This chapter begins with an etymological and historical elucidation of the terms conscientia and synderesis. Philosophical and theological reflection on these terms, beginning with St. Jerome and proceeding through thinkers such as Peter Lombard and Philip the Chancellor, constitutes the background against which St. Thomas Aquinas develops his understanding, not only of conscientia and synderesis, but also of objective right (ius) or rights. Much of the debate regarding synderesis, the infallible basis of conscientia, concerns whether it is a power or habitus. Aquinas settles on understanding synderesis as a ‘habit’ of the potential intellect – which, following Aristotle, he understands as a sort of ‘blank slate’ upon which things can be written. One of the things written on the habit of synderesis is the practical version of the principle of non-contradiction: ‘good is to be done and pursued, evil avoided’. This allows him to develop a theory according to which objective rights are primary, although not to the exclusion of subjective rights. It also allows those in agreement with Aquinas to exclude subjective rights that contradict established objective rights.

John McGuckin examines early writings of Christian theologians about conscience. These relied upon the Scriptures and upon Plato, Aristotle, and others, and they regularly grappled with the positive and negative possibilities of conscience. But Christian writers made religious inquiries. St. Paul wrote about conscience in connection with a law “written on the heart,” a guide to everyone, but fallible and in need of the grace of Christ. The Greek fathers, led by Origen, emphasized conscience as an awareness of divine things, albeit impaired by man’s fall, yet still oriented to God and to obedience to moral norms. The Latin fathers, led by Augustine, emphasized humanity’s corruption after the fall and need for divine grace. Thus, conscience may convict, but might not provoke a person to goodness. Only loving God could do this. In both Latin and Greek thought, therefore, conscience was more than an inner voice instructing about right and wrong. It was a set of reflections on the spiritual identity of human beings.

What to focus on in an intellectual history of ius gentium et naturae for a volume on the relations between international law and Christianities? For centuries, (international) law and Christian theology maintained intensive and complex relations, which it is impossible to do justice to within the scope of this chapter. With the more recent “turn to history” in international legal scholarship, discussions of the relationship between ius gentium et naturae and Christianity generally center on secularization and/or empire. For obvious reasons both sets of histories deal with early modernity – the time that the so-called Respublica Christiana or Holy Roman Empire was profoundly affected by Reformations, gradually fragmented, and religious and theological fights were part of the politics of the newly emerging European nation-states.