In 1676 Leibniz found a pretext to visit Spinoza in The Hague, having learned that Spinoza was at work on a philosophical treatise of great importance. Spinoza showed Leibniz the manuscript of the Ethics, and the two men discussed philosophy together over several days. Although there is no written record of their conversation, it seems likely that these discussions were among the most rewarding in the whole history of philosophy.
When Leibniz met Spinoza
Nicholas Jolley, Leibniz (New York: Routledge, 2005), p. 18.