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he 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 $$x_1$$, $$y_1$$, $$z_1$$, 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.”

Similarly, the world of physical phenomena which was briefly called “world” by Minkowski