[FOM] How much of math is logic?

joeshipman@aol.com joeshipman at aol.com
Mon Feb 26 14:31:38 EST 2007

Responding to Raatikainen:

>> ... but there are alternative
>> formulations of number theory which are built up from pure logic, 
>> back to Russell. It is possible to define a mapping from sentences 
>> arithmetic to statements of logic, in such a way that the axioms and
>> theorems of Peano Arithmetic map to logical validities.
>It is possible to effectively map (almost) any r.e. set to any other 
>set, but that does not make an arbitrary r.e. set of formulas a set of
>logically valid sentences. (I am simplifying a bit, but nothing hinges 
on that)

This is a silly objection, the point is that the mapping **can be 
seen** to connect arithmetical statements to logical ones in a 
truth-preserving way. What do you think Russell thought he was doing?

>> However, this route does not seem to transcend Peano Arithmetic; as 
>> understand it, arithmetical statements whose proof (in ZF) requires 
>> use of the axiom of Infinity will not be reachable in the versions 
>> this setup that correspond in some way to "first order logic" ...

>There are two different senses of "an axiom of infinity".
>The axioms of ZFC without this
>axiom already make the domain infinite, but it is this
>axiom which gives ZFC its extreme power. It is much stronger 
>than an axiom of infinity in the first sense.
>Now I don't think that simple type theory and such can provide us much
>mathematics without the addition of an axiom of infinity in the first 
>And the claim that such an assumption is logically true is quite 

This is exactly what I was trying to say -- I was careful to 
distinguish the ZFC axiom of Infinity as something that was NOT 
"logical", in order to argue that those parts of math which do not need 
it CAN be thought of as simply logical.

>Almost all axioms (well, not extensionality) of ZFC are set existence
>axioms, and even a few of them constitute together an axiom of 
infinity in
>the first sense. Hence, they are not logically true.

Here we disagree; I take it as a logical truth that "something exists", 
namely (if nothing else) concepts, and in particular that concepts have 
certain basic properties (Pairing, Union, etc.). (By the way that does 
NOT mean I identify all concepts with sets, only that I claim some 
concepts are sets.) Thus, "the empty set" is a valid concept, and so 
"exists". If you refuse to grant logic any ontology whatsoever, then OF 
COURSE logicisim is false, but only in a trivial way unworthy of 
further discussion.

>> My third point, that there does not seem to be any interesting open
>> question outside of set theory which is not equivalent to the 
>> of a sentence of second-order logic with standard semantics, 
>> the slogan "almost all math is just (second-order) logic".

>Well, this takes us back to the old dispute on whether second-order 
>really is logic. I think not, but I would not like to open that worm 
can again.

Why not? In my opinion, many people who say that second-order logic is 
not logic are prejudiced because they find the view that statements 
like CH do not have an absolute truth value congenial; that view allows 
one to maintain the Tennantian anti-realist position that there do not 
exist truths which are in principle unknowable.

To be fair, Tennant himself does not share this prejudice; his attitude 
towards CH is that it is no different from other statements independent 
of earlier mathematical systems, which mathematical investigation and 
reflection eventually led to acceptance or rejection of. Discussing 
this near the end of chapter 6 of "The Taming of the True", he says "We 
have no way of foreclosing on future extensions of our collective 
intellectual insight."

-- JS
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