This article builds on my previous article on Dirk H. Th. Vollenhoven (Ive 2015) and provides an overview of the development of the systematic philosophy of Herman Dooyeweerd. This article seeks to provide an overview of the key developments in the thinking of Dooyeweerd, both in the convergences arising from the conversation of the two brothers-in-law and long-term colleagues and in their divergences. Dooyeweerd and Vollenhoven worked within the tradition of Abraham Kuyper, the father of Reformational philosophy. The development of (...) their respective philosophical systematics has tended to be addressed separately. However, I argue that the close cooperation between the two philosophers was critical in the formulation of the philosophy of the cosmonomic Idea. The philosophical conversation between the two persisted, despite critical divergences, largely aired privately. Their respective insights remained a source of mutual stimulus and complementarity foundational for Reformational philosophy. (shrink)