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Similarly, the world of physical phenomena which was briefly

called "world" by Minkowski is naturally four dimensional in the

space-time sense. For it is composed of individual events, each

of which is described by four numbers, namely, three space

co-ordinates x, y, z, and a time co-ordinate, the time value t.

The "world" is in this sense also a continuum; for to every event

there are as many "neighbouring" events (realised or at least

thinkable) as we care to choose, the co-ordinates x1, y1, z1, t1

of which differ by an indefinitely small amount from those of the

event x, y, z, t originally considered. That we have not been

accustomed to regard the world in this sense as a

four-dimensional continuum is due to the fact that in physics,

before the advent of the theory of relativity, time played a

different and more independent role, as compared with the space

coordinates. It is for this reason that we have been in the habit

of treating time as an independent continuum.