The problem of consistency for the predicate calculus is relatively simple. A world may be assumed in which there is only one object a. In this case, both the universally quantified and the existentially quantified sentences (∀x)A(x) and (∃ x)A(x) reduce to the simple sentence A(a), and all quantifiers can be eliminated. It may easily be confirmed that, after the reduction, all theorems of the calculus become tautologies (i.e., theorems in the propositional calculus). If F is any predicate, such a sentence as “Every x is F and not every x is F ”—i.e., (∀x)F(x) · ∼(∀x)F(x)—is then reduced to “a is both A and not-A”—A(a) · ∼A(a)—which is not a tautology; therefore, the original sentence is not a theorem; hence, no contradiction can be a theorem. If F is simple, then F and A are the same. If F is complex and contains (∀y) or (∃z), etc., then A is the result obtained by iterating the transformation of eliminating (∀y), etc. In fact, it can be proved quite directly not only that the calculus is consistent but also that all its theorems are valid.
The discoveries that the calculus is complete and undecidable are much more profound than the discovery of its consistency. Its completeness was proved by Gödel in 1930; its undecidability was established with quite different methods by Church and Turing in 1936. Given the general developments that occurred up to 1936, its undecidability also follows in another way from Theorem X of Gödel’s paper of 1931.
Completeness means that every valid sentence of the calculus is a theorem. It follows that if ∼A is not a theorem, then ∼A is not valid; and, therefore, A is satisfiable; i.e., it has an interpretation, or a model. But to say that A is consistent means nothing other than that ∼A is not a theorem. Hence, from the completeness, it follows that if A is consistent, then A is satisfiable. Therefore, the semantic concepts of validity and satisfiability are seen to coincide with the syntactic concepts of derivability and consistency.
The Löwenheim-Skolem theorem
A finding closely related to the completeness theorem is the Löwenheim-Skolem theorem (1915, 1920), named after Leopold Löwenheim, a German schoolteacher, and Skolem, which says that if a sentence (or a formal system) has any model, it has a countable or enumerable model (i.e., a model whose members can be matched with the positive integers). In the most direct method of proving this theorem, the logician is provided with very useful tools in model theory and in studies on relative consistency and independence in set theory.
In the predicate calculus there are certain reduction or normal-form theorems. One useful example is the prenex normal form: every sentence can be reduced to an equivalent sentence expressed in the prenex form—i.e., in a form such that all the quantifiers appear at the beginning. This form is especially useful for displaying the central ideas of some of the proofs of the Löwenheim-Skolem theorem.
As an illustration, one may consider a simple schema in prenex form, “For every x, there is some y such that x bears the (arbitrary) relation M to y”; i.e.,
(3) (∀x)(∃y)Mxy.
If (3) now has a model with a nonempty domain D, then, by a principle from set theory (the axiom of choice), there exists a function f of x, written f(x), that singles out for each x a corresponding y. Hence, “For every x, x bears the relation M to f(x)”; i.e.,
(4) (∀x)Mxf(x).
If a is now any object in D, then the countable subdomain {a, f (a), f [ f(a)], . . .} already contains enough objects to satisfy (4) and therefore to satisfy (3). Hence, if (3) has any model, it has a countable model, which is in fact a submodel of the original.
An alternative proof, developed by Skolem in 1922 to avoid appealing to the principles of set theory, has turned out to be useful also for establishing the completeness of the calculus. Instead of using the function f as before, a can be arbitrarily denoted by 1. Since equation (3) is true, there must be some object y such that the number 1 bears the relation M to y, or symbolically M1y, and one of these y’s may be called 2. When this process is repeated indefinitely, one obtains
(5) M12; M12 · M23; M12 · M23 · M34; . . . ,
all of which are true in the given model. The argument is elementary, because in each instance one merely argues from “There exists some y such that n is M of y”—i.e., (∃y)Mny—to “Let one such y be n + 1.” Consequently, every member of (5) is true in some model. It is then possible to infer that all members of (5) are simultaneously true in some model—i.e., that there is some way of assigning truth values to all its atomic parts so that all members of (5) will be true. Hence, it follows that (3) is true in some countable model.
The completeness theorem
Gödel’s original proof of the completeness theorem is closely related to the second proof above. Consideration may again be given to all the sentences in (5) that contain no more quantifiers. If they are all satisfiable, then, as before, they are simultaneously satisfiable and (3) has a model. On the other hand, if (3) has no model, some of its terms—say M12 · . . . · M89—are not satisfiable; i.e., their negations are tautologies (theorems of the propositional calculus). Thus, ∼M12 ∨ . . . ∨ ∼M89 is a tautology, and this remains true if 1, 2, . . ., 9 are replaced by variables, such as r, s, . . ., z; hence, ∼Mrs ∨ . . . ∨ ∼Myz, being a tautology expressed in the predicate calculus as usually formulated, is a theorem in it. It is then easy to use the usual rules of the predicate calculus to derive also the statement, “There exists an x such that, for every y, x is not M of y”; i.e., (∃ x)(∀y)∼Mxy. In other words, the negation of (3) is a theorem of the predicate calculus. Hence, if (3) has no model, then its negation is a theorem of the predicate calculus. And, finally, if a sentence is valid (i.e., if its negation has no model), then it is itself a theorem of the predicate calculus.
The undecidability theorem and reduction classes
Given the completeness theorem, it follows that the task of deciding whether any sentence is a theorem of the predicate calculus is equivalent to that of deciding whether any sentence is valid or whether its negation is satisfiable.
Turing’s method of proving that this class of problems is undecidable is particularly suggestive. Once the concept of mechanical procedure was crystallized, it was relatively easy to find absolutely unsolvable problems—e.g., the halting problem, which asks for each Turing machine the question of whether it will ever stop, beginning with a blank tape. In other words, each Turing machine operates in a predetermined manner according to what is given initially on the (input) tape; we consider now the special case of a blank tape and ask the special question whether the machine will eventually stop. This infinite class of questions (one for each machine) is known to be unsolvable.
Turing’s method shows that each such question about a single Turing machine can be expressed by a single sentence of the predicate calculus so that the machine will stop if and only if that sentence is not satisfiable. Hence, if there were a decision procedure of validity (or satisfiability) for all sentences of the predicate calculus, then the halting problem would be solvable.
In more recent years (1962), Turing’s formulation has been improved to the extent that all that is needed are sentences of the relatively simple form (∀x)(∃y)(∀z)Mxyz, in which all the quantifiers are at the beginning; i.e., M contains no more quantifiers. Hence, given the unsolvability of the halting problem, it follows that, even for the simple class of sentences in the predicate calculus having the quantifiers ∀ ∃ ∀, the decision problem is unsolvable. Moreover, the method of proof also yields a procedure by which every sentence of the predicate calculus can be correlated with one in the simple form given above. Thus, the class of ∀ ∃ ∀ sentences forms a “reduction class.” (There are also various other reduction classes.)