Fruin, a lifelong bachelor, studied classical philology at Leiden University from 1842, and received his Ph.D. on 18 December 1847 with a dissertation on Manetho, entitled De Manethone Sebennytha.
In this debate Fruin took the liberal side and he conducted a learned polemic with a fellow eminent Dutch historian, who was also a leader of the Conservative opposition to the new political ideas, Guillaume Groen van Prinsterer.
[3] In his inaugural oration (held on 1 July 1860) after his appointment as the first professor of Dutch national history at Leiden University on 20 February 1860 (which was preceded by his promotion to prorector of his gymnasium in 1859), which was entitled Onpartijdigheid van den geschiedschrijver (Impartiality of the historian), Fruin defended the point of view that historians needed to be impartial in their historiography.
He specialized in certain historical periods and subjects, like the early years of the Dutch Republic and the stadtholderate of stadtholder William III.
Beside his interpretive historical work, he also published two important compilations of sources: His defense of impartiality in historiography as a matter of principle did not prevent him from taking a stand occasionally, if he felt truth and fairness made it necessary.
[8] Another characterizing example is the altercation he had with general Willem Jan Knoop, a Dutch military historian, about the question whether stadtholder William III had known of the fact that the Treaty of Nijmegen had already been signed, when he fought the bloody Battle of Saint-Denis (1678).
However, new material brought to light that he must have known, and this so shocked Fruin's faith in William that he everafter displayed a distinct coolness to this heretofore revered subject.