This chapter establishes the theory of consent-based monism. The definition of law as well as its origins start for the purpose of this book in the “legal desert.” The legal desert is to be understood as a legal vacuum, a neutral, prelegal status without any further specifications. As we are not focusing on how a just society can be conceived, the following outline will suffice: in the legal desert, a consensus between two or more individuals is widely considered to be the possibility that allows the establishment of a binding legal rule to organize social life. Consensus is thereby understood to serve as a tool for unifying individual interests and does not aim to establish any values or tools that might indicate how a just society should be organized. Choosing this basis aims at modeling the structural relationship between international, EU and national law without saying how society or the State or a supranational organization should be organized (or without arguing, for instance, whether the EU has a constitution or not). This is illustrated by figures, which graphically display the most important consequences of structurally ordering the relationship between legal orders according to consent-based monism.