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Topic: Extensional Logic
Replies: 2   Last Post: May 23, 2013 2:24 PM

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Posts: 2,665
Registered: 6/29/07
Extensional Logic
Posted: May 22, 2013 3:16 PM
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I think it is plausible to study extensions of n_th order predicates:

A zero order predicate is an "object"
A first order predicate is a predicate that only hold of objects.
A second order predicate is a predicate that hold of zero or first
order predicates.
In general and n_th order predicate is a predicate that holds of 0 or
1st or ... or (n-1)_th order predicates.

Now lets add a monadic symbol e to second order logic, and stipulate
the following rule:

if P is an n_th order predicate then eP is a term.

eP is read as: "extension of P"

And axiomatize the following:

If P is an n_th order predicate and Q is an m_th order predicate then:

eP=eQ iff (for all x. P(x) <-> Q(x))

Of course one can define a general membership relation in the
following manner:

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

However it is clear that E is not an n_th predicate, so eE is not a
term! thus avoiding Russell's paradox!

This way can enable us to define every natural number

0 = eP
1 = eP*
2 = eP**

were P, P*, P** are defined as:

For all x. P(x) iff ~x=x

For all x. P*(x) iff x=eP

For all x. P**(x) iff x=eP*

Successor relation of x can be defined as {x} after relation E.

Should we have infinitely long formulas then definitely we can define
Number and thus prove PA in logic.

Anyhow I'm not really sure of this method yet. But it does show that
some mathematics does follow from pure logical theories like the
extensional logic defined above.


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