In order to analyze the relationship between international, EU and national law properly, this chapter establishes a working hypothesis: a common denominator of international, EU and national law. Analogous to the historical idea of social contract theory, this common denominator is consensus. After revisiting the major criticism of consent as a (moral) obligation to obey the law, the chapter elaborates on the moral duty to adhere to the pacta sunt servanda principle. After discussing the thesis of an innate Universal Moral Grammar (UMG) that relies on an analogy to the thesis of a universal grammar of the human faculty of language in linguistics, this chapter submits that Colwyn Trevarthen’s concept of primary, secondary and tertiary intersubjectivity is more suitable for conceptualizing the origins and development of the biological setup of human morality. This chapter refers to intersubjectivity, as the decisive concept to align morality and its development with core concepts of (developmental) psychology in order to lay bare the origins of moral normativity, which is essential for us as a basis for the pacta sunt servanda principle.

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.