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The reader may think that such a description of the world would

be quite inadequate. What does it mean to assign to an event the

particular co-ordinates x1, x2, x3, x4, if in themselves these

co-ordinates have no significance? More careful consideration

shows, however, that this anxiety is unfounded. Let us consider,

for instance, a material point with any kind of motion. If this

point had only a momentary existence without duration, then it

would to described in space-time by a single system of values x1,

x2, x3, x4. Thus its permanent existence must be characterised by

an infinitely large number of such systems of values, the

co-ordinate values of which are so close together as to give

continuity; corresponding to the material point, we thus have a

(uni-dimensional) line in the four-dimensional continuum. In the

same way, any such lines in our continuum correspond to many

points in motion.