But, however artificial the conception of a plane being may be, it is none the less to be used in passing to the conception of a greater dimensionality than ours, and hence the validity of the first part of this objection altogether disappears directly we find evidence for such a state of being.
The second part of the objection has more weight. How is it possible to conceive that in a four-dimensional space any creatures should be confined to a three-dimensional existence?
In reply I would say that we know as a matter of fact that life is essentially a phenomenon of surface. The amplitude of the movements which we can make is much greater along the surface of the earth than it is up or down.
Now we have but to conceive the extent of a solid surface increased, while the motions possible tranverse to it are diminished in the same proportion, to obtain the image of a three-dimensional world in four-dimensional space.
And as our habitat is the meeting of air and earth on the world, so we must think of the meeting place of two as affording the condition for our universe. The meeting of what two? What can that vastness be in the higher space which stretches in such a perfect level that our astronomical observations fail to detect the slightest curvature?
The perfection of the level suggests a liquid—a lake amidst what vast scenery!—whereon the matter of the universe floats speck-like.
But this aspect of the problem is like what are called in mathematics boundary conditions.
We can trace out all the consequences of four-dimensional movements down to their last detail. Then, knowing the mode of action which would be characteristic of the minutest particles, if they were free, we can draw conclusions from what they actually do of what the constraint on them is. Of the two things, the material conditions and the motion, one is known, and the other can be inferred. If the place of this universe is a meeting of two, there would be a one-sideness to space. If it lies so that what stretches away in one direction in the unknown is unlike what stretches away in the other, then, as far as the movements which participate in that dimension are concerned, there would be a difference as to which way the motion took place. This would be shown in the dissimilarity of phenomena, which, so far as all three-space movements are concerned, were perfectly symmetrical. To take an instance, merely, for the sake of precising our ideas, not for any inherent probability in it; if it could be shown that the electric current in the positive direction were exactly like the electric current in the negative direction, except for a reversal of the components of the motion in three-dimensional space, then the dissimilarity of the discharge from the positive and negative poles would be an indication of a one-sideness to our space. The only cause of difference in the two discharges would be due to a component in the fourth dimension, which directed in one direction transverse to our space, met with a different resistance to that which it met when directed in the opposite direction.
CHAPTER VII
THE EVIDENCES FOR A FOURTH DIMENSION
The method necessarily to be employed in the search for the evidences of a fourth dimension, consists primarily in the formation of the conceptions of four-dimensional shapes and motions. When we are in possession of these it is possible to call in the aid of observation, without them we may have been all our lives in the familiar presence of a four-dimensional phenomenon without ever recognising its nature.