Introduction: In mathematical as well as in everyday experiences, pairs of objects

are compared to one another by an abstract device called a binary relation, although in this text it will simply be called a relation. For example, the objects may be pairs of persons while the device could be the relation, "is the same age as," or, "is a parent of." In the first case, a specific pair of persons a and b is distinguished from other pairs when it is really true that a is the same age as b. If the sentence, "a is the same age as b," is true, we say the pair consisting of a and b belongs to the relation, "is the same age as." Of course, in the second relation, the pair would be so distinguished if, "a is a parent of b," and the relation could be called the, "is the parent of," relation.