[FOM] Second-order logic and neo-logicism

[Ran Lanzet]

I am probably missing something here, and will be glad if you could clarify.

As far as I understand, your main argument against neo-logicism is roughly this:
1. The rules of 2nd-order logic (SOL) employed by the neo-logicist are very strong, in the sense of entailing some serious mathematical content. In particular:
	a. They are provably equivalent to the "basic rules" of SOL plus the unrestricted impredicative comprehension scheme.
	b. Once we accept those rules as the background logic, we get immediately from the very weak Q+ to the very strong PA2.
2. Hence, it does not seem reasonable to accept the neo-logicist's version of SOL as logic.

Now I believe the neo-logicist would happily accept (1): after all, her basic claim is that, essentially, all of ordinary mathematics is derivable from logic (more precisely: from her favorite version of SOL plus Hume's principle (HP); and I'm sure she will happily accept that SOL and not HP does the majority of work here). She will, though, undoubtedly object to your step from (1) to (2). She might argue as follows: the move from (1) to (2) is unwarranted, unless we accept the following principle: 
(*) 	if a set of rules entails substantial mathematical theorems, then it is unreasonable to regard that set of rules as part of logic. 
But accepting this principle -- so she might argue -- is to beg the question against logicism.

Question: what did I miss here? Or, more specifically: why is the suggested reply ineffective against your argument?

Best,
Ran        

-----Original Message-----
From: fom-bounces at cs.nyu.edu [mailto:fom-bounces at cs.nyu.edu] On Behalf Of Panu Raatikainen
Sent: Sunday, March 22, 2015 09:16
To: Foundations of Mathematics
Subject: [FOM] Second-order logic and neo-logicism


The following new paper might interest some here:

Panu Raatikainen: "Neo-logicism and its logic", History and Philosophy of Logic (forthcoming)

http://philpapers.org/rec/RAANAI


It has greatly benefited from certain old discussions here in FOM on the second-order logic; special thanks to Martin Davis!


All the Best

Panu



Abstract:
The rather unrestrained use of second-order logic in the neo-logicist program is critically examined. It is argued in some detail that it brings with it genuine set-theoretical existence assumptions, and that the mathematical power that Hume’s Principle seems to provide, in the derivation of Frege’s Theorem, comes largely from the “logic” assumed rather than from Hume’s principle. It is shown that Hume’s principle is in reality not stronger than the very weak Robinson Arithmetic Q.  
Consequently, only few rudimentary facts of arithmetic are logically derivable from Hume’s principle. And that hardly counts as a vindication of logicism.
--
Panu Raatikainen

Ph.D., Adjunct Professor in Theoretical Philosophy

Theoretical Philosophy
Department of Philosophy, History, Culture and Art Studies P.O. Box 24  (Unioninkatu 38 A)
FIN-00014 University of Helsinki
Finland

E-mail: panu.raatikainen at helsinki.fi

http://www.mv.helsinki.fi/home/praatika/

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