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.

The chapter outlines a mentalist theory of ethics and law. It clarifies its background in the cognitive revolution of the twentieth century. It discusses mayor conceptual elements like the distinction of competence/performance, performance errors and experimental design and the poverty of stimulus argument. It outlines a detailed descriptive account of principles of moral cognition generating richly structured moral judgments. The content of justice, duties to care for others and respect for human beings are specified. New findings from child psychology indicate that children in early, preverbal states of development operate with normative principles. Approaches like the social intuitionist model and recent theories of moral ontogeny are considered, including models of statistical learning. The epistemology of ethics is a central concern of this chapter, particularly the epistemological merits of universalist accounts of human rights. The argument shows that epistemological universalism does not exclude the possibility of a legitimate pluralism of concrete attempts to bring to life the imperfectly understood (at least by this author) meaning of justice, solidarity and dignity.