As Newton had by now entered his eighth decade, it can be no surprise that the correspondence in this sixth volume shows a marked decline in his activity and intellectual vigour. While the number of extant letters written by him on other that Mint business is relatively small, the majority of them are devoted to his controversy with Leibniz - Newton's dominant interest during this period. The correspondence of Newton shades gradually into the correspondence of the Newtonians. Thus notably Keill, De Moivre, Chamberlayne, Brook Taylor, the Abbe Conti and Des Maizeaux interested themselves in the calculus dispute, all of them (except the first) having frequent opportunities for personal conversation with Newton.