According to Van Ginneken (opcit, 15/5, p.263) one of the letters can be dated just before the Sint Lucas exhibition of 24 April 1910. The letter dated 7.7.'10 Laren is an interesting letter from an art-historical point of view. He tells Spoor about Conrad Kikkerts' plan to organize a new art society in which he, Spoor, Toorop and Mondriaan and others should participate. This became later 'De Moderne kunstkring'. He also criticizes the Dutch art scene "dit ogenblik is het wel de geschikste tijd tot het vormen eener combinatie, .... die eens zal laten zien, dat er in ons land nog wat anders gefokt wordt dan Eerelmannetjes, Schwartzetjes, Prinsjes Wallpeeneetjes...., Schaapjes, Gortertjes enz."
Although Sluyters admires Mondriaan as a person and his work, he stated in this letter that probably Mondriaan isn't equal to his sudden fame: "Piet Mondriaan doet zoo vreemd tegenwoordig, die voelt zich zoo geweldig..." (op.cit 15/5, p.264)