Only a sketchy account of set theory is given here. Set theory is a logic of classes—i.e., of collections (finite or infinite) or aggregations of objects of any kind, which are known as the members of the classes in question. Some logicians use the terms “class” and “set” interchangeably; others distinguish between them, defining a set (for example) as a class that is itself a member of some class and defining a proper class as one that is not a member of any class. It is usual to write ∊ for “is a member of” and to abbreviate ∼(x ∊ y) to x ∉ y. A particular class may be specified either by listing all its members or by stating some condition of membership, in which (latter) case the class consists of all and only those things that satisfy that condition (it is used, for example, when one speaks of the class of inhabitants of London or the class of prime numbers). Clearly, the former method is available only for finite classes and may be very inconvenient even then; the latter, however, is of more general applicability. Two classes that have precisely the same members are regarded as the same class or are said to be identical with each other, even if they are specified by different conditions; i.e., identity of classes is identity of membership, not identity of specifying conditions. This principle is known as the principle of extensionality. A class with no members, such as the class of atheistic popes, is said to be null. Since the membership of all such classes is the same, there is only one null class, which is therefore usually called the null class (or sometimes the empty class); it is symbolized by Λ or ø. The notation x = y is used for “x is identical with y,” and ∼(x = y) is usually abbreviated as x ≠ y. The expression x = Λ therefore means that the class x has no members, and x ≠ Λ means that x has at least one member.

A member of a class may itself be a class. The class of dogs, for example, is a member of the class of species of animals. An individual dog, however, though a member of the former class, is not a member of the latter—because an individual dog is not a species of animal (if the number of dogs increases, the number of species of animals does not thereby increase). Class membership is therefore not a transitive relation. The relation of class inclusion, however (to be carefully distinguished from class membership), is transitive. A class x is said to be included in a class y (written x ⊆ y) if and only if every member of x is also a member of y. (This is not meant to exclude the possibility that x and y may be identical.) If x is included in, but is not identical with, y—i.e., if every member of x is a member of y but some members of y are not members of x—x is said to be properly included in y (written x ⊂ y).

It is perhaps natural to assume that for every statable condition there is a class (null or otherwise) of objects that satisfy that condition. This assumption is known as the principle of comprehension. In the unrestricted form just mentioned, however, this principle has been found to lead to inconsistencies and hence cannot be accepted as it stands. One statable condition, for example, is non-self-membership—i.e., the property possessed by a class if and only if it is not a member of itself. This in fact appears to be a condition that most classes do fulfill; the class of dogs, for example, is not itself a dog and hence is not a member of the class of dogs.

Let it now be assumed that the class of all classes that are not members of themselves can be formed and let this class be y. Then any class x will be a member of y if and only if it is not a member of itself; i.e., for any class x, (x ∊ y) ≡ (x ∉ x). The question can then be asked whether y is a member of itself or not, with the following awkward result: if it is a member of itself, then it fails to fulfill the condition of membership of y, and hence it is not a member of y—i.e., not a member of itself. On the other hand, if y is not a member of itself, then it does fulfill the required condition, and therefore it is a member of y—i.e., of itself. Hence the equivalence (y ∊ y) ≡ (y ∉ y) results, which is self-contradictory. This perplexing conclusion, which was pointed out by Russell, is known as Russell’s paradox. Russell’s own solution to it and to other similar difficulties was to regard classes as forming a hierarchy of types and to posit that a class could only be regarded sensibly as a member, or a nonmember, of a class at the next higher level in the hierarchy. The effect of this theory is to make x ∊ x, and therefore x ∉ x, ill-formed. Another kind of solution, however, is based upon the distinction made earlier between two kinds of classes, those that are sets and those that are not—a set being defined as a class that is itself a member of some class. The unrestricted principle of comprehension is then replaced by the weaker principle that for every condition there is a class the members of which are the individuals or sets that fulfill that condition. Other solutions have also been devised, but none has won universal acceptance, with the result that several different versions of set theory are found in the literature of the subject.

Formally, set theory can be derived by the addition of various special axioms to a rather modest form of LPC that contains no predicate variables and only a single primitive dyadic predicate constant (∊) to represent membership. Sometimes LPC-with-identity is used, and there are then two primitive dyadic predicate constants (∊ and =). In some versions the variables x, y, … are taken to range only over sets or classes; in other versions they range over individuals as well. The special axioms vary, but the basis normally includes the principle of extensionality and some restricted form of the principle of comprehension, or some elements from which these can be deduced.

A notation to express theorems about classes can be either defined in various ways (not detailed here) in terms of the primitives mentioned above or else introduced independently. The main elements of one widely used notation are the following: if α is an expression containing some free occurrence of x, the expression {x : α} is used to stand for the class of objects fulfilling the condition expressed by α. For example, {x : x is a prime number} represents the class of prime numbers; {x} represents the class the only member of which is x; {x, y} the class the only members of which are x and y; and so on. <x, y> represents the class the members of which are x and y in that order (thus, {x, y} and {y, x} are identical, but <x, y> and <y, x> are in general not identical). Let x and y be any classes, as (for example) those of the dots on the two arms of a stippled cross. The intersection of x and y, symbolized as x ∩ y, is the class the members of which are the objects common to x and y—in this case the dots within the area where the arms cross—i.e., {z : z ∊ x · z ∊ y}. Similarly, the union of x and y, symbolized as x ∪ y, is the class the members of which are the members of x together with those of y—in this case all the dots on the cross—i.e., {z : z ∊ x ∨ z ∊ y}; the complement of x, symbolized as -x, is the class the members of which are all those objects that are not members of x—i.e., {y : y ∉ x}; the complement of y in x, symbolized as x − y, is the class of all objects that are members of x but not of y—i.e., {z : z ∊ x · z ∉ y}; the universal class, symbolized as V, is the class of which everything is a member, definable as the complement of the null class—i.e., as -Λ. Λ itself is sometimes taken as a primitive individual constant, sometimes defined as {x : x ≠ x}—the class of objects that are not identical with themselves.

Among the simpler theorems of set theory are

• (∀x)(x ∩ x = x),
• (∀x)(∀y)(x ∩ y = y ∩ x);

corresponding theorems for ∪:

• (∀x)(∀y)(∀z)[x ∩ (y ∪ z) = (x ∩ y) ∪ (x ∩ z)],
• (∀x)(∀y)[-(x ∩ y) = -x ∪ -y];

and corresponding theorems with ∩ and ∪ interchanged:

• (∀x)(- -x = x),
• (∀x)(∀y)(x - y = x ∩ -y),
• (∀x)(Λ ⊂ x),
• (∀x)(x ∩ Λ = Λ),
• (∀x)(x ∪ Λ = x).

In these theorems, the variables range over classes. In several cases, there are obvious analogies to valid wffs of PC.

Apart from its own intrinsic interest, set theory has an importance for the foundations of mathematics in that it is widely held that the natural numbers can be adequately defined in set-theoretic terms. Moreover, given suitable axioms, standard postulates for natural-number arithmetic can be derived as theorems within set theory.