Thoralf Skolem


Pioneer	Journal where reappraisal appeared	Title	Link to article containing reappraisal
Thoralf Skolem	Foundations of Science	"Tools, objects, and chimeras: Connes on the role of hyperreals in mathematics"	13c, Section 3.2

Thoralf Skolem (1887-1963) was a Swedish logician. Skolem's extensions provide a modern illustration of the Leibnizian concept of infinitum terminatum (bounded infinity). In 1933, Skolem developed extensions of ℕ. Such an extension, say M, satisfies the axioms of Peano Arithmetic (and in this sense is indistinguishable from ℕ). Yet M is a proper extension, of which ℕ is an initial segment (more precisely, ℕ is isomorphic to an initial segment of M). Each element of the complement M ∖ ℕ is greater than each element of ℕ and in this sense can be said to be infinite.
Depending on the background logical system, one can view Skolem's extensions as either potentially or actually infinite (of course in the former case neither ℕ nor M exists as a completed whole). The sense in which an element of the complement M ∖ ℕ is 'infinite' is unrelated to the Aristotelian distinction, and provides a modern formalisation of the infinitum terminatum.

In his 1966 book Non-standard Analysis, page vii, Robinson wrote: "The resulting subject was called by me Non-standard Analysis since it involves and was, in part, inspired by the so-called Non-standard models of Arithmetic whose existence was first pointed out by T. Skolem."