If the state did not exist would it be necessary to invent it? Would one be needed, and would it have to be invented? These questions arise for political philosophy and for a theory explaining political phenomena, and are answered by investigating the “state of nature,” to use the terminology of traditional political theory. The justification for resuscitating this archaic notion would have to be the fruitfulness, interest, and far-reaching implications of the theory that results. For the (less trusting) readers who desire some assurance in advance, this chapter discusses reasons why it is important to pursue state-of-nature theory, reasons for thinking that theory would be a fruitful one. These reasons necessarily are somewhat abstract and metatheoretical. The best reason is the developed theory itself.