The influence of mathematics on philosophy
and vice versâ can be inferred from the historical
progress of both studies, though it
has not been possible till about within the last
fifteen years to give a logical explanation for
the relations between them. As long as it
was believed, according to the Kantian view,
that the science of mathematics was based
on intuitions of time and space, the alliance
between philosophy and mathematics could
not be proved to be closer than that between
philosophy and experimental science,
although the historical fact remained that philosophy
and mathematics exercised a mutual
stimulus, and developed at the same periods
of history.
But mathematics, as now defined, is independent
of intuitions of space and time, and
also of axioms and hypotheses.[1] Mathematics,
as now understood, is based, like formal logic,
on the prerequisites of thought, not on the
notions of space and time. Here there is
no definition of number or space, but the
conception of number and space,[2] which is
more complicated, can be derived from them.
All other complicated mind processes can, in
the same way, be reduced to the simple elements
of the prerequisites of thought.
Such a science might exist out of conditions
of time and space as we know them. It is a
science of relations rather than of mere number.
Founded, then, on the laws of symbolic logic, it
is a valuable aid and illustration to philosophy;
philosophy, on the other hand, can imagine lines
for the exercise of the constructive power involved
in mathematics. It is the object of this
paper to show that the close though apparently
accidental union of philosophy and mathematics
throughout the history of thought can now be
explained, and that the problems with which
pure mathematics is now concerned are those
which lie at the core of philosophic thought and
speculation. (Symbolic logic has developed to
meet the new demands made upon it. It does
not now reduce itself to the syllogism, as
Aristotle thought it did; the prerequisites of
thought are shown to be manifold instead of
single.[3])
The use of the word philosophic in this
connection suggests a necessity for further
definition. Philosophy is held to include at
least two great branches—Metaphysics and
Ethics. The influence of mathematics is
most evident on the metaphysical side of
philosophy; in fact, the grouping of mathematics
and metaphysics as allied sciences tends
to bring out the essential distinction between
metaphysics and ethics, and—though not by
any means to imply a break in their real
relation—to show where this has been misunderstood.
No philosophy has been equally
strong on both sides; they represent different
forms of activity of the human mind; but it
is still true, and from the conditions always
must be, that an ethical system grows out of
metaphysics as practice follows precept and
conduct implies belief. The new definition
of mathematics does not touch these consequences;
it merely marks the limits within
which philosophy on the metaphysical side
can submit to, or rest upon, the conclusions
of mathematics.
As to the historical relation between metaphysics
and mathematics, the subject is so vast
that we shall only attempt a very rapid generalisation
of its results on the growth of the
conception of the Finite and the Infinite. (Of
course, there are many other sides of the relation
which might be studied.) The general
result of the inquiry has been, as far as we
can judge, that metaphysics has exercised an
inspiring force on mathematics, and mathematics
has defined and strengthened the conceptions
of metaphysics at every critical stage in the
history of philosophy. But where metaphysics
has been treated as the proof of science, where
it has been laid down as the foundation for
exact knowledge, the results have not corresponded
with the truth of experience, and the
quality of thought has become degenerate.
Progress depends on the right perception of
the relations between the sciences and parts
of philosophy.
Such progress is especially evident in the
early Greek and in the modern periods, while
the large period from the Christian era to the
Renaissance gives examples of the unfortunate
reversal of the parts of metaphysics and science
and consequent confusion of thought.[4]
The problems of the metaphysician are no
doubt in a sense always the same; but this
is equally true of the problems of any other
science. The methods by which the problems
are attacked and the adequacy of the solutions
they receive vary, from age to age, in close
correspondence with the general development
of science. Every great metaphysical conception
has exercised its influence on the
general history of science, and in return every
important movement in science has affected
the development of metaphysics. The metaphysician
could not if he would, and would
not if he could, escape the duty of estimating
the bearing of the great scientific theories of
his time upon our ultimate conceptions of the
nature of the world as a whole. Every fundamental
advance in science thus calls for a
restatement and reconsideration of the old
metaphysical problems in the light of the new
discovery.
During the Greek period mathematics was
the only branch of science which was at all
developed, and its development coincided with
the age of the philosophers. Thus when
Plato spoke of science he always meant mathematics.
And even later, when the physical
sciences had begun to develop, Aristotle put
mathematical ideas into close connection with
metaphysical ones when he stated that they
occupied the middle term between the ideal
and the sensible. Both Plato and Aristotle
referred to and depended upon mathematical
proofs and illustrations of philosophical questions.
During this Greek period the conception
of Infinity took shape. The pre-Platonic
notion, reproduced again later in the decline
of Socratic theory by the Stoics, was that
the Infinite was the aggregate of the Finite;
the Platonic and Aristotelian theory, that,
namely, of the most vigorous moment of Greek
thought, was that the Infinite was more than
the aggregate of the Finite; that it had a
self-determined existence from which the Finite
had been derived. Existence, as known to
man, was treated as a compromise between the
Finite and the Infinite.
Neo-Platonism altogether separated the Infinite
from the Finite. In the Alexandrine
metaphysics, which represented a decadent
stage of philosophy and its deviation from the
sciences, the conception of the Infinite became
less clear and logical; it diverged from the
view which had been affected by mathematical
thought, and tended to assimilate to itself the
ideas of perfection and universality, which,
philosophically speaking, are conceptions distinct
from that of Infinity—universality referring
to a common principle of unity, and
perfection involving the moral ideal. Real
progress was deferred by the too rapid coherence
of ideas only partially analysed and
understood. Thinkers passed quickly from the
exclusive contemplation of subject to that of
object and back again,[5] each new period
negativing all previous experience, till the
result was the exclusion of an imperfectly
analysed Relative and Finite from an insufficiently
apprehended Absolute and Infinite.
After the Christian era Greek philosophy
drifted off into scholasticism and lost touch
of reality, the grammar of Aristotelian logic
replacing the vital connection of ideas. St.
Anselm, it is true, attempted to find a rational
proof of the existence of God, and identified
Him with the Infinite of Greek thought; but
St. Thomas Aquinas led away the argument
to a discussion as to how far form and matter,
separately considered, shared in the quality
of Infinity. (He thought form did, but not
matter.) An overpowering sense of mystery,
joined to a premature desire for definition
without scientific analysis, sapped the vigour
of mediæval thought.
Throughout the middle ages, then, we see
the conditions of the Greek period reversed:
philosophy during the second period is not,
as in the first, engaged in giving a stimulus
to the efforts of pure reason; rather the intuitions
of philosophy are treated as axiomatic,
and a false superstructure of knowledge, alien
to experience and reality, is erected upon these
foundations. Philosophy, in fact, is used as
a general basis for science. The parts of
philosophy and mathematics, correctly though
imperfectly seen by the Greeks, are in the
second period exchanged, and the result
is confusion of idea. The notion of the Infinite,
as in the Alexandrine metaphysics,
is held to include perfection and universality,
and does not exist as a conception apart from
these.
After the Renaissance, the scholastic philosophy
falling into disuse, the attempt to find
an explanation of the Cosmos, a synthesis of
the universe, was abandoned, and replaced by
the Cartesian idea—the inference of existence
from thought, and the limitation of the sphere
of inquiry to that which could be known by
the ego. New scientific and mathematical discoveries
kept pace with this new analysis and
development of thought,[6] and the surer ground
in philosophy was definitely allied with the
work of the mathematical mind. The philosophical
thesis developed from “The Infinite
is the negation of the Finite,” to “The Infinite
presupposes the Finite and does not exclude
it.” The problem of the Finite and the Infinite
became the great idea of the age, and there
was a reversion to the Greek notion of existence
as a compromise between the two, and
almost the hint of a coming explanation of
them. In the decline of Cartesian philosophy,
when it drifted off into Pantheism, there was
only a vague conception of the Infinite, and
we trace a tendency to identify the notion
of Infinity with that of the Cosmos. In
mediæval thought the idea of the Infinite had
become confused with that of the Perfect
and Universal; in the modern period the
effort to give a concrete expression to the
notions of Infinity, Perfection, and Universality
diverted the ideas from their relation
to the Creator and applied them to the
Creation.
Kant, who gave a new impulse to some
parts of the Cartesian idea, neglected both
mathematical proofs and the search for a
metaphysical Absolute. In avoiding the subject
he helped to perpetuate the vague descriptions
of the Finite and the Infinite, uncorrected
by mathematical thought, which had been the
currency of the philosophy of his age, and
which corrupted the philosophy of the succeeding
century. The nineteenth century
produced nothing more than guesses at truth,
which were, perhaps, not very far wrong, and
which the present century is engaged in correcting
and substantiating. The same vagueness
afflicted both mathematics (theory of
functions) and philosophy. Fichte, Schelling,
and Hegel, particularly Hegel, identify the
metaphysical Absolute with reality, infinity,
and the universal. The ideas of continuity
and infinity are not separated by them from
those of perfection and universality, nor from
one another, and their nature is not understood.
Leaving aside the French neo-critical School
(Renouvier) and the English School (Spencer)—the
first of whom deny the Infinite, thus acting
in opposition to mathematical reasoning, while
the second perpetuate Kant’s error of considering
the Infinite, though thinkable, as
unknowable (Dr. Caird has pointed out that
this position is illogical)—we arrive at a
moment in history which is more fruitful in
result on the mathematical side, and will, no
doubt, have an effect on metaphysics. For,
owing to recent discoveries in Germany and
England, mathematics is now in a position
to give greater support than before to the
intuitions of philosophy. Hitherto, philosophers
have been reluctant to allow full value
to the mathematical conceptions of Infinity,
and with some justice, as the notion had not
been sufficiently analysed. Philosophers, who
never attempted the analysis, have been inclined
to accept certain contradictions in their
conception as inherent in the nature of Infinity.
Within the last twenty-five years Cantor
and Dedekind have cleared up the notion
of continuity, and Russell has given greater
precision to the idea, and has applied this
reasoning to philosophy.
Present-day metaphysicians seem to be
divided into two groups; on the one side,
those who consider in philosophy the value
of a theory of being, and, on the other, those
who chiefly consider the value of a theory of
knowledge, i.e. the Epistemologists. The first
group, devoting themselves to psychology,
evolution, and history, have no necessary belief
in the Infinite. The Epistemologists, whose
work is founded on Kant, discuss the theory
of knowledge and enumerate the conditions
of knowledge. Their argument may not
touch, but does not exclude, the notion of the
Infinite. The position of the Epistemologist
has been made infinitely more secure by recent
mathematical work. That of the psychologist
remains almost untouched. It is
necessary now to examine more closely the
mathematical results to which reference has
been made.
In general terms it may be said that mathematics
has, as a study, led immediately from
the nature of the subject to the perception
of the Infinite, and to a knowledge of the
connection between the Infinite and the Finite.
The simplest form in which the idea can be
put is stated by St. Augustine, who said that
numbers considered individually were finite,
but considered as an aggregate were infinite.[7]
Before St. Augustine, and after him down
the long stream of philosophic thought, the
theologian and the philosopher have turned
to mathematics for illustrations of the infinitely
great and infinitely little, as developed
from the concrete processes of arithmetic and
geometry. The recurring decimal in arithmetic,
the properties of the circle and ellipse
in geometry, of the cone in conic sections, and
of the surd in algebra, all touch the problem
of number and space on the side of Infinity.
In higher mathematics it is possible to start
from the idea of the Finite and reach the conception
of the Infinite; or to reverse the process,
and from the Infinite to deduce the Finite.
Thus in the familiar puzzle of the subdivision
of the parts of a straight line by halving the
remainder, there will be a crowding and a
coalescing of the points of division towards
one end of the line, the points of division
getting infinitely nearer, but the steps will
never meet. Here in the centre of a straight
line—a limited straight line—we are confronted
with the problem of Infinity.
Again, from a series of finite numbers we
can gain the notion of an infinite series. Take
two series which have a correspondence with
one another. If for every element of the one
we can choose an element of the other, and of
the other there is an element for the one, when
at any point we cut off its progress to infinity,
this happens:—
One series, if summed up, will give a larger
numerical result than the other, and therefore
can be said to be greater than the second. Let
us call the first series A, and the second B.
Let us now imagine the two series, though
starting at a definite point, are never cut at
the further end. Then to all infinity series B
is without certain numbers which series A
possesses, and as an infinite series is smaller
than series A. But, on the other hand, when
neither series is cut, series B retains its correspondence
with series A. Thus we attain
a definition of an infinite series. It is such
that the part, while being less than the whole,
has yet a complete correspondence with the
whole. The whole is greater than the part,
but take away the part from the whole and
that which remains corresponds to it in
infinity, because the test of summing the
series (which would give a contrary result)
involves limitation, and thus cannot be applied.
Subtraction can take place in Infinity without
loss.
By reversing this process, and by starting
from the theory of the Infinite, we may gain
some idea of the discovery of the Finite. So
Dedekind and Russell define finite numbers
not only in the usual way as those which can
be reached by mathematical induction, starting
from 0 and increasing by 1 at each step, but also
as those of classes which are not similar to the
parts of themselves obtained by taking away
single terms. That is the reversal of the
process applied just now. Dedekind also
has deduced the Finite from the Infinite by
a novel process. He predicates a world of
thought which we each and all possess, filled
with thoughts and things, to each thing corresponding
a thought. There are thus two
“trans-finite” series in the minds of each and
all of us; we cannot say when the series of
thoughts and things will end; but they have
number, though it is infinite number. (Number
exists wherever there is a correspondence, one
to one, between two aggregates.) But in this
Gedankenwelt, says Dedekind, there is one
thing to which there is no corresponding
thought: that is the ego. Each man is part
of his own world of thought, but there is no
thought of himself in his mind corresponding
exactly to himself, as a thought in his mind
corresponds to another object.[8] Two important
results follow from Dedekind’s theory:
first, the existence of a finite number one, the
number of the ego, as deduced from the Gedankenwelt
of two infinite systems; second, by
putting together all the Gedankenwelts there
are or may be, we get the notion of series of
series, which seems to transcend Infinity, and
it gives us the conditions which are possibly
gathered up in the Absolute. Now the argument
from the Finite to the Infinite and the
converse process may both be employed in
mathematics (or both may be neglected, as in
the elementary methods of calculation used in
arithmetic). A discussion has taken place in
the Hibbert Journal on the relative value of
the two methods. Keyser, in an article called
the Axiom of Infinity, argued that one method,
that of Dedekind, should be exclusively developed.
Russell answered him, stating that
it was not necessary to hold exclusively to
either. If the Finite and the Infinite can in
turn be deduced from one another, neither
conception can be truly called an axiom. The
real axiom is existence, which includes both,
and which is defined by mathematicians as that
which is not self-contradictory.
Now the problem of Infinity includes also
that of continuity; in other words, the problem
of number includes that of cardinal and ordinal
number. It is time to get to the mathematical
definition of number, which we have found
as a conception can be attached both to the
Finite and to the Infinite. What is number
in mathematics?
Take any collection of things—we call that
an aggregate. If an aggregate corresponds one
to one with another aggregate, they are both
said to have a number, and the same number.
Subtract from the idea of an aggregate the idea
of quality or kind, and order or arrangement,
what is left is its cardinal number. If you
subtract quality and not order, the result is
an ordinal number. This reasoning applies
both to finite and infinite aggregates; in fact,
the Infinite may be said to possess most of the
properties which we attach to the Finite. Two
infinite aggregates, for example, can have an
ordinal correspondence, and infinite aggregates
submit, like finite ones, to arithmetical
processes.
The mathematician analyses still more closely
the relation between the Finite and the Infinite,
as follows:—
He starts from the aggregate, which he
analyses into the Finite and the Infinite, and
the latter he analyses into the Transfinite and
the Absolute. Of these two elements, one
only has till just lately been the subject of
mathematical treatment—the one called the
Transfinite. It is the transfinite subdivision
of the Infinite to which the idea of number
is applicable, and which is, therefore, in a sense
inseparable from the Finite. Infinite numbers
or series ought then to be more correctly described
as Transfinite. But the processes of
mathematics do not end here; they reach
up to the idea of the Absolute Infinite,
the conception of which has been attained
in recent years by mathematical work. The
results of this work may now be briefly summarised.
I. The Absolute appears to have the same
relation to the Transfinite as the Transfinite
to the Finite. If the Finite deals with numbers,
and the Transfinite with series of numbers, the
Absolute deals with series of series. Thus
there are at least two examples of the Infinite
within our grasp which lead up to the idea
of the Absolute. One is the class of all
classes of propositions; the other is the
series of all worlds of thought, in Dedekind’s
sense.
II. The Finite, Transfinite, and Absolute
can be further defined in this way. There is
no greatest finite number, but there is a least
transfinite number, which has been called Aleph
0, and which can be proved to be greater than
any possible finite number, however large, because
if there were a last number it must be
smaller than the sum of the whole series.
There are unending series of Alephs or infinite
numbers, which are as distinct from one another
in idea as 1 is from 0, and which can no more
be derived from one another by a mathematical
process than 1 can be derived from 0, but can
be reached in the same way by induction.
Beyond the Transfinite we cannot discover
in the Absolute the idea of least or of
greatest.
III. The relation of cardinal and ordinal
number also throws some light on the Finite,
Transfinite, and Absolute. In the Finite,
cardinals and ordinals are parallel to one another;
in the Transfinite they strikingly diverge;
in the Absolute we cannot trace any
connection between cardinals and ordinals, i.e.,
it is possible to have an ordinal series to which
there can be no corresponding cardinal number
or type.[9]
IV. If arithmetical processes are applied to
the Finite, Transfinite, or Absolute, we get
interesting results. We know the effect of
addition, multiplication, and raising to a power,
on the Finite. The first two processes have
been applied to the Alephs; the last has been
formulated, but the mathematical results have
not yet been brought to a satisfactory conclusion.
Broadly speaking, we may say that
the raising of an Aleph to a power may make
it transcend the Finite and the Transfinite
and melt into the Absolute. Thus all mathematical
processes which find their goal in the
Absolute would find their annihilation there.
No finite mathematical conception would be
applicable to it.
Now the conception of this Absolute Infinite,
of which the aggregate of all ordinal numbers
is perhaps a symbol,[10] has been subjected to
criticism. Some mathematicians[11] think that
it exists, but has no number. It is discovered
by a logical process, but defies analysis and
the application to it of the notion of number.
All mathematical conceptions find in it their
aim and conclusion. The importance of this
theory, its practical importance, lies in the
very much simpler mathematical formulæ that
can be produced now that the logical process
is shown to extend from the Finite to the
Absolute Infinite (in the same way that the
labour of summing a series arithmetically
by statement and addition is shortened
by the application of algebraical principles
which depend on larger knowledge). Its
philosophical importance is great: the Absolute
is here, as elsewhere, the goal of human
thought, and is the mathematician’s name
for the highest power discoverable by human
reason.
It would be very interesting to discuss the
probable attitude of a Pascal or a Hegel to
these mathematical conceptions, if they had
been aware of them. Take Pascal’s puzzle of
the Finite and the Infinite. He thought that
if the Finite could be subtracted from the
Infinite, the Infinite would thereby lose some
of its quality of infinity. How differently
would it have appeared to him had he realised
that an aggregate infinite cardinal can have
subtracted from it either finite or transfinite
terms: if transfinite terms, many different
answers result, giving different degrees of
transfinity: if only finite terms are taken
away, the Infinite remains in its entirety.
How, again, would Hegel have rejoiced in
a definition of thought and existence which
would bridge over the logical gulf in his
system! Hegel asserted that thought and
existence were one. He is objected to by
many philosophers, who ask where is the
tertium quid which makes it possible to reach
from one to the other, or predicate their
essential unity? But the mathematician defines
existence as something which is not self-contradictory.
Thought, then, to him is a
form of existence, for thought is not self-contradictory;
but the two, thought and
existence, are not necessarily conterminous.[12]
Hence, to say that non-contradiction is a fundamental
condition of true thinking is as much
as to say that it is a fundamental characteristic
of real existence, and he identifies thought
with reality.
Dr. Caird remarks that the secular conscience
conceives of the Infinite as opposed
to the Finite; the religious conscience treats
the Infinite as real, presupposed by the illusory
Finite. Where does the truth lie? Mathematics
does not admit the necessity of adopting
either view at the expense of the
other.
Metaphysics standing alone produces results
that may be disproved, but cannot be proved.
Mathematics standing alone produces results
that are susceptible of proof. Both are based
on logic, and rest on the prerequisites of
thought. Together they are a field for the
best powers of human reason: metaphysics
supplies insight, intuition, imagination; mathematics
offers the indubitable proof and translates
the ideal into the actual.
But the element in philosophical thought
which, employing the psychological method,
tends to the discussion of a theory of being
rather than that of knowledge, and thus to
the realisation of an ethical system rather than
to metaphysical discovery, is averse from accepting
these conclusions. It remains, therefore,
for us to examine the criticism offered
by the psychological school on what they call
the mathematising of philosophy; and it will
be found that the attack deals both with the
ground of the alliance and its results.
A typical exponent of this school is Moisant,
who, in the Revue Philosophique for January
1905, attacked what he considered to be the
characteristic of modern philosophy and also its
vice. It will be observed that at the outset
he reverses the rôles of philosophy and mathematics
as we have apprehended them. Philosophy,
he says, should expect to be inspired
by mathematics, but should avoid its method.
Next, he connects the modern movement with
the theories of Leibniz, who aimed at substituting
general formulæ for elementary forms
of reason and calculation. These short cuts,
which seem to the mathematician to liberate
the mind from a burden which prevents it
from employing its full activity, seem to the
psychologist to tend to a mechanical method,
in which the thinker is only aware of premises
and results, and in which the mathematical
concept tends to replace the real idea. Then
he attacks the new definition of mathematics
as the science of relations, asserting that it
still contains notions of space.[13]
Finally, he comes to the real question at
issue, and enters into the comparison of a
metaphysical and a mathematical problem.
He takes as his subject the argument from
the known to the unknown. Descartes had
said that argument should lead from the
known to the unknown, simple to complex,
and had defined the first as that which
could be known without the help of the
second. This logical order of reasoning has
been attributed to mathematics, but has been
considered to be inapplicable to philosophy.
Mathematics, in its recent development, by
the argument from the Finite to the Infinite
and back again, starts from two propositions,
neither of which can be said to be axiomatic,
because each in turn can be proved from the
other, but in the course of argument from either
mathematics makes use of the logical process.
The real axiom, as has been shown, is that of
existence or being. A metaphysical argument
has the same root—that of existence—but
a metaphysical problem deals with paradoxes,
with questions which are sometimes defined as
having two answers, each equally correct, and
sometimes as yielding no answer at all. The
method of thesis, antithesis, and synthesis is
in the Hegelian logic applied to their solution.
A mathematical and a metaphysical problem
are not, then, problems of the same kind to be
solved by the same method; nor is the conception
of the mathematical Absolute reached
in the same way as that of the metaphysical
Absolute. We are even unable to say how
far they correspond except in respect of their
absoluteness.[14] But the contention of the mathematician
to-day and of the epistemologist school
of philosophy is not the identity of methods
and results in the two sciences. It is the axiom
of existence on which they both depend: the
law of thought by which all methods are developed,
and, above all, the correlative value of
each science to the other, which allows us, in
developing our knowledge from the standpoint
of the two sciences, to recognise something
of the greatness of the Absolute principle to
which they both reach up, and in which their
being consists.