But if we refuse to be restricted to a flat surface we find that it is possible to draw a third line through O which is quite "independent" of the directions of the two lines we have previously drawn. We can do this by drawing it vertically, that is to say, perpendicular to the plane of the paper. Call this line COC´.
Fig. 3
I have shown it in perspective in Fig. 3. This line fulfils the definition we gave of an independent direction in space for it is at right angles both to AOA´ and to BOB´. But we have now exhausted our resources. Try as we will we are unable to draw a fourth line which shall be at right angles to AOA´, BOB´, and COC´ simultaneously.
On other words—In the space we know we find only three dimensions and consequently we can refer to it as "Space of three dimensions" or "Three-dimensional space."
Now the idea of a fourth dimension of space is simply this: That, whereas in three-dimensional space, we can draw, through any point in it, three, and only three, lines mutually at right angles: in four-dimensional space, it would be possible to draw, through any point in it, four, and only four, lines mutually at right angles.
Extending the idea to "Higher space" in general, we may say that,—In space of "n" dimensions we can draw, through any point in it, "n," and only "n," lines mutually at right angles.
Now I admit, that, at first sight, the idea that it might be possible, under any circumstances, to draw more than three such lines through a point, seems utterly staggering and inconceivable. And indeed the more one thinks of it and the more thoroughly one grasps what it means, the more absolutely impossible does it appear.
All the same, as I hope to show very soon, it is, as a matter of fact, quite possible that there may be another independent direction fulfilling the prescribed conditions, in spite of the fact that we are at present ignorant of it.
This we can only realize by a consideration of the time-honoured but indispensable analogy of a two-dimensional world, or "Flatland."