Previous chapter: DEFEND AGAINST PHYSICAL THREATS | Multipolar Active Shields Security in the physical realm is an extraordinarily hard problem, but an inescapable one. Our civilization now also rests on software infrastructure. Security in the digital realm is now also an inescapable problem. But the current software infrastructure is not just insecure, it is insecurable. Digital security is also extraordinarily hard, but it is differently hard. Reasoning by analogy with physical security is fraught with peril. Let’s begin by explaining how these realms differ, and how to approach computer security.