Date: Dec 5, 2012 3:44 PM
Author: Zaljohar@gmail.com
Subject: What are sets? A correction
Language: FOL + P, Rp

P stands for "is part of"

Rp stands for "represents"

Axioms: Identity theory axioms +

I. Part-hood: P partially orders the universe.

ll. Supplementation: x P y & ~ y P x -> Exist z. z P y & ~ Exist v. v

P x & v P z.

Def.) atom(x) <-> for all y. y P x -> x P y

Def.) x atom of y <-> atom(x) & x P y.

Def.) c is a collection of atoms iff [for all y. y P c -> Exist z. z

atom of y].

Def.) c is atomless <-> ~ Exist x. x atom of c

lll. Representation: x Rp c & y Rp d -> (x=y<->c=d)

lV. Representatives: x Rp c -> atom(x)

V. Null: Exist! x. (Exist c. x Rp c & c is atomless)

A Set is an atom that uniquely represents a collection of atoms or

absence of atoms.

Def.) Set(x) <-> Exist c. (c is a collection of atoms or c is

atomless) & x Rp c & atom(x)

Here in this theory because of lV there is no need to mention atom(x)

in the above definition.

Set membership is being an atom of a collection of atoms that is

uniquely represented by an atom.

Def.) x e y iff Exist c. c is a collection of atoms & y Rp c & x atom

of c & atom(y)

Here in this theory because of lV there is no need to mention atom(y)

in the above definition.

Vl. Composition: if phi is a formula in which y is free but x not,

then [Exist y. atom(y) & phi] -> [Exist x. x is a collection of atoms

&

(for all y. y atom of x <-> atom(y) & phi)] is an axiom.

Vll. Pairing: for all atoms c,d Exist x for all y. y e x <-> y=c or

y=d

/

Zuhair