And here certain aspects of the problem must be noted. In the first place, 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
is a time which can only be measured with a clock, yet
, being the product of a velocity by a time, is a spatial length since it represents the spatial distance covered by light in the time
. For this reason we may consider our four-dimensional continuum to possess the qualifications of an extensional space.
Our next problem is to determine the nature of the geometry of this mysterious continuum. The mere fact that it has been possible to write the expression of this square of the interval with squared magnitudes, such as
,