There are at least two independent arguments in favor of a small set theory like PST.
One can get the impression from mathematical practice outside set theory that there are “only two infinite cardinals which demonstrably ‘occur in nature’,” therefore “set theory produces far more superstructure than is needed to support classical mathematics.” Although it may be an exaggeration, with some technical tricks a considerable portion of mathematics can be reconstructed within PST; certainly enough for most of its practical applications.
A second argument arises from foundational considerations. Most of mathematics can be implemented in standard set theory or one of its large alternatives. Set theories, on the other hand, are introduced in terms of a logical system; in most cases it is first-order logic. The syntax and semantics of first-order logic, on the other hand, is built on set-theoretical grounds. Thus, there is a foundational circularity, which forces us to choose as weak a theory as possible for bootstrapping. This line of thought, again, leads to small set theories.
Thus, there are reasons to think that Cantor's infinite hierarchy of the infinites is superfluous. Pocket set theory is a “minimalistic” set theory that allows for only two infinites: the cardinality of the natural numbers and the cardinality of the reals.
The theory
PST uses standard first-order language with identity and the binary relation symbol. Ordinary variables are upper caseX, Y, etc. In the intended interpretation, the variables these stand for classes, and the atomic formula means "class X is an element of class Y". A set is a class that is an element of a class. Small case variables x, y, etc. stand for sets. A proper class is a class that is not a set. Two classes are equinumerous iff a bijection exists between them. A class is infinite iff it is equinumerous with one of its proper subclasses. The axioms of PST are
Remarks on the axioms
Although different kinds of variables are used for classes and sets, the language is not many-sorted; sets are identified with classes having the same extension. Small case variables are used as mere abbreviations for various contexts; e.g.,
Since the quantification in A2 ranges over classes, i.e., is not set-bound, A2 is the comprehension scheme of Morse–Kelley set theory, not that of Von Neumann–Bernays–Gödel set theory. This extra strength of A2 is employed in the definition of the ordinals.
Since there is no axiom of pairing, it must be proved that for any two sets x and y, the Kuratowski pair exists and is a set. Hence proving that there exists a one-to-one correspondence between two classes does not prove that they are equinumerous.
Pocket set theory is analogous to third order arithmetic, with the sets and classes corresponding to subsets of the natural numbers and subsets of the powerset of the natural numbers.
A model for pocket set theory is given by taking the sets of pocket set theory to be the constructible elements of HC, and the classes to be the constructible subsets of HC.
Some PST theorems
;1. The Russell class is a proper class. ;2. The empty class is a set. ;3. The singleton class is a set. ;4. is infinite. ;5. Every finite class is a set. Once the above facts are settled, the following results can be proved: ;6. The class V of sets consists of all hereditarily countable sets. ;7. Every proper class has the cardinality. ;8. The union class of a set is a set. PST also verifies the:
Continuum hypothesis. This follows from and above;
Axiom of replacement. This is a consequence of ;
Axiom of choice. Proof. The class Ord of all ordinals is well-ordered by definition. Ord and the class V of all sets are both proper classes, because of the Burali-Forti paradox and Cantor's paradox, respectively. Therefore there exists a bijection between V and Ord, which well-orders V. ∎
The well-foundedness of all sets is neither provable nor disprovable in PST.
Possible extensions
Adding the so-called axiom of free construction to PST, any consistent system of set-theoretical axioms will have an inner model in the resulting system.
It is an unfriendly feature of PST that it cannot handle classes of sets of real numbers or classes of sets of real functions. However, it is not a necessary one. can be modified various ways to allow for various portions of the usual hierarchy of infinites, with or without supporting the continuum hypothesis. One example is
