Later, in the seventeenth century, several Dutch legal scholars, including Christian Rodenburg, Paulus Voet, Johannes Voet, and Ulrik Huber, further expounded the jurisprudence of conflict of laws. Their key conceptual contributions were twofold: First, nations are wholly sovereign within their borders and therefore cannot be compelled to enforce foreign law in their own courts. Second, in order for international conflicts of law to work rationally, nations must exercise comity in enforcing others' laws, because it is in their mutual interest to do so.