In his book on The Foundation of the General Theory of Relativity Einstein gives two motives which called for the extension of the primary relativity postulate upon which the special theory was based. The first was the well-known fact of the equality of inertial mass and gravitational mass, i.e. a fact of a purely physical nature, whereas the other was a consideration of an epistemological nature, a consideration about the principle of causality. It is precisely that philosophical argument drawn from epistemology, an "erkenntnistheoretisches Argument" that is placed in the foreground by Einstein. In order to see what it looks like, let us present the relevant passage of Einstein"s book, in which he develops his idea using a very simple example: ¹⁴