From autumn 1916 onward, Einstein exchanged letters with de Sitter – they discuss the consequences of the Theory of Relativity for cosmology and for Mach’s principle. De Sitter, an opponent of the principle, disclosed to Einstein that the cosmological constant did not fulfill its intended purpose – even with it Einstein’s equations allow for a universe that does not follow Mach’s principle. If a single test particle existed in the world, that is, there were no sun and stars, etc., it would have inertia.