Frege on the introduction of real and complex numbers by abstraction and cross-sortal identity claims

Abstract

In this article, I try to shed new light on Frege’s envisaged definitional introduction of real and complex numbers in Die Grundlagen der Arithmetik (1884) and the status of cross-sortal identity claims with side glances at Grundgesetze der Arithmetik (vol. I 1893, vol. II 1903). As far as I can see, this topic has not yet been discussed in the context of Grundlagen. I show why Frege’s strategy in the case of the projected definitions of real and complex numbers in Grundlagen is modelled on his definitional introduction of cardinal numbers in two steps, tentatively via a contextual definition and finally and definitively via an explicit definition. I argue that the strategy leaves a few important questions open, in particular one relating to the status of the envisioned abstraction principles for the real and complex numbers and another concerning the proper handling of cross-sortal identity claims.