Adriaan Koerbagh (Keurbagh) must have belonged to the inner core of Spinoza’s Amsterdam circle of friends, although we are largely ignorant about the details of their relationship. His name appears to have been carefully removed from Spinoza’s correspondence published in the OP, and Spinoza’s early biographers, including Pierre Bayle, also remained silent about Koerbagh. He was born in 1632 in Amsterdam, the son of a well-to-do potter and corn carrier, who died in 1644, after which Lambert Reijnst, future sheriff and mayor of Amsterdam, came to serve as the guardian of Adriaan, his sister Lucia and his younger brother Johannes, born in 1634. For most of their lives, Adriaan and Johannes lived at their mother’s house at the Singel, no. 49. Neither of them ever married and they remained close throughout their lives. In 1653 they jointly matriculated at Utrecht University. Two years later they moved to Franeker University and in 1656 they continued their studies at the University of Leiden. While Adriaan took doctorates in medicine (1659) and law (1661), Johannes, in 1659, took up theology, at Groningen University.