Hilbert's second problem
Hilbert's problem and its interpretation
In one English translation, Hilbert asks:
"When we are engaged in investigating the foundations of a science, we must set up a system of axioms which contains an exact and complete description of the relations subsisting between the elementary ideas of that science. ... But above all I wish to designate the following as the most important among the numerous questions which can be asked with regard to the axioms: To prove that they are not contradictory, that is, that a definite number of logical steps based upon them can never lead to contradictory results. In geometry, the proof of the compatibility of the axioms can be effected by constructing a suitable field of numbers, such that analogous relations between the numbers of this field correspond to the geometrical axioms. ... On the other hand a direct method is needed for the proof of the compatibility of the arithmetical axioms."
It is now common to interpret Hilbert's second question as asking in particular for a proof that Peano arithmetic is consistent.
There are many known proofs that Peano arithmetic is consistent that can be carried out in strong systems such as Zermelo-Fraenkel set theory. These do not provide a resolution to Hilbert's second question, however, because someone who doubts the consistency of Peano arithmetic is unlikely to accept the axioms of set theory (which is much stronger) to prove its consistency. Thus a satisfactory answer to Hilbert's problem must be carried out using principles that would be acceptable to someone who does not already believe PA is consistent. Such principles are often called finitistic because they are completely constructive and do not presuppose a completed infinity of natural numbers. Gödel's incompleteness theorem places a severe limit on how weak a finitistic system can be while still proving the consistency of Peano arithmetic.
Gödel's incompleteness theorem
Gödel's second incompleteness theorem shows that it is not possible for any proof that Peano Arithmetic is consistent to be carried out within Peano arithmetic itself. This theorem shows that if the only acceptable proof procedures are those that can be formalized within arithmetic then Hilbert's call for a consistency proof cannot be answered. However, as Nagel and Newman (1958:96–99) explain, there is still room for a proof that cannot be formalized in arithmetic:
- "This imposing result of Godel's analysis should not be misunderstood: it does not exclude a meta-mathematical proof of the consistency of arithmetic. What it excludes is a proof of consistency that can be mirrored by the formal deductions of arithmetic. Meta-mathematical proofs of the consistency of arithmetic have, in fact, been constructed, notably by Gerhard Gentzen, a member of the Hilbert school, in 1936, and by others since then. ... But these meta-mathematical proofs cannot be represented within the arithmetical calculus; and, since they are not finitistic, they do not achieve the proclaimed objectives of Hilbert's original program. ... The possibility of constructing a finitistic absolute proof of consistency for arithmetic is not excluded by Gödel’s results. Gödel showed that no such proof is possible that can be represented within arithmetic. His argument does not eliminate the possibility of strictly finitistic proofs that cannot be represented within arithmetic. But no one today appears to have a clear idea of what a finitistic proof would be like that is not capable of formulation within arithmetic."
Gentzen's consistency proof
In 1936, Gentzen published a proof that Peano Arithmetic is consistent. Gentzen's result shows that a consistency proof can be obtained in a system that is much weaker than set theory.
Gentzen's proof proceeds by assigning to each proof in Peano arithmetic an ordinal number, based on the structure of the proof, with each of these ordinals less than ε0. He then proves by transfinite induction on these ordinals that no proof can conclude in a contradiction. The method used in this proof can also be used to prove a cut elimination result for Peano arithmetic in a stronger logic than first-order logic, but the consistency proof itself can be carried out in ordinary first-order logic using the axioms of primitive recursive arithmetic and a transfinite induction principle. Tait (2005) gives a game-theoretic interpretation of Gentzen's method.
Gentzen's consistency proof initiated the program of ordinal analysis in proof theory. In this program, formal theories of arithmetic or set theory are assigned ordinal numbers that measure the consistency strength of the theories. A theory will be unable to prove the consistency of another theory with a higher proof theoretic ordinal.
Modern viewpoints on the status of the problem
While the theorems of Gödel and Gentzen are now well understood by the mathematical logic community, no consensus has formed on whether (or in what way) these theorems answer Hilbert's second problem. Simpson (1988:sec. 3) argues that Gödel's incompleteness theorem shows that it is not possible to produce finitistic consistency proofs of strong theories. Kreisel (1976) states that although Gödel's results imply that no finitistic syntactic consistency proof can be obtained, semantic (in particular, second-order) arguments can be used to give convincing consistency proofs. Detlefsen (1990:p. 65) argues that Gödel's theorem does not prevent a consistency proof because its hypotheses might not apply to all the systems in which a consistency proof could be carried out. Dawson (2006:sec. 2) calls the belief that Gödel's theorem eliminates the possibility of a persuasive consistency proof "erroneous", citing the consistency proof given by Gentzen and a later one given by Gödel in 1958.
- From the English translation by M. Newson, 1902 provided by http://aleph0.clarku.edu/~djoyce/hilbert/problems.html .
- A similar quotation with minor variations in wording appears in the 2001 edition p.107-108, as revised by Douglas R. Hofstadter, New York University Press, NY, ISBN 0-8147-5816-9.
- Actually, the proof assigns a "notation" for an ordinal number to each proof. The notation is a finite string of symbols that intuitively stands for an ordinal number. By representing the ordinal in a finite way, Gentzen's proof does not presuppose strong axioms regarding ordinal numbers.
- Dawson, John W. (2006) "Shaken foundations or groundbreaking realignment? A Centennial Assessment of Kurt Gödel’s Impact on Logic, Mathematics, and Computer Science". 2006 21st Annual IEEE Symposium on Logic in Computer Science, IEEE, pp. 339–341. ISBN 0-7695-2631-4 doi:10.1109/LICS.2006.47
- Michael Detlefsen (1990). "On an alleged refutation of Hilbert's Program using Gödel's First Incompleteness Theorem". Journal of Philosophical Logic (Springer) 19 (4): 343–377. doi:10.1007/BF00263316.
- Torkel Franzen (2005), Godel's theorem: An Incomplete Guide to its Use and Abuse, A.K. Peters, Wellesley MA. ISBN 1-56881-238-8
- Gerhard Gentzen (1936). "Die Widerspruchsfreiheit der reinen Zahlentheorie." Mathematische Annalen, v. 112, pp. 493–565.
- Kurt Gödel (1931), Über formal unentscheidbare Sätze der Principia Mathematica und verwandter Systeme, I. Monatshefte für Mathematik und Physik 38: 173-98. Translated in Jean van Heijenoort, 1967. From Frege to Gödel: A Source Book on Mathematical Logic. Harvard University Press: 596-616.
- David Hilbert  (1901) "Mathematische Probleme". Archiv der Mathematik und Physik, v. 3 n. 1, pp. 44–63 and 213–237. English translation, Maby Winton, Bulletin of the American Mathematical Society 8 (1902), 437–479. Available online at http://aleph0.clarku.edu/~djoyce/hilbert/problems.html .
- George Kreisel (1976). "What have we learnt from Hilbert's second problem?". "Mathematical developments arising from Hilbert problems (Proc. Sympos. Pure Math., Northern Illinois Univ., De Kalb, Ill.,)". Providence, R. I.: Amer. Math. Soc. pp. 93–130. ISBN 0-8218-1428-1.
- Nagel, Ernest and Newman, James R., Godel's Proof, New York University Press, 1958.
- Stephen G. Simpson (1988). "Partial realizations of Hilbert's Program". Journal of Symbolic Logic 53 (2): 349–363. doi:10.2307/2274508. JSTOR 2274508. ISSN = 0022-4812. Available online at http://www.math.psu.edu/simpson/papers/hilbert.pdf .
- William W. Tait (2005). "Gödel's reformulation of Gentzen's first consistency proof of arithmetic: the no-counterexample interpretation." Bulletin of Symbolic Logic v. 11 n. 2, pp. 225–238.