rational action in the face of adversity.

However, the unknown presents a fundamentally different challenge. It provokes fear largely because it introduces uncertainty, randomness, and the absence of control. Unknown threats escape the grasp of our experience and predictions, making us vulnerable not just physically, but psychologically. This fear of the unknown can paralyze or motivate, depending on the individual’s resilience and mindset.

Heinlein’s insight suggests that much of human fear isn't just about the immediacy or severity of potential harm but often revolves around what is unpredictable and undefined. It aligns well with many psychological theories that claim uncertainty is one of the greatest sources of anxiety, surpassing actual danger in many cases.

This perspective invites readers to reflect on their personal encounters with fear. How often have we been more frightened by the uncertainty following an event than the event itself? It encourages embracing knowledge, risk assessment, and confronting the unknown with informed courage. Such courage is foundational not only in survival but in progress and innovation, where venturing into the unknown is a necessity.

In summary, this quote elegantly captures a fundamental truth about the human condition and challenges us to find strength in confronting uncertainty.