# The Early Development of Set Theory

*First published Tue Apr 10, 2007; substantive revision Wed Jul 6, 2011*

Set theory is one of the greatest achievements of modern mathematics. Basically all mathematical concepts, methods, and results admit of representation within axiomatic set theory. Thus set theory has served quite a unique role by systematizing modern mathematics, and approaching in a unified form all basic questions about admissible mathematical arguments — including the thorny question of existence principles. This entry covers in outline the convoluted process by which set theory came into being, covering roughly the years 1850 to 1930.

In 1910, Hilbert wrote that set theory is “that mathematical
discipline which today occupies an outstanding role in our science, and
radiates [*ausströmt*] its powerful influence into all
branches of mathematics”. This already suggests that, in order to
discuss the early history, it is necessary to distinguish two aspects
of set theory: its role as a fundamental language and repository of the
basic principles of modern mathematics; and its role as an independent
branch of mathematics, classified (today) as a branch of mathematical
logic. Both aspects are considered here.

The first section examines the
origins and *emergence* of set theoretic mathematics around
1870; this is followed by a discussion of the period of expansion and
*consolidation* of the theory up to 1900. Section 3 provides a
look at the *critical period* in the decade 1897 to 1908, and
Section 4 deals with the time from Zermelo to Gödel (from theory
to metatheory), with special attention to the often overlooked, but
crucial, *descriptive* set theory.

## 1. Emergence

The concept of a set appears deceivingly simple, at least to the trained mathematician, and to such an extent that it becomes difficult to judge and appreciate correctly the contributions of the pioneers. What cost them much effort to produce, and took the mathematical community considerable time to accept, may seem to us rather self-explanatory or even trivial. Three historical misconceptions that are widespread in the literature should be noted at the outset: (1) It is not the case that actual infinity was universally rejected before Cantor. (2) Set-theoretic views did not arise exclusively from analysis, but emerged also in algebra, number theory, and geometry. (3) In fact, the rise of set-theoretic mathematics preceded Cantor's crucial contributions. All of these points shall become clear in what follows.

The notion of a collection is as old as counting, and logical ideas about classes have existed since at least the “tree of Porphyry” (3rd century C.E.). Thus it becomes difficult to sort out the origins of the concept of set. But sets are neither collections in the everyday sense of this word, nor “classes” in the sense of logicians before the mid-19th century. Ernst Zermelo, a crucial figure in our story, said that the theory had historically been “created by Cantor and Dedekind”. This suggests a good pragmatic criterion: one should start from authors who have significantly influenced the conceptions of Cantor, Dedekind, and Zermelo. For the most part, this is the criterion adopted here. Nevertheless, as every rule calls for an exception, the case of Bolzano is important and instructive, even though Bolzano did not significantly influence later writers.

In 19th century German-speaking areas, there were some intellectual tendencies that promoted the acceptance of the actual infinite (e.g., a revival of Leibniz's thought). In spite of Gauss's warning that the infinite can only be a manner of speaking, some minor figures and three major ones (Bolzano, Riemann, Dedekind) preceded Cantor in fully accepting the actual infinite in mathematics. Those three authors were active in promoting a set-theoretic formulation of mathematical ideas, with Dedekind's contribution in a good number of classic writings (1871, 1872, 1876/77, 1888) being of central importance.

Chronologically, Bernard Bolzano was the first, but he exerted
almost no influence. The high quality of his work in logic and the
foundations of mathematics is well known. A book entitled
*Paradoxien des Unendlichen* was posthumously published in 1851.
Here Bolzano argued in detail that a host of paradoxes surrounding
infinity are logically harmless, and mounted a forceful defence of
actual infinity. He proposed an interesting argument attempting to
prove the existence of infinite sets, which bears comparison with
Dedekind's later argument (1888). Although he employed
complicated distinctions of different kinds of sets or classes, Bolzano
recognized clearly the possibility of putting two infinite sets in
one-to-one correspondence, as one can easily do, e.g., with the
intervals [0, 5] and [0, 12] by the function 5*y* =
12*x*. However, Bolzano resisted the conclusion that both sets
are “equal with respect to the multiplicity of their
parts”. In all likelihood, traditional ideas of measurement were
still too powerful in his way of thinking, and thus he missed the
discovery of the concept of cardinality.

The case of Bolzano suggests that a liberation from metric concepts
(which came with the development of theories of projective geometry and
especially of topology) was to have a crucial role in making possible
the abstract viewpoint of set theory. Bernhard Riemann proposed
visionary ideas about topology, and about basing all of mathematics on
the notion of set or “manifold” in the sense of class
(*Mannigfaltigkeit*), in his celebrated inaugural lecture
“On the Hypotheses which lie at the Foundations of
Geometry” (1854/1868). Also characteristic of Riemann was a great
emphasis on *conceptual* mathematics, particularly visible in
his approach to complex analysis (which again went deep into topology).
To give but the simplest example, Riemann was an enthusiastic follower
of Dirichlet's idea that a function has to be conceived as an
arbitrary injective correspondence between numerical values, be it
representable by a formula or not; this meant leaving behind the times
when a function was defined to be an analytic expression. Through this
new style of mathematics, and through his vision of a new role for sets
and a full program for developing topology, Riemann was a crucial
influence on both Dedekind and Cantor.

The five-year period 1868–1872 saw a mushrooming of set-theoretic proposals in Germany, so much so that we could regard it as the birth of set-theoretic mathematics. Riemann's geometry lecture, delivered in 1854, was published by Dedekind in 1868, jointly with Riemann's paper on trigonometric series (1854/1868b, which presented the Riemann integral). The latter was the starting point for deep work in real analysis, commencing the study of “seriously” discontinuous functions. The young Georg Cantor entered into this area, which led him to the study of point-sets. In 1872 Cantor introduced an operation upon point sets (see below) and soon he was ruminating about the possibility to iterate that operation to infinity and beyond: it was the first glimpse of the transfinite realm.

Meanwhile, another major development had been put forward by Richard
Dedekind in 1871. In the context of his work on algebraic number
theory, Dedekind introduced an essentially set-theoretic viewpoint,
defining fields and ideals of algebraic numbers. These ideas were
presented in a very mature form, making use of set operations and of
structure-preserving mappings (see a relevant passage in
Ferreirós 1999, 92–93; Cantor employed Dedekind's
terminology for the operations in his own work on set theory around
1880: *op. cit.*, 204). Considering the ring of integers in a
given field of algebraic numbers, Dedekind defined certain subsets
called “ideals” and operated on these sets as new objects.
This procedure was the key to his general approach to the topic. In
other works, he dealt very clearly and precisely with equivalence
relations, partition sets, homomorphisms, and automorphisms. Thus,
many of the usual set-theoretic procedures of twentieth-century
mathematics go back to his work. Several years later (in 1888),
Dedekind would publish a presentation of the basic elements of set
theory, making only a bit more explicit the operations on sets and
mappings he had been using since 1871.

The following year, Dedekind published a paper in which he provided an
axiomatic analysis of the structure of the set
**R** of real numbers. He defined it as an ordered field
that is also complete (in the sense that all Dedekind-cuts on
**R** correspond to an element in **R**);
completeness in that sense has the Archimedean axiom as a consequence.
Cantor too provided a definition of **R** in 1872,
employing Cauchy sequences of rational numbers, which was an elegant
simplification of the definition offered by Carl Weierstrass in his
lectures. The form of completeness axiom that Weierstrass preferred
was Bolzano's principle that a sequence of nested closed intervals in
**R** (a sequence such that
[*a*_{m+1},*b*_{m+1}]
⊂
[*a*_{m},*b*_{m}])
“contains” at least one real number (or, as we would say,
has a non-empty intersection).

The Cantor and Dedekind definitions of the real numbers relied
implicitly on set theory, and can be seen in retrospect to involve the
assumption of a Power Set principle. Both took as given the set of
rational numbers, and for the definition of **R** they
relied on a certain totality of infinite sets of rational numbers
(either sequences, or Dedekind cuts). With this, too, constructivistic
criticism of set theory began to emerge, as Leopold Kronecker started
to make objections to such infinitary procedures. Simultaneously,
there began a study of the topology of **R**, in
particular in the work of Weierstrass, Dedekind, and Cantor. The
set-theoretic approach was also exploited by Dedekind in his
pioneering work on algebraic number theory (and algebraic geometry in
1882), and by several authors in the fields of real analysis and
complex analysis.

Cantor's derived sets are of particular interest (for the context of
this idea in real analysis, see e.g. Dauben 1979, Hallett 1984, Lavine
1994, Kanamori 1996, Ferreirós 1999). Cantor took as given the
“conceptual sphere” of the real numbers, and he considered
arbitrary subsets *P*, which he called ‘point
sets’. A real number *r* is called a *limit point*
of *P*, when all neighbourhoods of *r* contain points of
*P*. This can only happen if *P* is infinite. With that
concept, introduced by Weierstrass, the *derived set*
*P*′ of *P* can be defined, as Cantor did, to be
the *set of all* the limit points of *P*. In general
P′ may be infinite and have its own limit points. Thus one can
iterate the operation and obtain further derived sets
*P*″, *P*″′…
*P*^{(n}^{)} … It is easy to
give examples of a set *P* that will give rise to non-empty
derived sets *P*^{(n}^{)} for all
finite *n*. (A rather trivial example is *P* =
**Q**_{[0,1]}, the set of rational numbers in the
unit interval; in this case *P*′ = [0,1] =
*P*″.) Thus one can define
*P*^{(∞)} as the intersection of all
*P*^{(n)} for finite *n*. This was Cantor's
first encounter with transfinite iterations.

Then, in late 1873, came a surprising discovery that fully opened the
realm of the transfinite. In correspondence with Dedekind (see Ewald
1996, vol. 2), Cantor asked the question whether the infinite sets
**N** of the natural numbers and **R** of
real numbers can be placed in one-to-one correspondence. In reply,
Dedekind offered a surprising proof that the set *A* of all
algebraic numbers is denumerable (i.e., there is a one-to-one
correspondence with **N**). A few days later, Cantor was
able to prove that the assumption that **R** is
denumerable leads to a contradiction. To this end, he employed the
Bolzano-Weierstrass principle of completeness mentioned above. Thus he
had shown that there are more elements in **R** than in
**N** or **Q** or *A*, in the precise
sense that the cardinality of **R** is strictly greater
than that of **N**.

## 2. Consolidation

Set theory was beginning to become an essential ingredient of the new “modern” approach to mathematics. But this viewpoint was contested, and its consolidation took a rather long time. Dedekind's algebraic style only began to find followers in the 1890s; David Hilbert was among them. The soil was better prepared for the modern theories of real functions: Italian, German, French and British mathematicians contributed during the 1880s. And the new foundational views were taken up by Peano and his followers, by Frege to some extent, by Hilbert in the 1890s, and later by Russell.

Meanwhile, Cantor spent the years 1878 to 1885 publishing key works
that helped turn set theory into an autonomous branch of mathematics.
Let's write *A* ≡ *B* in order to express that the
two sets *A*, *B* can be put in one-to-one
correspondence (have the same cardinality). After proving that the
irrational numbers can be put in one-to-one correspondence with
**R**, and, surprisingly, that also
**R**^{n} ≡ **R**,
Cantor conjectured in 1878 that any subset of **R** would
be either denumerable (≡ **N**) or ≡
**R**. This is the first and weakest form of the
celebrated *Continuum Hypothesis*. During the following years,
Cantor explored the world of point sets, introducing several important
topological ideas (e.g., perfect set, closed set, isolated set), and
arrived at results such as the Cantor-Bendixson theorem.

A point set *P* is *closed* iff its derived set
*P*′ ⊆ *P*, and *perfect* iff
*P* = *P*′. The Cantor-Bendixson theorem then
states that a closed point set can be decomposed into two subsets
*R* and *S*, such that *R* is denumerable and
*S* is perfect (indeed, *S* is the
*a*^{th} derived set of *P*, for a countable
ordinal *a*). Because of this, closed sets are said to have
the perfect set property. Furthermore, Cantor was able to prove, in
(1884), that perfect sets have the power of the continuum. Both
results implied that the Continuum Hypothesis is valid for all closed
point sets. Many years later, in 1916, Pavel Aleksandrov and Felix
Hausdorff were able to show that the broader class of Borel sets have
the perfect set property too.

His work on points sets led Cantor, in 1882, to conceive of the
*transfinite numbers* (see Ferreirós 1999,
267*ff*). This was a turning point in his research, for from
then onward he studied *abstract* set theory independently of
more specific questions having to do with point sets and their
topology (until the mid-1880s, these questions had been prominent in
his agenda). Subsequently, Cantor focused on the transfinite cardinal
and ordinal numbers, and on general order types, independently of the
topological properties of **R**.

The transfinite ordinals were introduced as new numbers in an
important mathematico-philosophical paper of 1883, *Grundlagen
einer allgemeinen Mannichfaltgkeitslehre* (notice that Cantor
still uses Riemann's term). Cantor defined them by means of two
“generating principles”: the first (1) yields the
successor *a*+1 for any given number *a*, while the
second (2) stipulates that there is a number *b* which follows
immediately after any given sequence of numbers without a last
element. Thus, after all the finite numbers comes, by (2), the first
transfinite number, ω (read: omega); and this is
followed by ω+1, ω+2, …,
ω+ω = ω·2, …,
ω·*n*, ω·*n* +1,
…,ω^{2}, ω^{2}+1,
…, ω^{ω}, … and so on
and on. Whenever a sequence without last element appears, one can go
on and, so to say, jump to a higher stage by (2).

The introduction of these new numbers seemed like idle speculation to
most of his contemporaries, but for Cantor they served two very
important functions. To this end, he classified the transfinite
ordinals as follows: the “first number class” consisted of
the finite ordinals, the set **N** of natural numbers;
the “second number class” was formed by ω and all numbers
following it (including ω^{ω}, and
many more) that have *only a denumerable* *set of
predecessors*. This crucial condition was suggested by the problem
of proving the Cantor-Bendixson theorem. On that basis, Cantor could
establish the results that the cardinality of the “second number
class” is greater than that of **N**; and that no
intermediate cardinality exists. Thus, if you write
*card*(**N**) = ℵ_{0} (read: aleph
zero), his theorems justified calling the cardinality of the
“second number class” ℵ_{1}.

After the second number class comes a “third number class”
(all transfinite ordinals whose set of predecessors has cardinality
ℵ_{1}); the cardinality of this new number class can
be proved to be ℵ_{2}. And so on. The first function
of the transfinite ordinals was, thus, to establish a well-defined
scale of increasing transfinite cardinalities. (The aleph notation
used above was introduced by Cantor only in 1895.) This made it
possible to formulate much more precisely the problem of the
continuum; Cantor's conjecture became the hypothesis that
*card*(**R**) =
ℵ_{1}. Furthermore, relying on the transfinite
ordinals, Cantor was able to prove the Cantor-Bendixson theorem,
rounding out the results on point sets that he had been elaborating
during these crucial years.

The study of the transfinite ordinals directed Cantor's attention
towards ordered sets, and in particular *well-ordered sets*. A
set *S* is well-ordered by a relation < iff < is a total
order and every subset of
*S* has a least element in the <-ordering. (The real numbers
are not well-ordered in their usual order: just consider an open
interval. Meanwhile, **N** is the simplest infinite
well-ordered set.) Cantor argued that the transfinite ordinals truly
deserve the name of *numbers*, because they express the
“type of order” of any possible well-ordered set. Notice
also that it was easy for Cantor to indicate how to reorder the
natural numbers so as to make them correspond to the order types
ω+1, ω+2, …, ω·2,
…, ω·*n*, …,
ω^{2}, …,
ω^{ω}, … and so on. (For
instance, reordering **N** in the form: 2, 4, 6,
…, 5, 15, 25, 35, …, 1, 3, 7, 9, … we obtain a
set that has order type ω·3.)

Notice too that the Continuum Hypothesis, if true, would entail that
the set **R** of real numbers can indeed be
well-ordered. Cantor was so committed to this viewpoint, that he
presented the further hypothesis that *every* set can be
well-ordered as “a fundamental and momentous law of
thought”. Some years later, Hilbert called attention to both the
Continuum Hypothesis and the well-ordering problem as Problem 1 in his
celebrated list of ‘Mathematische Probleme’ (1900). Doing
so was an intelligent way of emphasizing the importance of set theory
for the future of mathematics, and the fruitfulness of its new methods
and problems.

In 1895 and 1897, Cantor published his last two articles. They were a
well-organized presentation of his results on the transfinite numbers
(cardinals and ordinals) and their theory, and also on order types and
well-ordered sets. However, these papers did not advance significant
new ideas. Unfortunately, Cantor had doubts about a third part he had
prepared, which would have discussed very important issues having to
do with the problem of well-ordering and the paradoxes (see below).
Surprisingly, Cantor also failed to include in the 1895/97 papers a
theorem which he had published some years before which is known simply
as Cantor's Theorem: given any set *S*, there exists another
set whose cardinality is greater (this is the power set
** p**(

*S*), as we now say — Cantor used instead the set of functions

*f*:

*S*→ {0, 1}). In the same short paper (1892), Cantor presented his famous proof that

**R**is non-denumerable by the method of diagonalisation, a method which he then extended to prove Cantor's Theorem. (A related form of argument had appeared earlier in the work of P. du Bois-Reymond.)

Meanwhile, other authors were exploring the possibilities opened by
set theory for the foundations of mathematics. Most important was
Dedekind's contribution (1888) with a deep presentation of the theory
of the natural numbers. He formulated some basic principles of set
(and mapping) theory; gave axioms for the natural number system;
proved that mathematical induction is conclusive and recursive
definitions are flawless; developed the basic theory of arithmetic;
introduced the finite cardinals; and proved that his axiom system is
categorical. His system had four axioms. Given a function φ
defined on *S*, a set *N* ⊆ *S*, and a
distinguished element 1 ∈ *N*, they are as follows:

(α) φ(N) ⊂N

(β)N= φ_{o}{1}

(γ) 1 ∉ φ(N)

(δ) the function φ is injective.

Condition (β) is crucial since it ensures minimality for the set
of natural numbers, which accounts for the validity of proofs by
mathematical induction. *N* = φ_{o}{1} is read:
*N* is the *chain of* singleton {1} under the function
φ, that is, the minimal closure of {1} under the function φ. In
general, one considers the chain of a set *A* under an
arbitrary mapping γ, denoted by
γ_{o}(*A*); in his booklet Dedekind developed
an interesting theory of such chains, which allowed him to prove the
Cantor-Bernstein theorem. The theory was later generalized by
Zermelo.

In the following years, Giuseppe Peano gave a more superficial (but
also more famous) treatment of the natural numbers, employing the new
symbolic language of logic, and Gottlob Frege elaborated his own ideas,
which however fell prey to the paradoxes. An important book inspired by
the set-theoretic style of thinking was Hilbert's *Grundlagen
der Geometrie* (1899), which took the “mathematics of
axioms” one step beyond Dedekind through a rich study of
geometric systems motivated by questions concerning the independence of
his axioms. Hilbert's book made clear the new axiomatic
methodology that had been shaping up in connection with the novel
methods of set theory, and he combined it with the axiomatic trends
coming from projective geometry.

## 3. Critical Period

In the late nineteenth century, it was a widespread idea that pure
mathematics is nothing but an elaborate form of arithmetic. Thus it was
usual to talk about the “*arithmetisation*” of
mathematics, and how it had brought about the highest standards of
rigor. With Dedekind and Hilbert, this viewpoint led to the idea of
*grounding all of pure mathematics in set theory*. The most
difficult steps in bringing forth this viewpoint had been the
establishment of a theory of the real numbers, and a set-theoretic
reduction of the natural numbers. Both problems had been solved by the
work of Cantor and Dedekind. But precisely when mathematicians were
celebrating that “full rigor” had been finally attained,
serious problems emerged for the foundations of set theory. First
Cantor, and then Russell, discovered the paradoxes in set theory.

Cantor was led to the paradoxes by having introduced the
“conceptual sphere” of the transfinite numbers. Each
transfinite ordinal is the order type of the set of its predecessors;
e.g., ω is the order type of {0, 1, 2, 3, …}, and
ω+2 is the order type of {0, 1, 2, 3, …,
ω, ω+1}. Thus, to each initial segment of
the series of ordinals, there corresponds an immediately greater
ordinal. Now, the “whole series” of all transfinite
ordinals would form a well-ordered set, and to it there would
correspond a new ordinal number. This is unacceptable, for this
ordinal *o* would have to be greater than all members of the
“whole series”, and in particular *o* <
*o*. This is usually called the *Burali-Forti paradox*,
or paradox of the ordinals (although Burali-Forti himself failed to
formulate it clearly, see Moore & Garciadiego 1981).

Although Cantor might have found that paradox as early as 1883,
immediately after introducing the transfinite ordinals (for arguments
in favour of this idea see Purkert & Ilgauds 1987 and Tait 2000),
the evidence indicates clearly that it was not until 1896/97 that he
found this paradoxical argument and realized its implications. By this
time, he was also able to employ Cantor's Theorem to yield the
*Cantor paradox*, or paradox of the alephs: if there existed a
“set of all” cardinal numbers (alephs), Cantor's Theorem applied to
it would give a new aleph ℵ, such that ℵ < ℵ.
The great set theorist realized perfectly well that these paradoxes
were a fatal blow to the “logical” approaches to sets
favoured by Frege and Dedekind. Cantor emphasized that his views were
“*in diametrical opposition*” to Dedekind's,
and in particular to his “naïve assumption that *all
well-defined collections, or systems*, are also
‘*consistent systems*’ ” (see the letter to
Hilbert, Nov. 15, 1899, in Purkert & Ilgauds 1987, 154). (Contrary
to what has often been claimed, Cantor's ambiguous definition of
set in his paper of 1895 was intended to be “diametrically
opposite” to the logicists’ understanding of sets –
often called “naïve” set theory, or can more
properly be called the dichotomy conception of sets, following a
suggestion of Gödel.)

Cantor thought he could solve the problem of the paradoxes by distinguishing between “consistent multiplicities” or sets, and “inconsistent multiplicities”. But, in the absence of explicit criteria for the distinction, this was simply a verbal answer to the problem. Being aware of deficiencies in his new ideas, Cantor never published a last paper he had been preparing, in which he planned to discuss the paradoxes and the problem of well-ordering (we know quite well the contents of this unpublished paper, as Cantor discussed it in correspondence with Dedekind and Hilbert; see the 1899 letters to Dedekind in Cantor 1932, Ewald 1996, vol. II, or Cantor 2005). Cantor presented an argument that relied on the “Burali-Forti” paradox of the ordinals, and aimed to prove that every set can be well-ordered. This argument was later rediscovered by the British mathematician P. E. B. Jourdain, but it is open to criticism because it works with “inconsistent multiplicities”.

Cantor's paradoxes convinced Hilbert and Dedekind that there were important doubts concerning the foundations of set theory. Hilbert formulated a paradox of his own (Peckhaus & Kahle 2002), and discussed the problem with mathematicians in his Göttingen circle. Ernst Zermelo was thus led to discover the paradox of the “set” of all sets that are not members of themselves (Rang & Thomas 1981). This was independently discovered by Bertrand Russell, who was led to it by a careful study of Cantor's Theorem, which conflicted deeply with Russell's belief in a universal set. Some time later, in June 1902, he communicated the “contradiction” to Gottlob Frege, who was completing his own logical foundation of arithmetic, in a well-known letter. Frege's reaction made very clear the profound impact of this contradiction upon the logicist program. “Can I always speak of a class, of the extension of a concept? And if not, how can I know the exceptions?” Faced with this, “I cannot see how arithmetic could be given a scientific foundation, how numbers could be conceived as logical objects” (Frege 1903, 253).

The publication of Volume II of Frege's *Grundgesetze*, and
above all Russell's work *The Principles of Mathematics*, made
the mathematical community fully aware of the existence of the
set-theoretic paradoxes, of their impact and importance. There is
evidence that, up to then, even Hilbert and Zermelo had not fully
appreciated the damage. Notice that the Russell-Zermelo paradox
operates with very basic notions – negation and set membership
– concepts that had widely been regarded as purely logical. The
“set” *R* = {*x*: *x* ∉
*x*} exists according to the principle of comprehension (which
allows any open sentence to determine a class), but if so, *R*
∈ *R* *iff* *R* ∉ *R*. It is a
direct contradiction to the principle favoured by Frege and
Russell.

It was obviously necessary to clarify the foundations of set theory,
but the overall situation did not make this an easy task. The different
competing viewpoints were widely divergent. Cantor had a metaphysical
understanding of set theory and, although he had one of the sharpest
views of the field, he could not offer a precise foundation. It was
clear to him (as it had been, somewhat mysteriously, to Ernst
Schröder in his *Vorlesungen über die Algebra der
Logik*, 1891) that one has to reject the idea of a Universal Set,
favoured by Frege and Dedekind. Frege and Russell based their approach
on the principle of comprehension, which was shown contradictory.
Dedekind avoided that principle, but he postulated that the Absolute
Universe was a set, a “thing” in his technical sense of
*Gedankending*; and he coupled that assumption with full
acceptance of arbitrary subsets.

This idea of admitting arbitrary subsets had been one of the deep
inspirations of both Cantor and Dedekind, but none of them had
thematized it. (Here, their modern understanding of analysis played a
crucial but implicit background role, since they worked within the
Dirichlet-Riemann tradition of “arbitrary” functions.) As
for the now famous iterative conception there were some elements of it
(particularly in Dedekind's work, with his iterative development of
the number system, and his views on “systems” and
“things”), but it was conspicuously absent from many of
the relevant authors. Typically, e. g., Cantor did not iterate the
process of set formation: he tended to consider sets
of *homogeneous* elements, elements which were taken to belong
“in some conceptual sphere” (either numbers, or points, or
functions, or even physical particles – but not
intermingled). The iterative conception was first suggested by Kurt
Gödel in 1933, in connection with technical work by von Neumann
and Zermelo a few years earlier; Gödel would insist on the idea
in his well-known paper on Cantor's continuum problem. It came
only *post facto*, after very substantial amounts of set theory
had been developed and fully systematized.

This variety of conflicting viewpoints contributed much to the overall
confusion, but there was more. In addition to the paradoxes discussed
above (set-theoretic paradoxes, as we say), the list of
“logical” paradoxes included a whole array of further ones
(later called “semantic”). Among these are paradoxes due
to Russell, Richard, König, Berry, Grelling, etc., as well as the
ancient liar paradox due to Epimenides. And the diagnoses and proposed
cures for the damage were tremendously varied. Some authors, like
Russell, thought it was essential to find a new logical system that
could solve all the paradoxes at once. This led him into the ramified
type theory that formed the basis of
*Principia Mathematica*
(3 volumes, 1910–1913),
his joint work with
Alfred Whitehead.
Other authors, like Zermelo, believed that most of those paradoxes
dissolved as soon as one worked within a restricted axiomatic
system. They concentrated on the “set-theoretic” paradoxes
(as we have done above), and were led to search for axiomatic systems
of set theory.

Even more importantly, the questions left open by Cantor and
emphasized by Hilbert in his first problem of 1900 caused heated
debate. At the International Congress of Mathematicians at
Heidelberg, 1904, Gyula (Julius) König proposed a very detailed
proof that the cardinality of the
continuum *cannot* *be* any of Cantor's alephs. His
proof was only flawed because he had relied on a result previously
“proven” by Felix Bernstein, a student of Cantor and
Hilbert. It took some months for Felix Hausdorff to identify the flaw
and correct it by properly stating the special conditions under which
Bernstein's result was valid (see Hausdorff 2002). Once thus
corrected, König's theorem became one of the very few results
restricting the possible solutions of the continuum problem, implying
e.g. that Card(**R**) is not equal to
ℵ_{ω}. Meanwhile, Zermelo was able to present a
proof that every set can be well-ordered, using the Axiom of
Choice. During the following year, prominent mathematicians in
Germany, France, Italy and England discussed the Axiom of Choice and
its acceptability.

This started a whole era during which the Axiom of Choice was treated most carefully as a dubious hypothesis (see the monumental study by Moore 1982). And that is ironic, for, among all of the usual principles of set theory, the Axiom of Choice is the only one that explicitly enforces the existence of some arbitrary subsets. But, important as this idea had been in motivating Cantor and Dedekind, and however entangled it is with classical analysis, infinite arbitrary subsets were rejected by many other authors. Among the most influential ones in the following period, one ought to emphasize the names of Russell, Hermann Weyl, and of course Brouwer.

The impressive polemics which surrounded his Well-Ordering Theorem, and the most interesting and difficult problem posed by the foundations of mathematics, led Zermelo to concentrate on axiomatic set theory. As a result of his incisive analysis, in 1908 he published his axiom system, showing how it blocked the known paradoxes and yet allowed for a masterful development of the theory of cardinals (and ordinals). This, however, is the topic of another entry.

## 4. From Zermelo to Gödel

In the period 1900–1930, the rubric “set theory” was still understood to include topics in topology and the theory of functions. Although Cantor, Dedekind, and Zermelo had left that stage behind to concentrate on pure set theory, for mathematicians at large this would still take a long time. Thus, at the first International Congress of Mathematicians, 1897, keynote speeches given by Hadamard and Hurwitz defended set theory on the basis of its importance for analysis. Around 1900, motivated by topics in analysis, important work was done by three French experts: Borel, Baire and Lebesgue. Their work inaugurated the development of descriptive set theory by extending Cantor's studies on definable sets of real numbers (in which he had established that the Continuum Hypothesis is valid for closed sets). They introduced the hierarchy of Borel sets, the Baire hierarchy of functions, and the concept of Lebesgue measure — a crucial concept of modern analysis.

Descriptive set theory (DST) is the study of certain kinds of
definable sets of real numbers, which are obtained from simple kinds
(like the open sets and the closed sets) by well-understood operations
like complementation or projection. The *Borel sets* were the
first hierarchy of definable sets, introduced in the 1898 book of
Émile Borel; they are obtained from the open sets by iterated
application of the operations of countable union and
complementation. In 1905 Lebesgue studied the Borel sets in an epochal
memoir, showing that their hierarchy has levels for all countable
ordinals, and analyzing the Baire functions as counterparts of the
Borel sets. The main aim of descriptive set theory is to find
structural properties common to all such definable sets: for instance,
the Borel sets were shown to have the perfect set property (if
uncountable, they have a perfect subset) and thus to comply with the
continuum hypothesis (CH). This result was established in 1916 by
Hausdorff and by Alexandroff, working independently. Other important
“regularity properties” studied in DST are the property of
being Lebesgue measurable, and the so-called property of Baire (to
differ from an open set by a so-called meager set, or set of first
category).

Also crucial at the time was the study of the *analytic* sets,
namely the continuous images of Borel sets, or equivalently, the
projections of Borel sets. The young Russian mathematician Mikhail
Suslin found a mistake in Lebesgue's 1905 memoir when he realized that
the projection of a Borel set is not Borel in general. However, he was
able to establish that the analytic sets, too, possess the perfect set
property and thus verify CH. By 1923 Nikolai Lusin and Waclaw
Sierpiński were studying the *co-analytic* sets, and this
was to lead them to a new hierarchy of *projective* sets, which
starts with the analytic sets (Σ^{1}_{1}), their
complements (co-analytic, Π^{1}_{1} sets), the
projections of these last (Σ^{1}_{2} sets), their
complements (Π^{1}_{2} sets), and so on. During
the 1920s much work was done on these new types of sets, mainly by
Polish mathematicians around Sierpiński and by the Russian school
of Lusin and his students. A crucial result obtained by
Sierpiński was that every Σ^{1}_{2} set is
the union of ℵ_{1} Borel sets (the same holds for
Σ^{1}_{1} sets), but this kind of traditional
research on the topic would stagnate after around 1940.

Soon Lusin, Sierpiński and their colleagues were finding extreme difficulties in their work. Lusin was so much in despair that, in a paper of 1925, he came to the “totally unexpected” conclusion that “one does not know and one will never know” whether the projective sets have the desired regularity properties (quoted in Kanamori [1993], 250). Such comments are highly interesting in the light of later developments, which have led to hypotheses that solve all the relevant questions (Projective Determinacy). They underscore the difficult methodological and philosophical issues raised by these more recent hypotheses, namely the problem concerning the kind of evidence that backs them.

Lusin summarized the state of the art in his 1930
book *Leçons sur les ensembles analytiques* (Paris,
Gauthier-Villars), which was to be a key reference for years to
come. Since this work, it has become customary to present results in
DST for the Baire space ω^{ω} of infinite sequences
of natural numbers, which in effect had been introduced by René
Baire in his dissertation of 1899 (anticipated by Dedekind in
1891). Baire space is endowed with a certain topology that makes it
homeomorphic to the set of the irrational numbers, and it is regarded
by experts to be “perhaps the most fundamental object of study
of set theory” next to the set of natural numbers (Moschovakis
1994).

This stream of work on DST must be counted among the most important contributions made by set theory to analysis and topology. But what had begun as an attempt to prove the Continuum Hypothesis could not reach this goal. Soon it was shown using the Axiom of Choice that there are non-Lebesgue measurable sets of reals (Vitali, 1905), and also uncountable sets of reals with no perfect subset (Bernstein, 1908). Such results made clear the impossibility of reaching the goal of CH by concentrating on definable and “well-behaved” sets of reals.

Also, with Gödel's work around 1940 (and also with forcing in the
1960s) it became clear why the research of the 1920s and 30s had
stagnated: the fundamental new independence results showed that the
theorems established by Suslin (perfect set property for analytic
sets), Sierpinski (Σ^{1}_{2} sets as unions of
ℵ_{1} Borel sets) and a few others were the best
possible results on the basis of axiom system ZFC. This is important
philosophically: already an exploration of the world of sets definable
from the open (or closed) sets by complement, countable union, and
projection had sufficed to reach the limits of the ZFC system. Hence
the need for new axioms, that Gödel emphasized after World War
II.

Let us now turn to Cantor's other main legacy, the study of
transfinite numbers. By 1908 Hausdorff was working on uncountable
order types and introduced the Generalized Continuum Hypothesis
(2^{ℵa} =
ℵ_{a+1}). He was also the first to consider
the possibility of an “exorbitant” cardinal, namely a
weakly inaccessible, i.e., a regular cardinal that is not a successor
(a cardinal α is called regular if decomposing α into a sum
of smaller cardinals requires α-many such numbers). Few years
later, in the early 1910s, Paul Mahlo was studying hierarchies of such
large cardinals in work that pioneered what was to become a central
area of set theory; he obtained a succession of inaccessible cardinals
by employing a certain operation that involves the notion of a
stationary subset; they are called Mahlo cardinals. But the study of
large cardinals developed slowly. Meanwhile, Hausdorff's textbook
*Grundzüge der Mengenlehre* (1914; see his 2002)
introduced two generations of mathematicians into set theory and
general topology.

The next crucial steps into the “very high” infinite were
done in 1930. The notion of strongly inaccessible cardinals was then
isolated by Sierpinński & Tarski, and by Zermelo. A strong
inaccessible is a regular cardinal α such that 2^{x} is
less than α whenever x < α. While weak inaccessibles merely
involve closure under the successor operation, strong inaccessibles
involve a much stronger notion of closure under the powerset
operation. That same year, in a path-breaking paper on models of ZFC,
Zermelo established a link between the uncountable (strongly)
inaccessible cardinals and certain “natural” models of ZFC
(in which work he assumed, so to say, that the powerset operation is
fully determined).

In that same year, Stanislaw Ulam was led by considerations coming out of analysis (measure theory) to a concept that was to become central: measurable cardinals. It turned out that such cardinals, defined by a measure-theoretic property, had to be (strongly) inaccessible. Indeed, many years later it would be established (by Hanf, working upon Tarski's earlier work) that the first inaccessible cardinal is not measurable, showing that these new cardinals were even more “exorbitant”. As one can see, the Polish school led by Sierpiński had a very central role in the development of set theory between the Wars. Measurable cardinals came to special prominence in the late 1960s when it became clear that the existence of a measurable cardinal contradicts Gödel's axiom of constructibility (V = L in the class notation). This again vindicated Gödel's convictions, expressed in what is sometimes called “Gödel's program” for new axioms.

Set-theoretic mathematics continued its development into the powerful
axiomatic and structural approach that was to dominate much of the
20^{th} century. To give just a couple of examples, Hilbert's
early axiomatic work (e.g. in his arch-famous *Foundations of
Geometry*) was deeply set-theoretic; Ernst Steinitz published in
1910 his research on abstract field theory, making essential use of
the Axiom of Choice; and around the same time the study of function
spaces began with work by Hilbert, Maurice Fréchet, and others.
During the 1920s and 30s, the first specialized mathematics
journal, *Fundamenta Mathematicae*, was devoted to set theory
as then understood (centrally including topology and function
theory). In those decades structural algebra came of age, abstract
topology was gradually becoming an independent branch of study, and
the study of set theory initiated its metatheoretic turn. Ever since,
“set theory” has generally been identified with the branch
of mathematical logic that studies transfinite sets, originating in
Cantor's result that **R** has a greater cardinality
than **N**. But, as the foregoing discussion shows, set
theory was both effect and cause of the rise of modern mathematics:
the traces of this origin are indelibly stamped on its axiomatic
structure.

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## Other Internet Resources

- A History of Set Theory, by J.J. O'Connor and E.F. Robertson, in The MacTutor History of Mathematics archive. Note that their reconstruction conflicts at some points with the one provided here.
- Godel's Program (PowerPoint), an interesting talk by John R. Steel (Mathematics, U.C./Berkeley).
- A Home Page for the Axiom of Choice, maintained by Eric Schechter (Mathematics, Vanderbilt University).

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