This chapter is an intellectual history of ius gentium et naturae for a volume on the relations between international law and Christianities. It examines how various early modern conceptions of conscientia relate to the emergence of ius gentium et naturae in the so-called Age of Conscience. It zooms in on the development of 'modern' ius gentium et naturae amidst the early modern fight over the human conscience, in particular through the work of Hugo Grotius.