The early counting numbers were obtained by the process of abstraction, in the same way as a child grasps their meaning. (For instance, the concept of ‘four’ is learnt by picking out the common property from a variety of sets of four items.) Then, gradually, names were given to the numbers that were obtained by adding 1, i.e., 10+1, 11+1, …. At one stage, ‘zero’ was introduced, which may have come from a process of counting backwards. From here on, a process of generalisation in the abstract world seems to have set in from which the sets of integers and rational numbers were formed.