The Principles of Mathematics (1903)

§ 115

The general definition of multiplication is due to Mr. A. N. Whitehead[78]. It is as follows. Let k be a class of classes, no two of which have any term in common. Form what is called the multiplicative class of k, i.e. the class each of whose terms is a class formed by choosing one and only one term from each of the classes belonging to k. Then the number of terms in the multiplicative class of k is the product of all the numbers of the various classes composing k. This definition, like that of addition given above, has two merits, which make it preferable to any other hitherto suggested. In the first place, it introduces no order among the numbers multiplied, so that there is no need of the commutative law, which, here as in the case of addition, is concerned rather with the symbols than with what is symbolized. In the second place, the above definition does not require us to decide, concerning any of the numbers involved, whether they are finite or infinite. Cantor has given[79] definitions of the sum and product of two numbers, which do not require a decision as to whether these numbers are finite or infinite. These definitions can be extended to the sum and product of any finite number of finite or infinite numbers; but they do not, as they stand, allow the definition of the sum or product of an infinite number of numbers. This grave defect is remedied in the above definitions, which enable us to pursue Arithmetic, as it ought to be pursued, without introducing the distinction of finite and infinite until we wish to study it. Cantor's definitions have also the formal defect of introducing an order among the numbers summed or multiplied: but this is, in his case, a mere defect in the symbols chosen, not in the ideas which he symbolizes. Moreover it is not practically desirable, in the case of the sum or product of two numbers, to avoid this formal defect, since the resulting cumbrousness becomes intolerable.(§ 115 ¶ 1)

§ 115 n. 1. American Journal of Mathematics, Oct. 1902.

§ 115 n. 2. Math. Annalen, Vol. XLVI, § 3.