Sloterdijk tells the story of how his Sphären project was inspired by Heidegger’s ‘artwork’.

I was also fascinated by a chalkboard drawing Martin Heidegger made around 1960, in a seminar in Switzerland, in order to help psychiatrists better understand his ontological theses. As far as I know, this is the only time that Heidegger made use of visual means to illustrate logical facts; he otherwise rejected such antiphilosophical aids. In the drawing, one can see five arrows, each of which is rushing toward a single semicircular horizon—a magnificently abstract symbolization of the term Dasein as the state of being cast in the direction of an always-receding world horizon (unfortunately, it’s not known how the psychiatrists reacted to it). But I still recall how my antenna began to buzz back then, and during the following years a veritable archaeology of spatial thought emerged from this impulse.

“Against Gravity: Bettina Funcke talks with Peter Sloterdijk”, Book Forum, 2005.

This was the Zollikoner Seminare with Medard Boss, and the diagram is shown on p. 3 of the German edition.