We have but to suppose the increase in surface and the diminution in mass carried on to a certain extent to find a region which, though without mobility of the constituents, would have to be described as two-dimensional. 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.