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: My final formal answer as to what classes are and what class
membership is!

Replies: 7   Last Post: Apr 7, 2013 4:34 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
My final formal answer as to what classes are and what class
membership is!

Posted: Mar 28, 2013 3:02 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply


Below is the full quote from the above link.

What Are Classes!

This account supplies THE final answer as to what classes are,
and what is class membership relation, those are defined in
a rigorous system with highly appealing well understood primitive
notions that are fairly natural and easy to grasp. It is aimed to be
the most convincing answer to this question. The formulations are
carried out in first order logic with Identity, Part-hood and Naming
binary relations. Identity theory axioms are assumed and they are
part of the background logical language of this theory. The
axioms are those of GEM (Generalized Extensional Mereology), they are
the standard ones. The two axioms of naming are very trivial.
The definitions of classes and their membership are coined with the
utmost care to require the least possible assumptions so they don't
require grounds of Atomic Mereology or unique naming or the alike..,
so they can work under more general situations. Also utmost care was
taken to ensure that those definitions are nearer to the reality of
issue and not just a technical fix. I simply think that what is given
do supply the TRUE and FINAL answer to what classes are and to
what is their membership!

The General approach is due to David Lewis. Slight modifications are
adopted here to assure more general and nearer to truth grounds.

Language: FOL(=,P,name)

Axioms: ID axioms +

1.Reflexive: x P x

2.Transitive: x P y & y P z -> x P z

3.Antisymmetric: x P y & y P x -> x=y

Define: x O y iff Exist z. z P y & z P x

4.Supplementation: ~y P x -> Exist z. z P y & ~ z O x

5.Composition: if phi is a formula, then ((Exist k. phi) ->
Exist x (for all y. y O x iff Exist z. phi(z) & y O z)) is an axiom.

Definition: x is a collection of phi-ers iff
for all y. y O x iff Exist z. phi(z) & y O z

6.Naming: n name of y & n name of x -> y=x

Definition: n is a name iff Exist x. n name of x

7.Discreteness: n,m are names & ~n=m -> ~n O m

Definitions of "Class" and "Class membership":

Define: x E y iff Class(y) & Exist n. n P y & n name of x.

1. Class(x) iff x is a collection of names.

2. Class(x) iff x is a collection of names Or x never overlap with a

when x never overlaps with a name then it is to be called an inert

Definition: x is inert iff ~Exist n. n is a name & x O n

3. Class(x) iff
x is a sum of an inert object and (an inert object or a collection of

Sum defined as:

Sum(x,y) = z iff for all q. q O z iff q O x Or q O y.

1 is incompatible with the empty class.
2 is incompatible with the subclass principle that is :
"Every subclass of x is a part of x".

3 does the job but it encourages gross violation of Extensionality
over classes
since having multiple names for an object is the natural expectation!

If we assume the subclass principle and use definition 3 then full
over classes is in place and it follows that the empty Class is an
Although attractive on the face of it (since the empty set is just a
technical fix),
however it is not that convincing since there is no real
for such atom-hood.

If we strengthen the subclass principle into the principle that:
"For all classes X,Y (Y subclass of X iff Y P X)", then only
definition 1
can survive such a harsh condition, and this would force all names to
be atoms and shuns the existence empty classes altogether! such
a demanding commitment that despite the clear aesthetic gain of
having internally pure classes in the sense that all classes are only
composed of parts that are classes, yet still this is a very
commitment that do not seem to agree with basic natural expectations
about naming.

So a definition of classes that proves Extensionality over them
restricting multiple naming per object is what is demanded.

Define: x is an equivalence collection of names iff
there exist y such that x is the collection of all names of y.

Define: y is a fusion of equivalence collections of names iff
y is a collection of names & for all a,b,c (a P y & a name of b & c
name of b -> c P y)

Define V' as the collection of ALL inert objects.

4. Definition: Class(x) iff
x is a sum of V' and (V' or a fusion of equivalence collections of

As far as the concept of class is concerned Extensionality is at the
core of it,
so 4. is the right definition of classes.

It is nice to see that the *Empty Class* is just the collection of all
inert objects.

For the sake of completion of this approach, we may say that
Definition 4.
is an Equivalence rendering of Definition 3. Similarly we can
introduce two
further definitions that are Equivalence renderings of Definition 1
and Definition 2.
But those are rarely applicable in class\set theories.

Now one can easily define a set as a class that is an element of a
An Ur-element is defined as an element of a class that is not a
Or alternatively a non-class object. All kinds of circular membership
can be explained;
paradoxes can be easily understood. Also non definability of some
can be understood.

This account explains membership and classes in a rigorous manner.
And actually supplies the FINAL answer!

Somehow those definitions might be helpful in orienting thought about
philosophical questions about mathematics founded in set theory. For
identity and part-hood are expected natural relations and they can be
about as being human independent, but Naming might present some
definitely it favors human dependency but still it can be human
Philosophical debate about the nature of sets would become a debate
about the
nature of naming procedures.

Zuhair Al-Johar
March 21 2013

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.