We will now analyze more closely, from the standpoint of modern algebra, the fundamental notions of “set” (or class) and “subset,” briefly introduced in §1.11. Suppose that I is any set, while X, Y, Z, … denote subsets of I. Thus I might be a square, and X, Y, Z three overlapping congruents disks located in I, as in the “Venn diagram” of Figure 1.