The Math Forum

Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Math Forum » Discussions » sci.math.* » sci.math

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Topic: A logically motivated theory
Replies: 15   Last Post: May 21, 2013 8:22 AM

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]

Posts: 2,665
Registered: 6/29/07
Re: A logically motivated theory
Posted: May 20, 2013 3:37 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

On May 20, 2:59 am, fom <> wrote:
> On 5/18/2013 11:19 PM, Zuhair wrote:

> > On May 19, 1:47 am, fom <> wrote:
> >> On 5/18/2013 2:52 PM, Zuhair wrote:
> >>> On May 18, 10:38 pm, fom <> wrote:
> >>>> On 5/18/2013 2:21 PM, Zuhair wrote:
> >>>>> On May 18, 8:58 pm, fom <> wrote:
> >>>>>> On 5/18/2013 10:40 AM, Zuhair wrote:
> >>>>>>> In this theory Sets are nothing but object extensions of some
> >>>>>>> predicate. This theory propose that for every first order predicate
> >>>>>>> there is an object extending it defined after some extensional
> >>>>>>> relation.

> >>>>>>> This goes in the following manner:
> >>>>>>> Define: E is extensional iff for all x,y: (for all z. z E x iff z E y)
> >>>>>>> -> x=y

> >>>>>>> where E is a primitive binary relation symbol.
> >>>>>> So,
> >>>>>> <X,E>
> >>>>>> is a model of the axiom of extensionality.
> >>>>>>> Now sets are defined as
> >>>>>>> x is a set iff Exist E,P: E is extensional & for all y. y E x <-> P(y)
> >>>>>> So,
> >>>>>> xEX <-> ...
> >>>>>> where
> >>>>>> ... is a statement quantifying over relations and predicates.
> >>>>> No ... is a statement quantifying over objects.
> >>>> How so?  The formula seems to have an
> >>>> existential quantifier applying to a
> >>>> relation and a subformula with the
> >>>> quantified 'E' as a free variable:

> >>>> 'E is extensional'
> >>>> Using 'R' for "Relation", I read
> >>>> Ax(Set(x) <-> EREP(extensional(R) /\ Ay(yRx <-> P(y))))
> >>> I meant that P must be first order. There is no General so to say
> >>> membership relation E, there are separate different membership
> >>> relations all of which are 'primitive' relations.

> >>> Of course one might contemplate something like the following:
> >>> Define(E): x E X iff Exist R Exist P( extensional(R) /\ Ay(yRX <-
> >>>> P(y) /\ P(x) )
> >>> This E relation would be something like a 'general' membership
> >>> relation, but this is not acceptable here, because it is 'defined'
> >>> membership relation and not 'primitive'. When I'm speaking about
> >>> membership relations in the axioms I'm speaking about ones represented
> >>> by 'primitive' symbols and not definable ones.

> >>> Zuhair
> >> I will refrain from making a long posting based on
> >> my earlier mistaken impressions.

> >> However, your remarks here suggest that you should take
> >> a look at Quine's "New Foundations" and the interpretation
> >> of stratified formulas.  If you have Quine's book "Set Theory
> >> and Its Logic" available to you, a couple of hours reading the
> >> appropriate chapters and flipping forward to the definitions
> >> in earlier chapters should give you some sense of the matter
> >> as he saw it.

> >> I believe it is Thomas Forster who is making his book
> >> available online concerning NF, if you should become more
> >> interested in Quine's theory.

> > Yes I'm familiar with NF, actually I managed to further simplify it. I
> > coined the Acyclicity criterion, after which we can forsake
> > stratification altogether. See the joint article of Bowler, Randall
> > Holmes and myself on that subject, you can find it on Randal Holmes
> > home page and also on my website. Actually see:
> >

> > Anyhow here I'm trying to achieve something else, that of seeing that
> > PA can be interpreted in a LOGICAL theory. I view all the extensional
> > primitive relations in this theory as logical since all what they
> > function is to extend predicates. If we regard the second order
> > quantifier as logical, then that's it the major bulk of traditional
> > mathematics belongs to logic. I'm not sure if we can get without the
> > second order quantifier.

> > Anyhow I'm not sure of the remarks I've presented here, I might be
> > well mistaken, but matters look to go along that side. I'm just
> > conjecturing here.

> > Zuhair
> What exactly do you take to be "logical"?
> For example, on the Fregean view, one is interpreting
> the syllogistic hierarchy as extensional.  This is a
> typical mathematical view.  However, Leibniz interprets
> the syllogistic hierarchy as intensional, and, Lesniewski's
> criticisms of Frege and Russell also lead to an intensional
> interpretation.
> What you are referring to as "second-order" is, for me,
> a directional issue (extensional=bottom-up, intensional=top-down)
> with respect to priority in the syllogistic hierarchy.
> One often takes such questions for granted because
> our textbooks provide such little background information.
> We focus our attentions according to what we are taught.
> John MacFarlane has written on this demarcation issue:
> Historically, Aristotle is "intensional".  This follows
> from his claim that genera are prior to species.  But,
> the problems arise with the issue of "substance".  The
> notion of "substance" is associated with individuals and
> grounds the syllogistic hierarchy predicatively.
> So, there is an implicit tension in foundational studies
> because of "first-order"/"second-order" or
> "extensional"/"intensional" dichotomies.
> That is why I am asking for some clarification as to
> what you take to be "logical".
> Thanks.

I don't have a principled approach as regards demarcation of logic
yet. For now I'm content with saying that all fragments of first order
logic with identity (including all substitution instances by concrete
objects) are logical. I also maintain that having object extensions of
first order predicates is by itself logical since it just copies the
predicative content into the object world. A simple trial to do that
is to add a monadic symbol like "e" to the above language and
stipulate that

if G is a predicate symbol then eG is a term.

eG is read as "the extension of G".

stipulate the axiom:

eF=eG iff (for all x. F(x) <-> G(x))

To me this approach is perfectly logical.

We can use second order quantification to define a membership

x E y iff Exist G: G(x) & y=eG

where G ranges over first order predicate symbols.

In general we can define an i_th membership relation as:

x Ei y iff Exist Gi: Gi(x) & y=eGi

where Gi ranges over the i_th order predicate symbols.

So the membership relations so defined (in second order) only reflect
fulfillment of predicates after their order.

Of course to justify such an approach one must show that fulfillment
of predicates differs after their order, which indeed is hard to show.
Since it seems that "is" in "Socrates is a man", is not really
different from "is" in "Triangle is a shape". Of course "is" here is
just another word for "fulfills being", so the sentences, completely
interpreted, are: "Socrates fulfills being a man", "Triangle fulfills
being a shape". Even more completely displayed those sentence are:

The object the name "Socrates" refers to -is- a man.

The predicate the name "Triangle" refers to -is- a shape.

It appears that the article "is" in both of the above sentences has
the same meaning, that of "fulfills being". And it seems that there is
no difference in this fulfillment per se between the two sentences.
However still it can be argued that fulfillment of predicates by
predicates is a different kind of concept from fulfillment of
predicates by objects, and that this difference is the same for higher
predicates fulfillment. And this can be a strong point since using
extensions in the same manner (that of concatenating the symbol e with
the predicate symbol) doesn't elucidate the difference between an
object and a predicate and between a predicate and a higher predicate,
which are agreeably must be Mirrored by different "sorts" of
extensions, so in absence of that difference we must show it by the
membership relation. Anyhow, the above stipulation of ordered
membership does in sense MIRROR the order of predicates, so in
principle it is inert and doesn't add something that is substantially
extra-logical, so it can be considered as logical. However saying its
logical really depends on whether the second order quantifier is inert
or not.



Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2018. All Rights Reserved.