  It is certain, that on the symbolic side, not space and time but four independent variables \(x, y, z, t\) appear;  one speaks of space, as one does of sounds and colours, only on the  side of conscious experience. A monochromatic light signal … has  now become a mathematical formula in which a certain symbol \(F\), called electromagnetic field strength, is expressed as a pure  arithmetically constructed function of four other symbols  \(x, y, z, t\), called spacetime coordinates.