 He was also the secretary of the Council of State in 1664 

Before becoming grand pensionary Van Slingelandt wrote several reports as preparation for the second Great Assembly (Dutch "Tweede Grote Vergadering", a kind of Constitutional Convention to reform the constitution of the Dutch Republic, 28 November 1716 – 14 September 1717), in which he proposed to give the Council of State ("Raad van State") more power.He was convinced of the necessity to restrict the power of the cities and the provinces in order to strengthen the central power of the republic.The Great Assembly however ended in failure when nothing came from Van Slingelandts proposed reforms.