Benedictus de Spinoza: Letter in Latin to Johannes Georgius Graevius (Epistolae 49), 14 December 1664. The present source https://spinozaweb.org/letters/91 states as the ultimate source: The Royal Library, Copenhagen, Thott 1266 4º. "Thott 1266 kvart Spinoza Johannes Georgius Graevius 14-12-1673".
File:Benedictus_de_Spinoza_-_Letter_in_Latin_to_Johannes_Georgius_Graevius_(Epistolae_49),_14_December_1664_-_Signature.jpg. spinozaweb.org. Retrieved on 7 July 2020.
Benedictus de Spinoza