## Notre Dame Journal of Formal Logic

### Extensionalizing Intensional Second-Order Logic

Jonathan Payne

#### Abstract

Neo-Fregean approaches to set theory, following Frege, have it that sets are the extensions of concepts, where concepts are the values of second-order variables. The idea is that, given a second-order entity $X$, there may be an object $\varepsilon X$, which is the extension of X. Other writers have also claimed a similar relationship between second-order logic and set theory, where sets arise from pluralities.

This paper considers two interpretations of second-order logic—as being either extensional or intensional—and whether either is more appropriate for this approach to the foundations of set theory. Although there seems to be a case for the extensional interpretation resulting from modal considerations, I show how there is no obstacle to starting with an intensional second-order logic. I do so by showing how the $\varepsilon$ operator can have the effect of “extensionalizing” intensional second-order entities.

#### Article information

Source
Notre Dame J. Formal Logic, Volume 56, Number 1 (2015), 243-261.

Dates
First available in Project Euclid: 24 March 2015

https://projecteuclid.org/euclid.ndjfl/1427202982

Digital Object Identifier
doi:10.1215/00294527-2835092

Mathematical Reviews number (MathSciNet)
MR3326597

Zentralblatt MATH identifier
1371.03015

#### Citation

Payne, Jonathan. Extensionalizing Intensional Second-Order Logic. Notre Dame J. Formal Logic 56 (2015), no. 1, 243--261. doi:10.1215/00294527-2835092. https://projecteuclid.org/euclid.ndjfl/1427202982

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