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Naive set theory was developed at the end of the 19th century (principally by Georg Cantor) in order to allow mathematicians to work with infinite sets consistently.
As it turned out, assuming that one could perform any operations on sets without restriction led to paradoxes such as Russell's paradox. In response, axiomatic set theory was developed to determine precisely what operations were allowed and when. Indeed, when research mathematicians talk about "set theory" today, they usually mean axiomatic set theory.
However, axiomatic set theory can be quite abstruse and yet has little effect on ordinary mathematics. Thus, it is useful to study sets in the original naive sense in order to develop facility for working with them. Furthermore, a firm grasp of naive set theory is important as a first stage in understanding the motivation for the axiomatic theory.
This article develops the naive theory. We begin by defining sets informally and investigating a few of their properties. Links in this article to specific axioms of set theory point out some of the relationships between the informal discussion here and the formal axiomatization of set theory, but we make no attempt to justify every statement on such a basis.

In naive set theory, a set is described as a collection of objects. Those objects that belong to a set are called its members. As objects we allow anything: numbers, people, other sets... For instance, 4 is a member of the set of all even integers. As you see, we allow sets to be infinite.
If x is a member of A, then we also say that x is an element of A, or that x belongs to A, or that x is in A, or that A owns x. In this case, we write x ∈ A. (The symbol "∈" is a derivation of the Greek letter Epsilon, "ε".)
We define two sets to be equal when they have precisely the same elements. (See Axiom of extension.) Thus a set is completely determined by its elements; the description is immaterial. For example, the set with elements 2, 3, and 5 is equal to the set of all prime numbers less than 6. If A and B are equal, then this is denoted symbolically as A = B (as usual).
We also allow for an empty set, a set without any members at all. Since a set is determined completely by its elements, there can only be one empty set. (See Axiom of empty set.)
The simplest way to describe a set is to list its elements between curly braces. Thus {1,2} denotes the set whose only elements are 1 and 2. (See Axiom of pairing.) Note the following points:
We can also use the notation {x : P(x)} to denote the set containing all objects for which the condition P holds. For example, {x : x is a real number} denotes the set of real numbers, {x : x has blonde hair} denotes the set of everything with blonde hair, and {x : x is a dog} denotes the set {dogs} of all dogs.
This notation is called "set builder notation" (or "set comprehension", particularly in the context of Functional programming). Some variants of set builder notation are:
Given two sets A and B we say that A is a subset of B, if every element of A is automatically an element of B. Notice that in particular, B is a subset of itself; a subset of B that isn't equal to B is called proper.
If A is a subset of B, then one can also say that B is a superset of A, or that A is contained in B, or that B contains A. In symbols, A ⊆ B means that A is a subset of B, and B ⊇ A means that B is a superset of A. Some authors use the symbols "⊂" and "⊃" for subsets, and others use these symbols only for proper subsets. In this encyclopedia, "⊆" and "⊇" are used for subsets while "⊂" and "⊃" are reserved for proper subsets.
As an illustration, let A be the set of real numbers, let B be the set of integers, let C be the set of odd integers, and let D be the set of current or former US Presidents. Then C is a subset of B, B is a subset of A, and C is a subset of A. Note that not all sets are comparable in this way. For example, it is not the case either that A is a subset of D nor that D is a subset of A.
The sets A, B and C above illustrate the
It is also the case that
Notice that one of the examples of set builder notation above, {x ∈ A : P(x)}, is specifically designed to construct a subset of A.
We also have
These proposition show that ⊆ is a partial order on the class of all sets, and {} is a bottom element[?]. (See Paradoxes below for more on the class of all sets.)
Universal sets and absolute complements
In certain contexts we may consider all of our sets as being subsets of some given universal set. For instance, if we are investigating properties of real numbers (and sets of reals), then we may take R, the set of all reals, as our universal set. It is important to realise that a universal set is only temporarily defined by the context; there is no such thing as a "universal" universal set, "the set of everything" (see Paradoxes below).
Given a universal set U and a subset A of U, we may define the complement of A (in U) as
The collection {A : A ⊆ U} of all subsets of a given universe U is called the power set of U. (See Axiom of power set.) It is denoted P(U); the "P" is sometimes in a fancy font.
Intersections, unions, and relative complements
Given two sets A and B, we may construct their union. This is the set consisting of all objects which are elements of A or of B or of both (see Axiom of union). It is denoted by A ∪ B. The intersection of A and B is the set of all objects which are both in A and in B. It is denoted by A ∩ B. Finally, the relative complement of B relative to A, also known as the set theoretic difference of A and B, is the set of all objects that belong to A but not to B. It is written as A \ B. Symbolically, these are respectively
Notice that A doesn't have to be a subset of B for B \ A to make sense; this is the difference between the relative complement and the absolute complement from the previous section.
To illustrate these ideas, let A be the set of lefthanded people, and let B be the set of people with blond hair. Then A ∩ B is the set of all lefthanded blondhaired people, while A ∪ B is the set of all people who are lefthanded or blondhaired or both. A \ B, on the other hand, is the set of all people that are lefthanded but not blondhaired, while B \ A is the set of all people that have blond hair but aren't lefthanded.
Now let E be the set of all human beings, and let F be the set of all living things over 1000 years old. What is E ∩ F in this case? No human being is over 1000 years old, so E ∩ F must be the empty set {}.
We list without proof several simple properties of these operations. These properties can be visualized with Venn diagrams.
We can also prove the distributive laws:
By Proposition 2, (1) and (2) together prove that LHS = RHS, as required.
The above propositions show that the power set P(U) is a Boolean lattice.
Given objects a and b the ordered pair containing a and b is denoted (a,b). For the time being we shall take this as a primitive notion (but see also Ordered pair). That is, we shall assume that (a,b) has the property that if (a,b) = (x,y), then a = x and b = y. The objects a and b are called respectively the first and second components of (a,b). Now, given two sets A and B, we define their Cartesian product to be
We can extend this definition to a set A × B × C of ordered triples, and more generally to sets of ordered ntuples for any positive integer n. It is even possible to define infinite Cartesian products, but to do this we need a more recondite definition of the product.
Cartesian products were first developed by René Descartes in the context of analytic geometry. If R denotes the set of all real numbers, then R^{2} := R × R represents the Euclidean plane and R^{3} := R × R × R represents threedimensional Euclidean space.
We referred earlier to the need for a formal, axiomatic approach. What problems arise in the treatment we have given? The problems relate to the formation of sets. One's first intuition might be that we can form any sets we want, but this view leads to inconsistencies. For any set we can ask whether x is a member of itself. Define
The penalty is a much more difficult development. In particular, it is problematic to speak of a set of everything, or to be (possibly) a bit less ambitious, even a set of all sets. In fact, in the standard axiomatisation of set theory, there is no set of all sets. In areas of mathematics that seem to require a set of all sets (such as category theory), one can sometimes make do with a universal set so large that all of ordinary mathematics can be done within it (see Universe (mathematics)[?]). Alternatively, one can make use of proper classes. Or, one can use a different axiomatisation of set theory, such as W. V. Quine's New Foundations[?], which allows for a set of all sets and avoids Russell's paradox in another way. The exact resolution employed rarely makes an ultimate difference.
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