A solid (three space) is bounded by Surfaces (two space).
We must conclude therefore that "higher solids" (four space) are bounded by Solids (three space).
Fig. 10
To take the special case with which we are already familiar. The line AB, is bounded by the points A and B. (Fig. 10). The square, A B C D, is bounded by four lines AB, BC, CD, DA. The cube, A B C D E F G H, is bounded by six surfaces, namely, ABCD, CDEF, EFGH, GHAB, ADEH, BCFG.
Similarly we must conclude that a tesseract is bounded by cubes.
We shall see later that there are eight of them.
(4)
We may put (3) in a slightly different way, by saying that: