Appendices: 1. Simple Derivation of the Lorentz Transformation 2. Minowski's Four-dimensional Space ("World") 3. The Experimental Confirmation of the General Theory of Relativity (a) Motion of the Perihelion of Mercury (b) Deflection of Light by a Gravitational Field (c) Displacement of Spectral Lines towards the Red 4. The Structure of Space according to the General Theory of Relativity 5. Relativity and the Problem of Space