5 OBJECTIONS TO THE AXIOMATIC APPROACH. OTHER PROPOSALS
What are the chief goals of axiomatic set theory, and to what extent have these goals been successfully
attained? In order to answer that question we must remember the circumstances which led
mathematicians, in the early years of the twentieth century, to search for an axiomatic basis to set
theory. The ideas of Cantor had already thoroughly permeated the fabric of modern mathematics, and
had become indispensable tools of the working mathematician. Algebra and analysis were formulated
within a framework of set theory, and some of the most elegant, powerful new results in these fields
were established by using the methods introduced by Cantor and his followers. Thus, when the
paradoxes were discovered and there arose doubts as to the basic validity of Cantor’s system, most
mathematicians were understandably reluctant to give it up; they trusted that some way would be
found to circumvent the contradictions and preserve, if not all, at least most of Cantor’s results.
Hilbert once wrote in this connection: “We will not be expelled from the paradise into which Cantor
has led us.”
With the discovery of newer paradoxes, and the failure of all the initial attempts to avoid them, it
became increasingly clear that it would not be possible to preserve intuitive set theory in its entirety.
Something—possibly quite a lot—would have to be relinquished. The best that one could hope for
was to retain as much of intuitive set theory as was needed to save the new results of modern
mathematics and provide an adequate framework for classical mathematics.
Briefly, then, axiomatic set theory was created to achieve a limited aim: it had to provide a firm
foundation for a system of set theory which—while it did not need to be as comprehensive as intuitive
set theory—must include all of Cantor’s basic results as well as the constructions (such as the number
systems, functions, and relations) needed for classical mathematics.
The systems of both Zermelo and von Neumann were successful in achieving this limited aim. But
the amount of intuitive set theory which they had to sacrifice was considerable. For example, in
Zermelo’s system, as we have already seen, the intuitive way of making sets—by naming a property
of objects and forming the set of all objects which have that property—does not take place at all. It is
replaced by the axiom of selection, in which properties are allowed only to determine subsets of
given sets. Furthermore, the only admissible “properties” are those which can be expressed entirely
in terms of the seven symbols ∈, ∨, ∧, ¬, ⇒, ∀, ∃ and variables x, y, z, … As a result, many of
the things that we normally think of as sets—for example, the “set of all apples,” the “set of all atoms
in the universe”—are not admissible as “sets” in axiomatic set theory. In fact, the only “sets” which
the axioms provide for are, first, the empty set Ø, and then constructions such as {Ø}, {Ø, {Ø}}, etc.,
which can be built up from the empty set. It is remarkable fact that all of mathematics can be based
upon such a meager concept of set.
While the various axiomatic systems of set theory saved mathematics from its immediate peril, they
failed to satisfy a great many people. In particular, many who were sensitive to the elegance and
universality of mathematics were quick to point out that the creations of Zermelo and von Neumann
must be regarded as provisional solutions—as expedients to solve a temporary problem; they will
have to be replaced, sooner or later, by a mathematical theory of broader scope, which treats the
concept of “set” in its full, intuitive generality.
This argument against axiomatic set theory—that it deals with an amputated version of our intuitive
conception of a set—has important philosophical ramifications; it is part of a far wider debate, on the
nature of mathematical “truth.” The debate centers around the following question: Are mathematical
concepts creations (that is, inventions) of the human mind, or do they exist independently of us in a
“platonic” realm of concepts, merely to be discovered by the mathematician? The latter opinion is
often referred to as “platonic realism” and is the dominant viewpoint of classical mathematics. We
illustrate these two opposing points of view by showing how they apply to a particular concept—the
notion of natural numbers. From the viewpoint of platonic realism, the concepts “one,” “two,”
“three,” and so on, exist in nature and existed before the first man began to count. If intelligent beings
exist elsewhere in the universe, then, no matter how different they are from us, they have no doubt
discovered the natural numbers and found them to have the same properties they have for us. On the
other hand, according to the opposing point of view, while three cows, three stones, or three trees
exist in nature, the natural number three is a creation of our minds; we have invented a procedure for
constructing the natural numbers (by starting from zero and adding 1 each time, thus producing
successively 1, 2, 3, etc.) and have in this manner fashioned a conceptual instrument of our own
making.
How does platonic realism affect the status of axiomatic set theory? From the point of view of
platonic realism, mathematical objects are given to us ready-made, with all their features and all their
properties. It follows that to say a mathematical theorem is true means it expresses a correct
statement about the relevant mathematical objects. (For example, the proposition 2 + 2 = 4 is not
merely a formal statement provable in arithmetic; it states an actual fact about numbers.) Now—if
we admit that mathematical objects are given to us with all their properties, it follows, in particular,
that the notion of set is a fixed, well-defined concept which we are not free to alter for our own
convenience. Thus the “sets” created by Zermelo and von Neumann do not exist, and theorems which
purport to describe these nonexistent objects are false! In conclusion, if we were to accept a strict
interpretation of platonic realism, we would be forced to reject the systems of Zermelo and von
Neumann as mathematically invalid.
Fortunately, the trend, for some time now, has been away from platonism and toward a more
flexible, more “agnostic” attitude toward mathematical “truth.” For one thing, developments in
mathematics have been conforming less and less to the pattern dictated by platonic philosophy. For
another, the cardinal requirement of platonism—that every mathematical object correspond to a
definite, distinct object of our intuition (just as “point” and “line” refer to well-defined objects of our
spatial intuition)—came to be an almost unbearable burden on the work of creative mathematicians
by the nineteenth century. They were dealing with a host of new concepts (such as complex numbers,
abstract laws of composition, and the general notion of function) which did not lend themselves to a
simple interpretation in concrete terms. The case of the complex numbers is a good illustration of
what was happening. Classical mathematics never felt at ease with the complex numbers, for it lacked
a suitable “interpretation” of them, and as a result there were nagging doubts as to whether such things
really “existed.” Real numbers may be interpreted as lengths or quantities, but the square root of a
negative real number—this did not seem to correspond to anything in the real world or in our intuition
of number. Yet the system of the complex numbers arises in a most natural way—as the smallest
number system which contains the real numbers and includes the roots of every algebraic equation
with real coefficients; whether or not the complex numbers have a physical or psychological
counterpart seems irrelevant.
The case of the complex numbers strikes a parallel with the problem of axiomatic set theory. For
the “sets” created by Zermelo and von Neumann arise quite naturally in a mathematical context. They
give us the simplest notion of set which is adequate for mathematics and yields a consistent axiomatic
theory. Whether or not we can interpret them intuitively may be relatively unimportant.
Be that as it may, many mathematicians in the early 1900’s were reluctant to make so sharp a break
with tradition as axiomatic set theory seemed to demand. Furthermore, they felt, on esthetic grounds,
that a mathematical theory of sets should describe all the things—and only those things—which our
intuition recognizes to be sets. Among them was Bertrand Russell; in his efforts to reinstate intuitive
set theory, Russell was led to the idea that we may consider sets to be ordered in a hierarchy of
“levels,” where, if A and are sets and A is an element of , then is “one level higher” than A.
For example, in plane geometry, a circle (regarded as a set of points) is one level below a family of
circles, which, in turn, is one level below a set of families of circles. This basic idea was built by
Russell into a theory called the theory of types, which can be described, in essence, as follows.
Every set has a natural number assigned to it, called its level. The simplest sets, those of level 0,
are called individuals—they do not have elements. A collection of individuals is a set of level 1; a
collection of sets of level 1 is a set of level 2; and so on. In the theory of types the expression a ∈ B
is only meaningful if, for some number n, a is a set of level n and B is a set of level n + 1. It follows
that the statement x ∈ x has no meaning in the theory of types, and as a result, Russell’s paradox
vanishes for the simple reason that it cannot even be formulated.
Russell’s theory of types is built upon a beautifully simple idea. Unfortunately, in order to make it
“work,” Russell was forced to add a host of new assumptions, until finally the resulting theory
became too cumbersome to work with and too complica
การคัดค้านการวิธี AXIOMATIC 5 ข้อเสนออื่น ๆเป้าหมายสำคัญของทฤษฎีเซต axiomatic คืออะไร และขอบเขตเป้าหมายเหล่านี้ได้สำเร็จบรรลุหรือไม่ เพื่อตอบคำถามที่เราต้องจำสถานการณ์ที่mathematicians ในช่วงปีแรก ๆ ของศตวรรษที่ยี่สิบ ค้นหาการเป็น axiomatic ตั้งทฤษฎีการ ความคิดของคันทอร์มี permeated ผ้าของคณิตศาสตร์สมัยใหม่ แล้วอย่างถี่ถ้วน และได้กลายเป็นเครื่องมือสำคัญของนักคณิตศาสตร์ทำงาน สูตรพีชคณิตและการวิเคราะห์ภายในกรอบของทฤษฎีเซต และบางที่สุดหรูหรา มีประสิทธิภาพผลลัพธ์ใหม่ในฟิลด์เหล่านี้ได้ก่อตั้งขึ้น โดยใช้วิธีที่แนะนำ โดยคันทอร์และลูกศิษย์ของเขา ดังนั้น เมื่อการพบ paradoxes และมีเกิดข้อสงสัยว่าใช้พื้นฐานของระบบของคันทอร์ มากที่สุดmathematicians ได้ช้า ๆ ไม่ให้ขึ้น พวกเขาเชื่อว่า จะเป็นอย่างพบเพื่อหลีกเลี่ยงกันข้ามและรักษา ถ้าไม่ทั้งหมด ที่สุดของผลลัพธ์ของคันทอร์ฮิลแบร์ทครั้งเขียนต่อ: "เราจะไม่ถูกขับไล่ออกจากสวรรค์ไปที่คันทอร์นำเรา"มีการค้นพบใหม่ paradoxes และความล้มเหลวทั้งหมดเริ่มต้นพยายามหลีกเลี่ยงพวกเขา มันกลายเป็นชัดเจนมากขึ้นว่า มันจะไม่สามารถรักษาทฤษฎีเซตง่ายทั้งนั้นสิ่ง — อาจจะค่อนข้างมากซึ่งจะต้องถูก relinquished ส่วนการไม่หวังwas to retain as much of intuitive set theory as was needed to save the new results of modernmathematics and provide an adequate framework for classical mathematics.Briefly, then, axiomatic set theory was created to achieve a limited aim: it had to provide a firmfoundation for a system of set theory which—while it did not need to be as comprehensive as intuitiveset theory—must include all of Cantor’s basic results as well as the constructions (such as the numbersystems, functions, and relations) needed for classical mathematics.The systems of both Zermelo and von Neumann were successful in achieving this limited aim. Butthe amount of intuitive set theory which they had to sacrifice was considerable. For example, inZermelo’s system, as we have already seen, the intuitive way of making sets—by naming a propertyof objects and forming the set of all objects which have that property—does not take place at all. It isreplaced by the axiom of selection, in which properties are allowed only to determine subsets ofgiven sets. Furthermore, the only admissible “properties” are those which can be expressed entirelyin terms of the seven symbols ∈, ∨, ∧, ¬, ⇒, ∀, ∃ and variables x, y, z, … As a result, many ofthe things that we normally think of as sets—for example, the “set of all apples,” the “set of all atomsin the universe”—are not admissible as “sets” in axiomatic set theory. In fact, the only “sets” whichสัจพจน์ให้มี แรก ว่างเปล่าตั้งØ และก่อสร้างเช่น {Ø}, {Ø {Ø} }, ฯลฯ .,ซึ่งสามารถสร้างขึ้นจากเซตว่าง มันเป็นน่าทึ่งจริงที่ทั้งหมดของวิชาคณิตศาสตร์สามารถยึดตามดังกล่าวไม่เพียงพอแนวคิดของชุดนี้ในขณะที่ระบบ axiomatic ต่าง ๆ ของคณิตศาสตร์ทฤษฎีเซตที่บันทึกจากเควสอย่างทันที พวกเขาไม่สามารถตอบสนองคนจำนวนมาก หลายคนมีความไวต่อความสง่างามโดยเฉพาะ และuniversality ของคณิตศาสตร์ได้อย่างรวดเร็วเพื่อชี้ให้เห็นว่าสร้างสรรค์น่า Zermelo Neumann ฟอนต้องถือเป็นโซลูชั่นสำรองซึ่งเป็น expedients แก้ปัญหาชั่วคราว พวกเขาจะต้องไม่ ช้า ทฤษฎีทางคณิตศาสตร์ของขอบเขตที่กว้างขึ้น การปฏิบัติการแนวคิดของ "ตั้งค่า" ในการ generality เต็ม ใช้งานง่ายอาร์กิวเมนต์นี้กับทฤษฎีเซต axiomatic — ที่จะเกี่ยวข้องกับรุ่น amputated ของเราง่ายความคิดของชุด — มี ramifications ปรัชญาสำคัญ เป็นส่วนหนึ่งของการอภิปรายกว้างไกล ในการธรรมชาติของคณิตศาสตร์ "ต้อง" ดำเนินการอภิปรายรอบคำถามต่อไปนี้: มีทางคณิตศาสตร์แนวคิดสร้างสรรค์ (นั่นคือ ประดิษฐ์) ของจิตใจมนุษย์ หรือทำพวกเขามีอิสระเราในการ"platonic" ขอบเขตของแนวคิด เพียงการค้นพบ โดยนักคณิตศาสตร์ที่ ความเห็นหลังได้มักเรียกว่า "สัจนิยม platonic" และเป็นจุดหลักของคณิตศาสตร์คลาสสิก เราแสดงเหล่านี้สองฝ่ายตรงข้ามเห็น ด้วยแสดงว่าพวกเขากับแนวคิดเฉพาะตัวnotion of natural numbers. From the viewpoint of platonic realism, the concepts “one,” “two,”“three,” and so on, exist in nature and existed before the first man began to count. If intelligent beingsexist elsewhere in the universe, then, no matter how different they are from us, they have no doubtdiscovered the natural numbers and found them to have the same properties they have for us. On theother hand, according to the opposing point of view, while three cows, three stones, or three treesexist in nature, the natural number three is a creation of our minds; we have invented a procedure forconstructing the natural numbers (by starting from zero and adding 1 each time, thus producingsuccessively 1, 2, 3, etc.) and have in this manner fashioned a conceptual instrument of our ownmaking.How does platonic realism affect the status of axiomatic set theory? From the point of view ofplatonic realism, mathematical objects are given to us ready-made, with all their features and all theirproperties. It follows that to say a mathematical theorem is true means it expresses a correctstatement about the relevant mathematical objects. (For example, the proposition 2 + 2 = 4 is notmerely a formal statement provable in arithmetic; it states an actual fact about numbers.) Now—ifwe admit that mathematical objects are given to us with all their properties, it follows, in particular,that the notion of set is a fixed, well-defined concept which we are not free to alter for our ownconvenience. Thus the “sets” created by Zermelo and von Neumann do not exist, and theorems whichpurport to describe these nonexistent objects are false! In conclusion, if we were to accept a strictinterpretation of platonic realism, we would be forced to reject the systems of Zermelo and vonNeumann as mathematically invalid.Fortunately, the trend, for some time now, has been away from platonism and toward a moreflexible, more “agnostic” attitude toward mathematical “truth.” For one thing, developments inmathematics have been conforming less and less to the pattern dictated by platonic philosophy. Foranother, the cardinal requirement of platonism—that every mathematical object correspond to adefinite, distinct object of our intuition (just as “point” and “line” refer to well-defined objects of ourspatial intuition)—came to be an almost unbearable burden on the work of creative mathematiciansby the nineteenth century. They were dealing with a host of new concepts (such as complex numbers,abstract laws of composition, and the general notion of function) which did not lend themselves to asimple interpretation in concrete terms. The case of the complex numbers is a good illustration ofwhat was happening. Classical mathematics never felt at ease with the complex numbers, for it lackeda suitable “interpretation” of them, and as a result there were nagging doubts as to whether such thingsreally “existed.” Real numbers may be interpreted as lengths or quantities, but the square root of a
negative real number—this did not seem to correspond to anything in the real world or in our intuition
of number. Yet the system of the complex numbers arises in a most natural way—as the smallest
number system which contains the real numbers and includes the roots of every algebraic equation
with real coefficients; whether or not the complex numbers have a physical or psychological
counterpart seems irrelevant.
The case of the complex numbers strikes a parallel with the problem of axiomatic set theory. For
the “sets” created by Zermelo and von Neumann arise quite naturally in a mathematical context. They
give us the simplest notion of set which is adequate for mathematics and yields a consistent axiomatic
theory. Whether or not we can interpret them intuitively may be relatively unimportant.
Be that as it may, many mathematicians in the early 1900’s were reluctant to make so sharp a break
with tradition as axiomatic set theory seemed to demand. Furthermore, they felt, on esthetic grounds,
that a mathematical theory of sets should describe all the things—and only those things—which our
intuition recognizes to be sets. Among them was Bertrand Russell; in his efforts to reinstate intuitive
set theory, Russell was led to the idea that we may consider sets to be ordered in a hierarchy of
“levels,” where, if A and are sets and A is an element of , then is “one level higher” than A.
For example, in plane geometry, a circle (regarded as a set of points) is one level below a family of
circles, which, in turn, is one level below a set of families of circles. This basic idea was built by
Russell into a theory called the theory of types, which can be described, in essence, as follows.
Every set has a natural number assigned to it, called its level. The simplest sets, those of level 0,
are called individuals—they do not have elements. A collection of individuals is a set of level 1; a
collection of sets of level 1 is a set of level 2; and so on. In the theory of types the expression a ∈ B
is only meaningful if, for some number n, a is a set of level n and B is a set of level n + 1. It follows
that the statement x ∈ x has no meaning in the theory of types, and as a result, Russell’s paradox
vanishes for the simple reason that it cannot even be formulated.
Russell’s theory of types is built upon a beautifully simple idea. Unfortunately, in order to make it
“work,” Russell was forced to add a host of new assumptions, until finally the resulting theory
became too cumbersome to work with and too complica
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