# By thedi agonalization of En we will mea.n the expre

In Smullyan. Gödel's Incompleteness Theorems.
I. Abstract Forms of Gõdel's and Tarski's Theorems
I.
5
Abstract Forms of Gõdel's and Tarski )s
Theorems
Ea.ch of the languages .C to which Gõdel's a.rgument is a.pplica.ble
contains at least the followi:ng items.
1. A denumera.ble set E whose elements are called the expressions
of .C.
2. A subset S of E whose elements are called the sentences of C.
3. A subset P of S whose elements a.re called the provable sentences of .C.
4. A subset 'R. of S whose elements are called the refutable (sometimes disprovable) sentences of .C.
5. A set 1-(. of expressions whose elements a.re called the predicates
of C. [These were called class names in Gõdel's introduction .
lnformally, each pred.icate H is thought of as being the name
of a. set of natural numbers.)
6. A function ~ that a.ssigns to every expression E a.nd every natural number n a.n expression E(n). The function is required to
obey the condition tha.t for every predica.te H and every natural number n, the expression H(n) is a. sentence. [Informally,
the sentence H(n) expresses the proposition that the number n
belongs to the set named by H .J
ln the fust incompleteness proof that we will give for a. par-
ticular system .C, we will use a. basic concept made precise by
Alfred Tarskí [1936]-viz. the notion of a. true sentence ( defined
quite differently than that of a. provable sentence of a. system).
And so we considera seventh and final item of our la.ngua.ge .C.
7. A set T of sentences whose elements are called the true sentences of .C.
This concludes our abstract descriptjon of the type of systems that
we will study in the next several chapters.
Expressihility in .C. The notion of expressibility in L, which we
are about to define, concerna the t.ruth set T but does not concern
either of the sets p and n.
The word number shall mean natural number for the rest of this
volume. We will say tha.t a. predica.te H is true for a number n or
tha.t n satisfies H if H (n) is a. true sentence (i.e. is an element of
T). By tbe set expressed by H , we mean the set of all n that sa.tisfy
6
Chapter 1. The General Ide& Behind Gõdel 's Proof
H. Thus for any set A of numbers, H expresses A if a.nd only if for
every number n:
H(n) E T
+-+- n
E A.
Definition. A set A is called ezpressible or nameable in C if A is
expressed by some predicate of e.
Since there are only denumerably many expressions of C, then
there a.re only fini t ely or denumerably many predicates of .C. But
by Cantor's well-known t heorem., there are non-denumerably many
sets of natural numbers. Therefore, not every set of numbers js
~rpesibl
in e.
Definition. The system C is called correct if every provable sentence
is true and every refutable sentence is false (not true). Thls means
that "P is a. subset of T a.nd n is disjoint from T. W e are now
interested in sufficient conditions that C , if correct , must contain a
true sentence not provable in e.
Gõdel Numbering and Diagonalization. We let g be a.1-1 function whlch assigns to each exptession E a naturalnumber g (E) called
the Gõdel number of E. The function g will be constan t for the rest
of this chapter. [ln the concrete systems to be studied in s ubsequent
chapters, a. specific Gôdel numbering will be given. Our present
purely abstract treatment, however, applies to an arbitra.ry Gõdel
num.bering.) It will be tech.nically convenient to assume that every number is the Gõdel number of a.n expression. [Gõdel's original
numbering did not have thls property, but the Gõdel num.bering we
will use in subsequent chapters will have this property. However,
the Tesults of this chapter can, with :minor mod.ifications, be proved
without this restriction (cf. Ex. 5).) A ssum.ing now that every num.ber n is the Gõdel number of a. unique expression, we let En be that
expression whose Gõdel n umber is n . Thlls, g(En) = n.
By the diagonalization of En we will mea.n the expression En(n).
If E.,. is a. predica.te, then its diagonalization is, of course, a sentence;
this sentence is true iff the predica.te E,,. is sa.tisfied by i ts own Gõdel
number n. (We write " iff" to mean if and only if; we use "+-+"
synonymously.]
For any n , we let d( n) be the Gõdel number of E,,.(n). The function d(x ) plays a key rôle in aJl that follows; we call it the diagonal
function of the system.
We use the term number-set to mean set of (natural) numbers.
For any number set A, by A* we shall mean the set of ali numbers
I. Abstra.ct Forma of Gõdel's a.nd Tarski's Theorems
7
n such tha.t d( n) E A . Thus for any n, the equiva.lence
n E A* ~
d(n) E A
holds by definition of A*. [A* could a.lso be written d- 1 (A ), since it
is the inverse image of A under the diagonal function d(x).]
An Abstract Forlll of Godel's Theorem. We let P be the set
oí Gõdel numbers of aJl the prova ble sentences. For any nurober set
A, b y its complement Ã, we mea.n the complement of A relative to
t he set N of natural numbers-i .e. Ã is t he set of all natural numbers
not in A.
Theoreni (GT)-After Godel w'ith sbades of Tarski. If the set
J5- is expressible in .C and .C is correct, then there is a true sentence
of .C not provable in ,C.
Proof. Suppose .C is correct a.nd J>· is expressible in ..C. Let H be a
pre dica.te that expresses J5• in .C, a.nd let h be the Godel number of
H . Let G be the diagonalization of H (i.e. the sen tence H(h)). We
will show that G is true but not provable in ..C.
Since H expresses P* in .C, then for any number n , It(n) is true +-+
n E P*. Since this equivalence holds for every n, then it holds in
partic ular for n the number h. So we take h for n ( and this is the part
of the argum.ent called diagonalizing) and we have t he equivalence:
H (h) is trtte +-+ h E J5•. Now,
h E j5• +-+ d (h) E
P +-+
d(h ) s Proof
10
that aJl the Athenians of the land always tell the truth and a.11
the Cretans of the land alwa.ys lie. Wha.t sta.tement could a.n
inhabita.nt make th.at would convince you that he alwa.ys tells
the truth but tha.t heis not a.n Athenia.n?
All he would need to say is: "l aro not an Athenian." A lia.r
couldn't make tha.t claim (beca.use alia.ris really not an Athenian ; only truth-tellers are Athenia.n). Therefore, hc must be
truthful. Hence his statement was true, which mea.ns tha.t he
is rea.lly not an Athenian. So he is a. truth teller but not an
Athenia.n.
li we think of the Athenians as pla.ying the rôle of the sente.nces
of .C, which are not only true but prova.ble in .C, t hen any inha.bitant who clalms he is not Athenia.n plays the rôle of Gõdel's
sentence G, which asserts its own non-prova.bility in C. (The
Creta.na, of course, pla.y the rôle of the refutable sentences of C,
but their function won't emerge till a. bit la.ter.]
II.
Undecidable Sentences of .C
So fa.r , the set n of refutable sentences ha.s pla.yed no rôle. N OW it
shall play a. key one.
e is called consistent if no sentence is both prova.ble a.nd refuta.ble
in
(i.e. the sets p and
are disjoint) and inconsistent otherwise.
The de:finition of consistency refers only to the sets 'P and n, not
to the set T . Nevertheless, if C is correct, then it is a.utomatically
consistent (beca.use if 'P is a. subset of T and Tis disjoint from n,
then P mn.st be disjoint from 'R). The converse is not necessarily
true ( we will. la.ter consider some systems that are consistent but not
correct).
A sentence X is called decidable in C if it is either provable or
refutable in C and undecidable in C otherwise. The system C is
called complete if every sentence is decidable in C and incomplete if
some sentence is undecidable in e.
Suppose now J:, satisfies the hypothesis of Theorem GT. Then
some sentence G js true but not provable in. C.. Since G is true, it is
not refuta.ble in C either (by the a.ssumption of correctness). Hence
G is undeddable in C. And so we at once ha.ve
e
n

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