In this chapter, we describe and explicate the theory of legitimacy in informal terms. However, while our discussion is much less formal than the presentation in the following chapter, a bit of notation will make the discussion here more precise and succinct. Specifically, the basic primitives of our theory are a set of feasible policies, denoted by X, and a set of principles, denoted by P. In addition, when we are discussing a particular principle in P, we will denote it by p. As described in more detail in this chapter, a principle p ∈ P is simply a generalized description of how the various possible policy choices are compared or ranked. The principle is akin to the collective ordering denoted by ≻ in the first part of the book, which we used to denote the output of an aggregative process, f. Thus, a principle represents a method of comparing alternatives that may take many factors into account and that may have been generated via an aggregative procedure. However, in the remainder of the book, we deliberately set aside the question of how the aggregative process of generating a principle worked, the axioms it satisfied, etc. What we take as given is that the principle, as it stands, is the standard by which alternatives are measured and represents the accepted way of choosing alternatives.