Benedictus de Spinoza: Letter in Latin to Johannes Georgius Graevius (Epistolae 49), 14 December 1664. Address Utrecht. The present source <a rel="nofollow" class="external free" href="https://spinozaweb.org/letters/91">https://spinozaweb.org/letters/91</a> states as the ultimate source: The Royal Library, Copenhagen, Thott 1266 4º. "Thott 1266 kvart Spinoza Johannes Georgius Graevius 14-12-1673".
<bdi><a href="https://en.wikipedia.org/wiki/en:Baruch_Spinoza" class="extiw" title="w:en:Baruch Spinoza"><span title="Dutch philosopher (1632–1677)">Benedictus de Spinoza</span></a>
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1673-12-14
<a rel="nofollow" class="external text" href="https://spinozaweb.org/letters/91">spinozaweb.org</a>. Retrieved on 7 July 2020.