With regard to this definition of a sum of numbers, it is to be observed that it cannot be freed from reference to classes which have the numbers in question. The number obtained by summation is essentially the number of the logical sum of a certain class of classes or of some similar class of similar classes. The necessity of this reference to classes emerges when one number occurs twice or oftener in the summation. It is to be observed that the numbers concerned have no *order* of summation, so that we have no such proposition as the commutative law: this proposition, as introduced in Arithmetic, results only from a defective symbolism, which causes an order among the symbols which has no correlative order in what is symbolized. But owing to the absence of order, if one number occurs twice in a summation, we cannot distinguish a first and a second occurrence of the said number. If we exclude a reference to classes which have the said number, there is no sense in the supposition of its occurring twice: the summation of a class of numbers can be defined, but in that case, no number can be repeated. In the above definition of a sum, the numbers concerned are defined as the numbers of certain classes, and therefore it is not necessary to decide whether any number is repeated or not. But in order to define, without reference to particular classes, a sum of numbers of which some are repeated, it is necessary first to define multiplication.(§ 114 ¶ 1)

This point may be made clearer by considering a special case, such as 1+1. It is plain that we cannot take the number 1 itself twice over, for there is one number 1, and there are not two instances of it. And if the logical addition of 1 to itself were in question, we should find that 1 and 1 is 1, according to the general principle of Symbolic Logic. Nor can we define 1+1 as the arithmetical sum of a certain class of numbers. This method can be employed as regards 1+2, or any sum in which no number is repeated; but as regards 1+1, the only class of numbers involved is the class whose only member is 1, and since this class has one member, not two, we cannot define 1+1 by its means. Thus the full definition of 1+1 is as follows: 1+1 is the number of a class `w` which is the logical sum of two classes `u` and `v` which have no common term and have each only one term. The chief point to be observed is, that logical addition of classes is the fundamental notion, while the arithmetical addition of numbers is wholly subsequent.(§ 114 ¶ 2)

The Principles of Mathematics was written by Bertrand Russell, and published in in 1903. It is now available in the Public Domain.