"The discovery of the invariantdx^2 + dy^2 + dz^2 -c^2dt^2whose value we shall designate ds^2 marks the date of immense importance in the history of natural philosophy. ...It mattered not whether we were situated in this frame or in that one; in every case ...it still maintained the same value when referred to any other frame. ...we were in the presence of something which, contrary to a distance in space or a duration in time, transcended our variable points of view ...a common absolute world underlying the relativity of physical space and time. Minkowski immediately recognised in the mathematical form of this invariant the expression of the square of the distance in a four-demensional continuum. This distance was termed the Einsteinian interval, or, more simply, the interval. ...The continuum was neither space nor time, but it pertained to both ...it may appear strange that measurements with clocks can be co-ordinated with measurements with rods or scales. This difficulty, however, need not arrest us; for although dt is a time which can only be measured with a clock, yet cdt, being the product of a velocity by a time, is a spatial length since it represents the distance covered by light in the time dt."