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The non-mathematician is seized by a mysterious shuddering when

he hears of "four-dimensional" things, by a feeling not unlike

that awakened by thoughts of the occult. And yet there is no more

common-place statement than that the world in which we live is a

four-dimensional space-time continuum.

Space is a three-dimensional continuum. By this we mean that it

is possible to describe the position of a point (at rest) by

means of three numbers (co-ordinates) x, y, z, and that there is

an indefinite number of points in the neighbourhood of this one,

the position of which can be described by co-ordinates such as

x1, y1, z1, which may be as near as we choose to the respective

values of the co-ordinates x, y, z, of the first point. In virtue

of the latter property we speak of a "continuum," and owing to

the fact that there are three co-ordinates we speak of it as

being "three-dimensional."