Wittgenstein states that “the exploration of logic means the exploration of everything that is subject to law” (TLP §6.3). This chapter argues that what makes a law a law is the necessity of its application. Stopping one’s automobile at a red traffic light is the law. There is no physical necessity whatsoever forcing one to stop the car. And yet, the law does not state that one may stop the car at a red light, it clearly states that one must. Laws are a sub-set of rules. Generally speaking, rules are to be followed. This entails two aspects of rules and necessity which are investigated in this chapter. The first is that rule-systems have their own internal logic (which is what is being followed). The second is that rules-systems bear a unique kind of force, logical in its nature, which we understand as the necessity to follow the rules, an understanding which leads to the compulsion to do so.