# [FOM] 158:Sentential Reflection

Harvey Friedman friedman at math.ohio-state.edu
Mon Mar 31 00:17:10 EST 2003

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SENTENTIAL REFLECTION
by
Harvey M. Friedman
Ohio State University
Princeton University
friedman at math.ohio-state.edu
http://www.math.ohio-state.edu/~friedman/
http://www.mathpreprints.com
January 5, 2003

Abstract. We present two forms of "sentential reflection", which are
shown to be mutually interpretable with Z_2 and ZFC, respectively.

1. Introduction.

We use "class" as a neutral term, without commitment to the developed
notions of "set" and "class" that have become standard in set theory
and mathematical logic. We use epsilon for membership.

This framework supports interpretations of sentential reflection that
may differ from conventional set theory or class theory. However, we
do not pursue this direction here.

This framework is intended to accommodate objects that are not
classes. Such nonclasses are treated as classes with no elements.
Thus we are careful not to assume extensionality. In fact, we will
not assume any form of extensionality.

All of our formal theories of classes are in the language L(epsilon),
which is the usual classical first order predicate calculus with only
the binary relation symbol epsilon (no equality).

We use "category of classes" or just "category" as a neutral term,
not specifically related to category theory. They are given by a
formula of L(epsilon) with a distinguished free variable, with
parameters allowed.

The first version of sentential reflection that we consider is
SR(epsilon), which informally asserts the following.

if a given sentence of L(epsilon) holds in a given category
then it holds in a subclass.

Here "holds in" means that it holds if the quantifiers are
relativized over the category or class.

THEOREM 1.1. SR(epsilon) is mutually interpretable with Z_2.

See [Si99] for copious material on Z_2, which is the standard two
sorted first order system for "second order arithmetic".

Z_2 is much weaker in interpretation power than ZFC. In particular,
ZFC is not interpretable in SR. So in order to achieve mutually
interpretability with ZFC, we strengthen the choice of language.

One way of strengthening SR(epsilon) is to strengthen the notion of "subclass".

We say that the class x is an inclusion subclass of the category K if
and only if

x is a subclass of K, and any element of K
that is a subclass of an element of K lies in x.

This can be restated by a single clause as follows.

any subclass of any element of x lies in K
if and only if it lies in x.

SRIS(epsilon) informally asserts the following.

if a given sentence of L(epsilon) holds in a given category
then it holds in an inclusion subclass.

Here SRIS(epsilon) is read "sentential reflection by inclusion
subclass for L(epsilon)".

THEOREM 1.2. SRIS(epsilon) is provable in ZF and interprets ZFC.

For the formal presentations of SR(epsilon) and SRIS(epsilon), let
phi be a formula of L(epsilon), and let psi be a sentence of
L(epsilon) with no variables in common with phi. Let y be a variable in
phi. Let psi[phi,y] be the result of replacing all quantifiers (Qz)
in phi by

(Qz|phi[y/z])

and expanding the result to a formula of L(epsilon).

Informally, psi[phi,y] is the (formula expressing the) result of
relativizing all quantifiers in psi to {y: phi(y)}. In particular,
psi[y epsilon x,y] is the (formula expressing the) result of
relativizing all quantifiers in psi to (the elements of) x.

SR(epsilon) is the formal system in L(epsilon) whose nonlogical axioms are

psi[phi,y] implies (therexists x)((forall y epsilon x)(phi) & psi[y
epsilon x,y])

where x,y are distinct variables, phi is a formula of L(epsilon) in
which x is not free, psi is a sentence of L(epsilon) with no
variables in common with phi, and y is a variable in phi.

SRIS(epsilon) is the formal system in L(epsilon) whose nonlogical axioms are

psi[phi,y] implies (therexists x)((forall y containedin z epsilon
x)(phi iff y epsilon x) & psi[y epsilon x,y])

where x,y,z are distinct variables, phi is a formula of L(epsilon) in
which x,z are not free, psi is a sentence of L(epsilon) with no
variables in common with phi, and y is a variable in phi.

2. SR(epsilon).

3. SRIS(epsilon).

*********************************************

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This is the 158th in a series of self contained postings to FOM covering
a wide range of topics in f.o.m. Previous ones counting from #100 are:

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102: Turing Degrees/2  4/8/01  5:20PM
103:Hilbert's Program for Consistency Proofs/1 4/11/01  11:10AM
104:Turing Degrees/3   4/12/01  3:19PM
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