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

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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 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 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 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 .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 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 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 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, 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 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 e n
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