5 Propositional Logic: Consistency and Completeness

Total Page:16

File Type:pdf, Size:1020Kb

5 Propositional Logic: Consistency and Completeness 5 Propositional Logic: Consistency and completeness Reading: Metalogic Part II, 24, 15, 28-31 Contents 5.1 Soundness . 61 5.2 Consistency . 62 5.3 Completeness . 63 5.3.1 An Axiomatization of Propositional Logic . 63 5.3.2 Kalmar's Proof: Informal Exposition . 66 5.3.3 Kalmar's Proof . 68 5.4 Homework Exercises . 70 5.4.1 Questions . 70 5.4.2 Answers . 70 5.1 Soundness In this section, we establish the soundness of the system, i.e., Theorem 3 (Soundness). Every theorem is a tautology, i.e., If ` A then j= A. Proof The proof is by induction the length of the proof of A. For the Basis step, we show that each of the axioms is a tautology. For the induction step, we show that if A and A ⊃ B are tautologies, then B is a tautology. Case 1 (PS1): AB B ⊃ A (A ⊃ (B ⊃ A)) TT TT TF TT FT FT FF TT Case 2 (PS2) 62 5 Propositional Logic: Consistency and completeness XYZ ABC B ⊃ C A ⊃ (B ⊃ C) A ⊃ B A ⊃ C Y ⊃ ZX ⊃ (Y ⊃ Z) TTT TTTTTT TTF FFTFFT TFT TTFTTT TFF TTFFTT FTT TTTTTT FTF FTTTTT FFT TTTTTT FFF TTTTTT Case 3 (PS3) AB » B » A » B ⊃∼ A A ⊃ B (» B ⊃∼ A) ⊃ (A ⊃ B) TT FFTTT TF TFFFT FT FTTTT FF TTTTT Case 4 (MP). If A is a tautology, i.e., true for every assignment of truth values to the atomic letters, and if A ⊃ B is a tautology, then there is no assignment which makes A T and B F. Since every assignment makes A T, every assignment must also make B T; so B must also be a tautology. ¤ 5.2 Consistency From Church (1956): The notion of consistency is semantically motivated in that one wants one's system to be free from absurdity or contradiction. It is, however, a syntactical notion, and there are several ways of de¯ning it. De¯nition 29 A logical system is Consistent with Respect to a partic- ular transformation by which each sentence or propositional form A is trans- formed into a sentence or propositional form A0, if there is no sentence or propositional form A such that ` A and ` A0. Lemma 5.2.1 The Propositional Calculus with respect to the transformation of A into » A. Proof. By the de¯nition of tautology, it is not the case both that A and » A are tautologies. In fact, if A is a tautology, » A is a contradiction. Since every theorem is a tautology, it cannot be both that ` A and `» A. QED. De¯nition 30 A logical system is Absolutely Consistent if not all its sen- tences and propositional forms are theorems. Lemma 5.2.2 The Propositional Calculus is absolutely consistent. 5.3 Completeness 63 Proof. The w® A^ » A is not a tautology, and since every theorem is a tautology, 6` (A^ » A). So there is at least one w® that is not provable. QED. De¯nition 31 A logical system is Consistent in the sense of Post (with respect to a certain category of primitive symbols designated as `propositional variables') if a w® consisting of a propositional variable alone is not a theorem. Lemma 5.2.3 The Propositional Calculus is consistent in the sense of Post Proof. A w® consisting of a propositional variable alone is not a tautology because its value for the value F of the variable is F. Since every theorem is a tautology, a propositional variable is not a theorem. QED. 5.3 Completeness Historical Note. Before the notion of a structure or a model had become clear, logicians had attempted to de¯ne the notion of completeness syntactically, although, as we all know, its motivation was semantical. Here (following Church) are some of the de¯nitions pro®ered. De¯nition 32 A Logical system is complete with respect to a given transforma- tion by which each sentence or propositional form A is transformed into a sentence or propositional form A0 if, for every sentence or propositional form B, either ` B or the system, upon addition of B to it as an axiom, becomes inconsistent with respect to the given transformation. De¯nition 33 A logical system is absolutely complete if, for every sentence or propositional form B, either ` B or the system, upon addition of B to it as an axiom, becomes absolutely inconsistent. De¯nition 34 A logical system is complete in the sense of Post if, for every sen- tence or propositional form B, either ` B or the system, upon addition of B to it as an axiom, becomes inconsistent in the sense of Post. 5.3.1 Kalmar's Proof: Informal Exposition So, we want to develop rules of inference for each of our connectives. For example, suppose our w® is A ⊃ B, wh ere A and are each as complicated as one wants. And suppose that we have our basic statement Xi. Here are the possibilities: Xi A B A ⊃ B Xi TTT Xi TFF Xi FTT Xi FFT 64 5 Propositional Logic: Consistency and completeness So, we have the following rules of inference, corresponding to the four lines on the table: Xi ⊃ A;Xi ⊃ B ` Xi ⊃ (A ⊃ B) Xi ⊃ A;Xi ⊃∼ B ` Xi ⊃∼ (A ⊃ B) Xi ⊃∼ A;Xi ⊃ B ` Xi ⊃ (A ⊃ B) Xi ⊃∼ A;Xi ⊃∼ B ` Xi ⊃ (A ⊃ B) Our job is to provide such rules for each of the connectives: CONJUNC- TION Xi A B A ^ B Xi TTT Xi TFF Xi FTF Xi FFF So, we have the following rules of inference, corresponding to the four lines on the table: Xi ⊃ A;Xi ⊃ B ` Xi ⊃ (A ^ B) Xi ⊃ A;Xi ⊃∼ B ` Xi ⊃∼ (A ^ B) Xi ⊃∼ A;Xi ⊃ B ` Xi ⊃∼ (A ^ B) Xi ⊃∼ A;Xi ⊃∼ B ` Xi ⊃∼ (A ^ B) DISJUNCTION Xi ABA_B Xi TTT Xi TFT Xi FTT Xi FFF So, we have the following rules of inference, corresponding to the four lines on the table: Xi ⊃ A;Xi ⊃ B ` Xi ⊃ (A_B) Xi ⊃ A;Xi ⊃∼ B ` Xi ⊃ (A_B) Xi ⊃∼ A;Xi ⊃ B ` Xi ⊃ (A_B) Xi ⊃∼ A;Xi ⊃∼ B ` Xi ⊃∼ (A_B) BICONDITIONAL Xi A B A ´ B Xi TTT Xi TFF Xi FTF Xi FFT 5.3 Completeness 65 So, we have the following rules of inference, corresponding to the four lines on the table: Xi ⊃ A;Xi ⊃ B ` Xi ⊃ (A ´ B) Xi ⊃ A;Xi ⊃∼ B ` Xi ⊃∼ (A ´ B) Xi ⊃∼ A;Xi ⊃ B ` Xi ⊃∼ (A ´ B) Xi ⊃∼ A;Xi ⊃∼ B ` Xi ⊃ (A ´ B) NEGATION Xi A » A »» A Xi TFT So, we have the following rules of inference: Xi ⊃ A ` Xi ⊃∼∼ A Next, we put in the following axiom schema: (X1 ^ X2 ^ ¢ ¢ ¢ ^ Xn) ⊃ Xi where 1 · i · n. Next, to work out tautologies, the second part of our requirement, we include the following rules: (A ^ B) ⊃ C; (A^ » B) ⊃ C ` A ⊃ C B ⊃ C; » B ⊃ C ` C This system is relatively cumbersome, but it can be simpli¯ed and then shown to be sound and complete. That is the heart of Kalmar's pro of. 5.3.2 Kalmar's Proof In this section, we are going to describe a completeness theorem due to Kalmar. This theorem is interesting because it shows how the notion of proof is intimately tied to the truth table idea. Let A1;:::; An be the atomic letters in a w® A, and let v be a truth value assignment to A1;:::; An. If v(Ai) = T then ®i = Ai; if v(Ai) = F then ®i =»Ai Then, we show ®1; : : : ; ®n ` ® For example, consider the w® A ⊃ B. We have the following truth table AB A ⊃ B TT T TF F FT T FF T 66 5 Propositional Logic: Consistency and completeness So, we have to show the following, corresponding to each line of the table: A; B ` A ⊃ B A; » B `» (A ⊃ B) »A; B ` A ⊃ B »A; » B ` A ⊃ B Supposing that we have this truth table lemma, we can prove completeness as follows: Claim (Completeness). If a w® A is a tautology, then it is a theorem. Proof. Let A be a tautology, and let A1;:::; An be the statement letters in it. For a given assignment v, we know that ®1; : : : ; ®n ` ® And since A is a tautology, we have ®1; : : : ; ®n ` A Now, since the assignment to each letter is independent, we have ®1; : : : ; ®n¡1; An ` A and ®1; : : : ; ®n¡1; »An ` A So, by the Deduction Theorem, we have ®1; : : : ; ®n¡1 ` An ⊃ A and ®1; : : : ; ®n¡1 `» An ⊃ A And, from the lemma and MP, we have ®1; : : : ; ®n¡1 ` A Next, we eliminate ®n¡1, and after n steps, we get ` A So, if A is a tautology, i.e., if j= A, then ` A. So, we only have to prove our lemma Lemma 5.3.1 Let A1;:::; An be the atomic letters in a w® A, and let v be a truth value assignment to A1;:::; An. If v(Ai) = T then ®i = Ai; if v(Ai) = F then ®i =»Ai Then, ®1; : : : ; ®n ` ® 5.3 Completeness 67 Proof by induction on the number of occurrences of logical connectives in A. Case 1 n=0. Then the lemma reduces to showing either A ` A or » A `» A and this is trivial. Case 2 A =»B, where B has fewer than n connectives. Case 2a v(B) = T . So, v(A) = F . By the induction hypothesis, ®1; : : : ; ®n ` B And by lemma x and MP, we have ®1; : : : ; ®n `»» B Now, since v(A) = F , ® =»A =»» B. QED Case 2b v(B) = F . So, v(A) = T . By the induction hypothesis, ®1; : : : ; ®n `» B But since v(A) = T , ® = A =»B.
Recommended publications
  • “The Church-Turing “Thesis” As a Special Corollary of Gödel's
    “The Church-Turing “Thesis” as a Special Corollary of Gödel’s Completeness Theorem,” in Computability: Turing, Gödel, Church, and Beyond, B. J. Copeland, C. Posy, and O. Shagrir (eds.), MIT Press (Cambridge), 2013, pp. 77-104. Saul A. Kripke This is the published version of the book chapter indicated above, which can be obtained from the publisher at https://mitpress.mit.edu/books/computability. It is reproduced here by permission of the publisher who holds the copyright. © The MIT Press The Church-Turing “ Thesis ” as a Special Corollary of G ö del ’ s 4 Completeness Theorem 1 Saul A. Kripke Traditionally, many writers, following Kleene (1952) , thought of the Church-Turing thesis as unprovable by its nature but having various strong arguments in its favor, including Turing ’ s analysis of human computation. More recently, the beauty, power, and obvious fundamental importance of this analysis — what Turing (1936) calls “ argument I ” — has led some writers to give an almost exclusive emphasis on this argument as the unique justification for the Church-Turing thesis. In this chapter I advocate an alternative justification, essentially presupposed by Turing himself in what he calls “ argument II. ” The idea is that computation is a special form of math- ematical deduction. Assuming the steps of the deduction can be stated in a first- order language, the Church-Turing thesis follows as a special case of G ö del ’ s completeness theorem (first-order algorithm theorem). I propose this idea as an alternative foundation for the Church-Turing thesis, both for human and machine computation. Clearly the relevant assumptions are justified for computations pres- ently known.
    [Show full text]
  • Completeness of the Propositions-As-Types Interpretation of Intuitionistic Logic Into Illative Combinatory Logic
    University of Wollongong Research Online Faculty of Engineering and Information Faculty of Engineering and Information Sciences - Papers: Part A Sciences 1-1-1998 Completeness of the propositions-as-types interpretation of intuitionistic logic into illative combinatory logic Wil Dekkers Catholic University, Netherlands Martin Bunder University of Wollongong, [email protected] Henk Barendregt Catholic University, Netherlands Follow this and additional works at: https://ro.uow.edu.au/eispapers Part of the Engineering Commons, and the Science and Technology Studies Commons Recommended Citation Dekkers, Wil; Bunder, Martin; and Barendregt, Henk, "Completeness of the propositions-as-types interpretation of intuitionistic logic into illative combinatory logic" (1998). Faculty of Engineering and Information Sciences - Papers: Part A. 1883. https://ro.uow.edu.au/eispapers/1883 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] Completeness of the propositions-as-types interpretation of intuitionistic logic into illative combinatory logic Abstract Illative combinatory logic consists of the theory of combinators or lambda calculus extended by extra constants (and corresponding axioms and rules) intended to capture inference. In a preceding paper, [2], we considered 4 systems of illative combinatory logic that are sound for first order intuitionistic prepositional and predicate logic. The interpretation from ordinary logic into the illative systems can be done in two ways: following the propositions-as-types paradigm, in which derivations become combinators, or in a more direct way, in which derivations are not translated. Both translations are closely related in a canonical way. In the cited paper we proved completeness of the two direct translations.
    [Show full text]
  • An Introduction to First-Order Logic
    Outline An Introduction to First-Order Logic K. Subramani1 1Lane Department of Computer Science and Electrical Engineering West Virginia University Completeness, Compactness and Inexpressibility Subramani First-Order Logic Outline Outline 1 Completeness of proof system for First-Order Logic The notion of Completeness The Completeness Proof 2 Consequences of the Completeness theorem Complexity of Validity Compactness Model Cardinality Lowenheim-Skolem¨ Theorem Inexpressibility of Reachability Subramani First-Order Logic Outline Outline 1 Completeness of proof system for First-Order Logic The notion of Completeness The Completeness Proof 2 Consequences of the Completeness theorem Complexity of Validity Compactness Model Cardinality Lowenheim-Skolem¨ Theorem Inexpressibility of Reachability Subramani First-Order Logic Completeness The notion of Completeness Consequences of the Completeness theorem The Completeness Proof Outline 1 Completeness of proof system for First-Order Logic The notion of Completeness The Completeness Proof 2 Consequences of the Completeness theorem Complexity of Validity Compactness Model Cardinality Lowenheim-Skolem¨ Theorem Inexpressibility of Reachability Subramani First-Order Logic Completeness The notion of Completeness Consequences of the Completeness theorem The Completeness Proof Soundness and Completeness Theorem Soundness: If ∆ ⊢ φ, then ∆ |= φ. Theorem Completeness (Godel’s¨ traditional form): If ∆ |= φ, then ∆ ⊢ φ. Theorem Completeness (Godel’s¨ altenate form): If ∆ is consistent, then it has a model. Subramani First-Order Logic Completeness The notion of Completeness Consequences of the Completeness theorem The Completeness Proof Soundness and Completeness (contd.) Theorem The traditional completeness theorem follows from the alternate form of the completeness theorem. Proof. Assume that ∆ |= φ. It follows that any model M that satisfies all the expressions in ∆, also satisfies φ and hence falsifies ¬φ.
    [Show full text]
  • Completeness and Compactness of First-Order Tableaux
    CS 486: Applied Logic Lecture 18, March 27, 2003 18 Completeness and Compactness of First-Order Tableaux 18.1 Completeness Proving the completeness of a first-order calculus gives us G¨odel’sfamous completeness result. G¨odelproved it for a slightly different proof calculus, and the proof that we will show here goes back to Beth and Hintikka. Let us briefly resume the propositional case. The key to the completeness proof was the use of Hintikka’s lemma, which states that every downward saturated set, finite or not, is satisfiable. We then showed that every open and complete path is in fact a Hintikka sequence. Putting these two things together we reasoned that the root of an open and complete tableau must be satisfiable. Thus a complete tableau for a valid formula cannot be open which means that every tableau for a valid formula will eventually close. We will prove the first order case along these lines, but have to keep in mind that several things have changed. ² The definition of a valuation now includes quantifiers. ² The definition of Hintikka sets must take γ and ± formulas into account. ² The notion of a complete tableau needs to be adjusted, because there is now the possibility of non-terminating proof attempts. Fortunately, we can easily make the necessary adjustments and then proceed as before. First, let us define first-order Hintikka sets. A Hintikka Set for a universe U is a set S of U-formulas such that for all closed U-formulas A, ®, ¯, γ, and ± the following conditions hold. 2 ¯ 62 H0 : A atomic and A S 7! A S 2 2 2 H1 : ® S 7! ®1 S ^ ®2 S 2 2 2 H2 : ¯ S 7! ¯1 S _ ¯2 S 2 2 2 H3 : γ S 7! 8k U.
    [Show full text]
  • Provability, Soundness and Completeness Deductive Rules of Inference Provide a Mechanism for Deriving True Conclusions from True Premises
    CSC 244/444 Lecture Notes Sept. 16, 2021 Provability, Soundness and Completeness Deductive rules of inference provide a mechanism for deriving true conclusions from true premises Rules of inference So far we have treated formulas as \given", and have shown how they can be related to a domain of discourse, and how the truth of a set of premises can guarantee (entail) the truth of a conclusion. However, our goal in logic and particularly in AI is to derive new conclusions from given facts. For this we need rules of inference (and later, strategies for applying such rules so as to derive a desired conclusion, if possible). In general, a \forward" inference rule consists of one or more premises and a con- clusion. Both the premises and the conclusion are generally schemas, i.e., they involve metavariables for formulas or terms that can be particularized in many ways (just as we saw in the case of valid formula schemas). We often put a horizontal line under the premises, and write the conclusion underneath the line. For instance, here is the rule of Modus Ponens φ, φ ) MP : This says that given a premise formula φ, and another formula of form φ ) , we may derive the conclusion . It is intuitively clear that this rule leads from true premises to a true conclusion { but this is an intuition we need to verify by proving the rule sound, as illustrated below. An example of using the rule is this: from Dog(Snoopy), Dog(Snoopy) ) Has-tail(Snoopy), we can conclude Has-tail(Snoopy).
    [Show full text]
  • METALOGIC METALOGIC an Introduction to the Metatheory of Standard First Order Logic
    METALOGIC METALOGIC An Introduction to the Metatheory of Standard First Order Logic Geoffrey Hunter Senior Lecturer in the Department of Logic and Metaphysics University of St Andrews PALGRA VE MACMILLAN © Geoffrey Hunter 1971 Softcover reprint of the hardcover 1st edition 1971 All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission. First published 1971 by MACMILLAN AND CO LTD London and Basingstoke Associated companies in New York Toronto Dublin Melbourne Johannesburg and Madras SBN 333 11589 9 (hard cover) 333 11590 2 (paper cover) ISBN 978-0-333-11590-9 ISBN 978-1-349-15428-9 (eBook) DOI 10.1007/978-1-349-15428-9 The Papermac edition of this book is sold subject to the condition that it shall not, by way of trade or otherwise, be lent, resold, hired out, or otherwise circulated without the publisher's prior consent, in any form of binding or cover other than that in which it is published and without a similar condition including this condition being imposed on the subsequent purchaser. To my mother and to the memory of my father, Joseph Walter Hunter Contents Preface xi Part One: Introduction: General Notions 1 Formal languages 4 2 Interpretations of formal languages. Model theory 6 3 Deductive apparatuses. Formal systems. Proof theory 7 4 'Syntactic', 'Semantic' 9 5 Metatheory. The metatheory of logic 10 6 Using and mentioning. Object language and metalang- uage. Proofs in a formal system and proofs about a formal system. Theorem and metatheorem 10 7 The notion of effective method in logic and mathematics 13 8 Decidable sets 16 9 1-1 correspondence.
    [Show full text]
  • The Entscheidungsproblem and Alan Turing
    The Entscheidungsproblem and Alan Turing Author: Laurel Brodkorb Advisor: Dr. Rachel Epstein Georgia College and State University December 18, 2019 1 Abstract Computability Theory is a branch of mathematics that was developed by Alonzo Church, Kurt G¨odel,and Alan Turing during the 1930s. This paper explores their work to formally define what it means for something to be computable. Most importantly, this paper gives an in-depth look at Turing's 1936 paper, \On Computable Numbers, with an Application to the Entscheidungsproblem." It further explores Turing's life and impact. 2 Historic Background Before 1930, not much was defined about computability theory because it was an unexplored field (Soare 4). From the late 1930s to the early 1940s, mathematicians worked to develop formal definitions of computable functions and sets, and applying those definitions to solve logic problems, but these problems were not new. It was not until the 1800s that mathematicians began to set an axiomatic system of math. This created problems like how to define computability. In the 1800s, Cantor defined what we call today \naive set theory" which was inconsistent. During the height of his mathematical period, Hilbert defended Cantor but suggested an axiomatic system be established, and characterized the Entscheidungsproblem as the \fundamental problem of mathemat- ical logic" (Soare 227). The Entscheidungsproblem was proposed by David Hilbert and Wilhelm Ackerman in 1928. The Entscheidungsproblem, or Decision Problem, states that given all the axioms of math, there is an algorithm that can tell if a proposition is provable. During the 1930s, Alonzo Church, Stephen Kleene, Kurt G¨odel,and Alan Turing worked to formalize how we compute anything from real numbers to sets of functions to solve the Entscheidungsproblem.
    [Show full text]
  • LOGIC I 1. the Completeness Theorem 1.1. on Consequences
    LOGIC I VICTORIA GITMAN 1. The Completeness Theorem The Completeness Theorem was proved by Kurt G¨odelin 1929. To state the theorem we must formally define the notion of proof. This is not because it is good to give formal proofs, but rather so that we can prove mathematical theorems about the concept of proof. {Arnold Miller 1.1. On consequences and proofs. Suppose that T is some first-order theory. What are the consequences of T ? The obvious answer is that they are statements provable from T (supposing for a second that we know what that means). But there is another possibility. The consequences of T could mean statements that hold true in every model of T . Do the proof theoretic and the model theoretic notions of consequence coincide? Once, we formally define proofs, it will be obvious, by the definition of truth, that a statement that is provable from T must hold in every model of T . Does the converse hold? The question was posed in the 1920's by David Hilbert (of the 23 problems fame). The answer is that remarkably, yes, it does! This result, known as the Completeness Theorem for first-order logic, was proved by Kurt G¨odel in 1929. According to the Completeness Theorem provability and semantic truth are indeed two very different aspects of the same phenomena. In order to prove the Completeness Theorem, we first need a formal notion of proof. As mathematicians, we all know that a proof is a series of deductions, where each statement proceeds by logical reasoning from the previous ones.
    [Show full text]
  • Chapter 7: Proof Systems: Soundness and Completeness
    Chapter 7: Proof Systems: Soundness and Completeness Proof systems are built to prove statements. Proof systems are an inference machine with special statements, called provable state- ments being its final products. The starting points of the inference are called axioms of the system. We distinguish two kinds of axioms: logi- calAL and specific SP . 1 Semantical link : we usually build a proof system for a given language and its seman- tics i.e. for a logic defined semantically. First step : we choose as a set of logical ax- ioms AL some subset of tautologies, i.e. statements always true. A proof system with only logical axioms AL is called a logic proof system. Building a proof system for which there is no known semantics we think about the logi- cal axioms as statements universally true. 2 We choose as axioms a finite set the state- ments we for sure want to be universally true, and whatever semantics follows they must be tautologies with respect to it. Logical axioms are hence not only tautolo- gies under an established semantics, but they also guide us how to establish a se- mantics, when it is yet unknown. The specific axioms SP are these formulas of the language that describe our knowl- edge of a universe we want to prove facts about. Specific axioms are not universally true, they are true only in the universe we are inter- ested to describe and investigate. 3 A proof system with logical axioms AL and specific axioms SP is called a formal the- ory. The inference machine is defined by a finite set of rules, called inference rules.
    [Show full text]
  • The Explanatory Power of a New Proof: Henkin's Completeness Proof
    The explanatory power of a new proof: Henkin’s completeness proof John Baldwin February 25, 2017 Mancosu writes But explanations in mathematics do not only come in the form of proofs. In some cases expla- nations are sought in a major recasting of an entire discipline. ([Mancosu, 2008], 142) This paper takes up both halves of that statement. On the one hand we provide a case study of the explanatory value of a particular milestone proof. In the process we examine how it began the recasting of a discipline. Hafner and Mancosu take a broad view towards the nature of mathematical explanation. They ar- gue that before attempting to establish a model of explanation, one should develop a ‘taxonomy of re- current types of mathematical explanation’ ([Hafner and Mancosu, 2005], 221) and preparatory to such a taxonomy propose to examine in depth various examples of proofs. In [Hafner and Mancosu, 2005] and [Hafner and Mancosu, 2008], they study deep arguments in real algebraic geometry and analysis to test the models of explanation of Kitcher and Steiner. In their discussion of Steiner’s model they challenge1 the as- sertion [Resnik and Kushner, 1987] that Henkin’s proof of the completeness theorem is explanatory, asking ‘what the explanatory features of this proof are supposed to consist of?’ As a model theorist the challenge to ‘explain’ the explanatory value of this fundamental argument is irresistible. In contrasting the proofs of Henkin and Godel,¨ we seek for the elements of Henkin’s proofs that permit its numerous generalizations. In Section 2 we try to make this analysis more precise through Steiner’s notion of characterizing property.
    [Show full text]
  • Lecture 26: Gödel Incompleteness Entscheidungsproblem Paradoxes
    Entscheidungsproblem • Does there exist an algorithm to decide, given Lecture 26: Gödel a set of axioms, whether a given statement is a Incompleteness theorem? • Church/Turing: No! FOL not decidable, but is semi-decidable CSCI 101 • Is it possible to axiomatize all of the Spring, 2019 mathematical structures of interest in such a way that every true statement becomes a Kim Bruce theorem? • Alow the set of axioms to be infinite, but it must be decidable. • Gödel: No: Incompleteness theorem — fails to be semi-decidable! Paradoxes? Definitions • Let T be a decidable set of statements and let φ be a formula of first-order logic. • This statement is not true. • T ⊢φ means there is a proof of φ using statements of T • This statement is not provable. as axioms • Assume all provable statements are true • T ⊨ φ means for every model in which all statements of T are true, then φ must be true as well. • Second is at heart of Gödel Incompleteness. • Example: Let T be axioms of number theory, and φ be ∀x. ∃y. y > x Gödel Incompleteness Incompleteness Theorems • T is consistent iff all provable statements are • Different notion of completeness -- w.r.t. model true. (T ⊢φ ⇒ T ⊨φ). Gödel Incompleteness 1: For every Equivalently, no false statement has a valid proof • • “interesting” system there are true statements • T is complete iff every true statement has a that cannot be proved. valid proof. (T ⊨φ ⇒ T ⊢φ). • Gödel Incompleteness 2: For every • Gödel Completeness: “interesting” system, the consistency of that In predicate logic, if T ⊨ φ, then T ⊢ φ.
    [Show full text]
  • Logical Theory & Completeness
    15-424: Foundations of Cyber-Physical Systems Lecture Notes on Logical Theory & Completeness Andre´ Platzer Carnegie Mellon University Lecture 24 1 Introduction This course has studied a number of logics, first-order logic FOL in Lecture 2, differ- ential dynamic logic dL [Pla08, Pla10a, Pla12c, Pla12b] in Lecture 3 and Lecture 4 and following, differential temporal dynamic logic dTL [Pla07, Pla10a, Chapter 4] in Lec- ture 16 and 17, as well as differential game logic dGL [Pla13] since Lecture 22. There are other logics for cyber-physical systems that have not been included in this course, but share similar principles for further dynamical aspects. Such logics include quantified differential dynamic logic QdL for distributed hybrid systems [Pla10b, Pla12a], which are systems that are simultaneously distributed systems and hybrid systems, as well as stochastic differential dynamic logic SdL for stochastic hybrid systems [Pla11], which simultaneously involve stochastic dynamics and hybrid dynamics. Logics play a stellar role not just in cyber-physical systems, but also many other contexts. Other important logics include propositional logic, restrictions of first-order logic to certain theories, such as first-order logic of real arithmetic [Tar51], and higher-order logic [And02]. But there are numerous other important and successful logics. In this lecture, we take a step back and study some common important concepts in logic. This study will necessarily be hopelessly incomplete for lack of time. But it should give you a flavor of important principles and concepts in logic that we have not already run across explicitly in earlier lectures of this course. We will also have the opportunity to apply these more general concepts to cyber-physical systems and learn more about them in the next lecture.
    [Show full text]