The mathematical objects discussed so far can be divided into two classes: elementary objects and composite objects. Elementary objects can be viewed as atoms that cannot be divided and are defined only in terms of the axioms associated with them. Composite objects, on the other hand, can be defined in terms of simpler objects. Thus, in the hierarchy of objects, the elementary objects are placed in the lowest level, whereas the composite objects are placed in the higher levels, so their definitions and functions may use more elementary objects from the lower levels.