consideration of some leading topics of the science will exemplify both
the plausibility and inadequacy of the above definition.
Arithmetic, algebra, and the infinitesimal calculus, are sciences
directly concerned with integral numbers, rational (or fractional)
numbers, and real numbers generally, which include incommensurable
numbers. It would seem that “the general theory
of discrete and continuous quantity” is the exact description of
the topics of these sciences. Furthermore, can we not complete
the circle of the mathematical sciences by adding geometry?
Now geometry deals with points, lines, planes and cubic contents.
Of these all except points are quantities: lines involve lengths,
planes involve areas, and cubic contents involve volumes. Also,
as the Cartesian geometry shows, all the relations between
points are expressible in terms of geometric quantities. Accordingly,
at first sight it seems reasonable to define geometry in
some such way as “the science of dimensional quantity.”
Thus every subdivision of mathematical science would appear
to deal with quantity, and the definition of mathematics as
“the science of quantity” would appear to be justified. We
have now to consider the reasons for rejecting this definition
as inadequate.

*Types of Critical Questions.*—What are numbers? We can
talk of five apples and ten pears. But what are “five” and
“ten” apart from the apples and pears? Also in addition to
the cardinal numbers there are the ordinal numbers: the fifth
apple and the tenth pear claim thought. What is the relation
of “the fifth” and “the tenth” to “five” and “ten”?
“The first rose of summer” and “the last rose of summer”
are parallel phrases, yet one explicitly introduces an ordinal
number and the other does not. Again, “half a foot” and
“half a pound” are easily defined. But in what sense is there
“a half,” which is the same for “half a foot” as “half a
pound”? Furthermore, incommensurable numbers are defined
as the limits arrived at as the result of certain procedures with
rational numbers. But how do we know that there is anything
to reach? We must know that √2 exists before we can prove
that any procedure will reach it. An expedition to the North
Pole has nothing to reach unless the earth rotates.

Also in geometry, what is a point? The straightness of a
straight line and the planeness of a plane require consideration.
Furthermore, “congruence” is a difficulty. For when a triangle
“moves,” the points do not move with it. So what is it that
keeps unaltered in the moving triangle? Thus the whole
method of measurement in geometry as described in the elementary
textbooks and the older treatises is obscure to the last
degree. Lastly, what are “dimensions”? All these topics
require thorough discussion before we can rest content with the
definition of mathematics as the general science of magnitude;
and by the time they are discussed the definition has evaporated.
An outline of the modern answers to questions such as the above
will now be given. A critical defence of them would require a
volume.^{[1]}

*Cardinal Numbers.*—A one-one relation between the members of two classes α and β is any method of correlating all the members
of α to all the members of β, so that any member of α has one and
only one correlate in β, and any member of β has one and only one
correlate in α. Two classes between which a one-one relation exists
have the same cardinal number and are called cardinally similar;
and the cardinal number of the class α is a certain class whose
members are themselves classes—namely, it is the class composed
of all those classes for which a one-one correlation with α exists.
Thus the cardinal number of α is itself a class, and furthermore α
is a member of it. For a one-one relation can be established between
the members of α and α by the simple process of correlating each
member of α with itself. Thus the cardinal number one is the class
of unit classes, the cardinal number two is the class of doublets,
and so on. Also a unit class is any class with the property that it
possesses a member *x* such that, if *y* is any member of the class,
then *x* and *y* are identical. A doublet is any class which possesses
a member *x* such that the modified class formed by all the other
members except *x* is a unit class. And so on for all the finite
cardinals, which are thus defined successively. The cardinal
number zero is the class of classes with no members; but there is
only one such class, namely—the null class. Thus this cardinal
number has only one member. The operations of addition and
multiplication of two given cardinal numbers can be defined by
taking two classes α and β, satisfying the conditions (1) that their
cardinal numbers are respectively the given numbers, and (2) that
they contain no member in common, and then by defining by reference
to α and β two other suitable classes whose cardinal numbers
are defined to be respectively the required sum and product of
the cardinal numbers in question. We need not here consider the
details of this process.

With these definitions it is now possible to *prove* the following
six premisses applying to finite cardinal numbers, from which
Peano^{[2]} has shown that all arithmetic can be deduced:—

i. Cardinal numbers form a class.

ii. Zero is a cardinal number.

iii. If *a* is a cardinal number, *a* + 1 is a cardinal number.

iv. If s is any class and zero is a member of it, also if when *x* is
a cardinal number and a member of *s*, also *x* + 1 is a member of *s*,
then the whole class of cardinal numbers is contained in *s*.

v. If *a* and *b* are cardinal numbers, and *a* + 1 = *b* + 1, then *a* = *b*.

vi. If *a* is a cardinal number, then *a* + 1 ≠ 0.

It may be noticed that (iv) is the familar principle of mathematical
induction. Peano in an historical note refers its first
explicit employment, although without a general enunciation, to
Maurolycus in his work, *Arithmeticorum libri duo* (Venice, 1575).

But now the difficulty of confining mathematics to being the science of number and quantity is immediately apparent. For there is no self-contained science of cardinal numbers. The proof of the six premisses requires an elaborate investigation into the general properties of classes and relations which can be deduced by the strictest reasoning from our ultimate logical principles. Also it is purely arbitrary to erect the consequences of these six principles into a separate science. They are excellent principles of the highest value, but they are in no sense the necessary premisses which must be proved before any other propositions of cardinal numbers can be established. On the contrary, the premisses of arithmetic can be put in other forms, and, furthermore, an indefinite number of propositions of arithmetic can be proved directly from logical principles without mentioning them. Thus, while arithmetic may be defined as that branch of deductive reasoning concerning classes and relations which is concerned with the establishment of propositions concerning cardinal numbers, it must be added that the introduction of cardinal numbers makes no great break in this general science. It is no more than an interesting subdivision in a general theory.

*Ordinal Numbers.*—We must first understand what is meant by
“order,” that is, by “serial arrangement.” An order of a set of
things is to be sought in that relation holding between members
of the set which constitutes that order. The set viewed as a class
has many orders. Thus the telegraph posts along a certain road
have a space-order very obvious to our senses; but they have also
a time-order according to dates of erection, perhaps more important
to the postal authorities who replace them after fixed intervals.
A set of cardinal numbers have an order of magnitude, often called
*the* order of the set because of its insistent obviousness to us; but,
if they are the numbers drawn in a lottery, their time-order of
occurrence in that drawing also ranges them in an order of some
importance. Thus the order is defined by the “serial” relation.
A relation (R) is serial^{[3]} when (1) it implies diversity, so that, if
x has the relation R to *y*, *x* is diverse from *y*; (2) it is transitive, so
that if *x* has the relation R to *y*, and *y* to *z*, then *x* has the relation
R to *z*; (3) it has the property of connexity, so that if *x* and *y* are
things to which any things bear the relation R, or which bear the
relation R to any things, then *either* *x* is identical with *y*, *or* *x* has
the relation R to *y*, *or* *y* has the relation R to *x*. These conditions
are necessary and sufficient to secure that our ordinary ideas of
“preceding” and “succeeding” hold in respect to the relation R.
The “field” of the relation R is the class of things ranged in order
by it. Two relations R and R′ are said to be ordinally similar, if
a one-one relation holds between the members of the two fields
of R and R′, such that if *x* and *y* are any two members of the field
of R, such that *x* has the relation R to *y*, and if *x*′ and *y*′ are the
correlates in the field of R′ of *x* and *y*, then in all such cases *x*′ has
the relation R′ to *y*′, and conversely, interchanging the dashes on
the letters, *i.e.* R and R′, *x* and *x*′, &c. It is evident that the ordinal
similarity of two relations implies the cardinal similarity of their
fields, but not conversely. Also, two relations need not be serial
in order to be ordinally similar; but if one is serial, so is the other.
The relation-number of a relation is the class whose members are
all those relations which are ordinally similar to it. This class will
include the original relation itself. The relation-number of a relation
should be compared with the cardinal number of a class. When a
relation is serial its relation-number is often called its serial type.
The addition and multiplication of two relation-numbers is defined

- ↑ Cf.
*The Principles of Mathematics*, by Bertrand Russell (Cambridge, 1903). - ↑ Cf.
*Formulaire mathématique*(Turin, ed. of 1903); earlier formulations of the bases of arithmetic are given by him in the editions of 1898 and of 1901. The variations are only trivial. - ↑ Cf. Russell,
*loc. cit.*, pp. 199-256.