We can determine the ratio of the units of length and time beforehand in such a way that the natural limit of velocity becomes c=1. If we introduce, further, Ö -1´t = t (tau) in place of t, the quadratic differential expression d2 = dx2 dy2 dz2 ds2 thus becomes perfectly symmetrical in x, y, z, s; and this symmetry is communicated to any law which does not contradict the world-postulate. Thus the essence of this postulate may be clothed mathematically in a very pregnant manner in the mystic formula: 3´105km=Ö-1 secs (our italics).
... Minkowsky's discovery [...] must be found mainly in the fact of his acknowledgment that the four-dimensional continuum of relativity [...] shows a pronounced relationship with the 3-dimensional continuum of Euclidean space. However, to give the right importance to this relationship we should replace the usual coordinate t with an imaginary value t=Ö -1´ct proportional to it. In such conditions, natural laws that satisfy the special theory of relativity exigencies assume mathematical forms where the time coordinates act precisely in the same way as space coordinates*.