[FOM] Logic/Syntax versus Arithmetic

Timothy Y. Chow tchow at math.princeton.edu
Fri Mar 6 22:44:23 EST 2020

Alan Weir wrote:

> Tim Chow wrote (FOM Vol 207 Issue 4.1): 'My hypothesis is that *if* you 
> can come up with some kind of nominalistically satisfactory account of 
> infinitely many wffs, and/or of syntactic operations, then you're going 
> to be able to come up with a nominalistically satisfactory account of 
> infinitely many numbers, and/or of arithmetic operations, essentially by 
> mimicking whatever philosophical moves you make in the syntactic case. 
> As to your hypothesis, yes I agree with the conditional and believe the 
> antecedent too.
> The original worry (looking back to try to remember what that was!) was 
> whether it made sense to challenge platonism in, say arithmetic, by 
> appeal to logical consequence, read for example as derivability, a 
> syntactic notion. For these logico-syntactic notions- proof, wff etc.- 
> seem to be as abstract, and the syntactic theory of the pretty much the 
> same power, e.g. entailing infinitely many distinct elements in a 
> structure, as arithmetic. That's a good objection which 
> nominalists/anti-platonists/formalists need to respond to, I agreed.

Thanks for your detailed reply.  I think that you and I are largely on the 
same page (though I don't consider myself a nominalist).

As for "the original worry," let me quote from the opening paragraph of 
Leng's article.  She quotes James Robert Brown as asking, "Is anyone 
really agnostic about 2+3=5, and willing only to give assent to 
PA->2+3=5?"  Leng affirms that she takes "precisely this attitude to 
mathematical claims."  Leng's attitude remains mysterious to me.  The 
question as phrased by Brown does not explicitly say that 2 is an abstract 
object and PA is not, nor does it explicitly say that PA->2+3=5 is a 
metalanguage statement while 2+3=5 is an object language statement.  It is 
Leng, not Brown, who regards PA->2+3=5 as directly affirming some 
nominalistically valid, non-abstract truth, yet for some reason does not 
regard 2+3=5 as directly affirming some nominalistically valid, 
non-abstract truth.  After all this discussion, I still can see no 
justifiable reason for this double standard.


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