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Johannes Casearius received his formal education at the Latin School of Franciscus van Enden in Amsterdam (cf. Meinsma, p. 182; Meijer, 1902a, p. 399). ‘Casuarius’ (Casearius) is quite probably the same individual as ‘Johannes Cassarius’ who matriculated as a philosophy student at Leiden University on 12 September 1659. Presumably, he is also the same as ‘Johannes Casear’ who studied theology (21 May 1661) in Leiden. After his studies in Leiden and Utrecht (1665), Casearius was appointed (January 1668) by the Dutch East Indian Company as a Reformed minister in Cochin (Indonesia). Together with Hendrik Adriaan van Reede tot Drakenstein, he edited an impressive botanical encyclopedia, called Hortus Indicus Malabaricus (Amsterdam, 1678–1703). A two-volume translation into Dutch, Malabaarsche kruidhof, was published in 1689. See: NNBW, vol. 5, cols 106–7; Meijer, 1902a.
It is quite likely that Casearius lived under one roof with Spinoza for at least some time in Rijnsburg. How and when relations between Spinoza and Casearius were established remains uncertain. It is also unknown how long they lived under the same roof at the Katwijker Laantje. Spinoza only mentions Casearius in his reply to De Vries (> 1663.02.24, Ep 9). What is of major biographical importance is that the letter of the 24th establishes the fact that Spinoza was teaching selections from Part 2 and 3 of Descartes’s ‘Principles of Philosophy’ to Casearius. He supplemented his lessons with metaphysical queries that cannot be found in Descartes. His private lessons would be ultimately brought to a head in his first book Renati Des Cartes Principiorum philosophiae pars I et II; Cogitata metaphysica, a portion of which he would finish in the late spring that same year in Amsterdam (cf. 1663.07.27, Ep 13).
In > 1663.02.24, Ep 9, Spinoza points out to De Vries that there is absolutely no need to envy his pupil Casearius: ‘No one is more troublesome to me, and there is no one with whom I have to be more on my guard’. Moreover, he also warns him, ‘and all our friends’, not to communicate any of his own philosophical notions to that young adolescent before he reaches the age of majority. Caesarius, Spinoza argues, is still too immature and childish to grasp my views (‘nimis adhuc est puer’), but ‘his talent induces me to like him’. Perhaps, his warning indicates that his pupil planned to return to his birthplace Amsterdam. It is documented that two years later, on 5 October 1665, the municipal Kerkenraad of Amsterdam gave approval to Johannes Caesarius to work in their congregation as a Reformed pastor.