A course of modern analysis

"The idea of a set of numbers is derived in the first instance from the consideration of the set of positive integral numbers, or positive integers;that is to say, the numbers 1, 2, 3, 4,... Positive integers have many properties, which will be found in treatises on the Theory of Integral Numbers; but at a very early stage in the development of mathematics it was found that the operations of Subtraction and Division could only be performed among them subject to inconvenient restrictions; and consequently, in elementary Arithmetic, classes of numbers are constructed such that the operations of subtraction and division can always be performed among them."