 Froebe developed a liking for the Ritsemas and invited them to stay the next winter at the Casa Eranos.From then on the Ritsemas would spend a month every year as Froebe’s guests, and a warm friendship developed among them (Froebe was also deeply interested in the "I Ching").In 1956 Ritsema began to collaborate with Froebe and the Swiss zoologist and natural philosopher Adolf Portmann (1897–1982) in the planning of the Eranos Conferences.