Theodorus Kerckring

Theodorus Kerckring studied medicine in Leiden (12 May 1659) under Dele Boë Sylvius. He first described the permanent transverse folds of the luminal surface of the small intestine. He was a collector of anatomical and pathological materials he displayed in his house at the Amsterdam Keizersgracht (cf. Kooijmans, 2004, p. 83). In 1678, Kerckring left the Netherlands. He spent time in England and France (where he converted to Catholicism) and finally settled in Hamburg. Kerckring wrote books on medical issues as well as on alchemy: Anthropogeniae ichnographia sive Conformatio foetus ab ovo usque ad ossificationis principia, in supplementum osteogeniae foetuum, Amsterdam, 1671; repr., Paris, 1672); Commentarius in currum triumphalem antimonii Basilii Valentini, Amsterdam, 1671; repr., Amsterdam: 1685. See: NNBW, vol. 2, col. 663; Nicholls, 1940; Van der Tak, 1982, Lindeboom,1984.

Theodorus Kerckring praised one of Spinoza's microscopes in Spicilegium anatomicum (Anatomical Gleanings), an illustrated collection of rare clinical observations, medical curiosities and autopsy discoveries, a copy of which was owned by Spinoza (perhaps a dedication copy given to him by Kerckring) (Offenberg, Spinoza’s Library, p. 318, no. 33). The second part of that same book focuses on foetal osteology (the study of the human bones of the foetus). One of the observations described by Kerckring in the first part of Spicilegium revolves around the lymphatic vascular bundles. Kerckring, according to his own testimony, made all of his observations with the help of an ‘excellent microscope’ made by Spinoza:

I, too, have an excellent microscope made by that noble mathematician and philosopher Benedictus Spinoza, through which the lymph vessels, when they enter their conglobate glands, seem to be split into various filaments, and when their extensions come out they seem to unite again into one… Because so far this is based only on the authority of the microscope, I rather want to suspend my opinion than to draw the wrong conclusion, because what is seen only seems so.

Source: (Kerckring, 1670, p. 178, no. 93).

It is uncertain how Kerckring came into the possession of that microscope. Perhaps he simply purchased the optical device from Spinoza, but he may also have received it from him as a gift. It is undocumented how and when contacts between the two men first emerged. They may have met in Amsterdam at the Latin School of Van den Enden during the mid-1650s, or later, when Kerckring enrolled as a medical student in Leiden and Spinoza settled (summer of 1661) in Rijnsburg. Alleged rumours put forward by biographer Johannes Colerus concerning rivalry between Kerckring and Spinoza over Van den Enden’s eldest daughter Clara Maria seem to have been based on ill-founded speculations which lack any historical evidence (cf. Walther and Czelinski, vol. 1, p. 100). With respect to their later contacts, the remnants of a (lost) letter of Spinoza to Jarig Jelles (see 1673.04.19, Ep 48B) proves that, in early 1673, the philosopher was still in contact with Kerckring and gave him some assignment in anatomy.