Deductive Systems in Traditional and Modern Logic
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Ling 130 Notes: a Guide to Natural Deduction Proofs
Ling 130 Notes: A guide to Natural Deduction proofs Sophia A. Malamud February 7, 2018 1 Strategy summary Given a set of premises, ∆, and the goal you are trying to prove, Γ, there are some simple rules of thumb that will be helpful in finding ND proofs for problems. These are not complete, but will get you through our problem sets. Here goes: Given Delta, prove Γ: 1. Apply the premises, pi in ∆, to prove Γ. Do any other obvious steps. 2. If you need to use a premise (or an assumption, or another resource formula) which is a disjunction (_-premise), then apply Proof By Cases (Disjunction Elimination, _E) rule and prove Γ for each disjunct. 3. Otherwise, work backwards from the type of goal you are proving: (a) If the goal Γ is a conditional (A ! B), then assume A and prove B; use arrow-introduction (Conditional Introduction, ! I) rule. (b) If the goal Γ is a a negative (:A), then assume ::A (or just assume A) and prove con- tradiction; use Reductio Ad Absurdum (RAA, proof by contradiction, Negation Intro- duction, :I) rule. (c) If the goal Γ is a conjunction (A ^ B), then prove A and prove B; use Conjunction Introduction (^I) rule. (d) If the goal Γ is a disjunction (A _ B), then prove one of A or B; use Disjunction Intro- duction (_I) rule. 4. If all else fails, use RAA (proof by contradiction). 2 Using rules for conditionals • Conditional Elimination (modus ponens) (! E) φ ! , φ ` This rule requires a conditional and its antecedent. -
Logic, Reasoning, and Revision
Logic, Reasoning, and Revision Abstract The traditional connection between logic and reasoning has been under pressure ever since Gilbert Harman attacked the received view that logic yields norms for what we should believe. In this paper I first place Harman’s challenge in the broader context of the dialectic between logical revisionists like Bob Meyer and sceptics about the role of logic in reasoning like Harman. I then develop a formal model based on contemporary epistemic and doxastic logic in which the relation between logic and norms for belief can be captured. introduction The canons of classical deductive logic provide, at least according to a widespread but presumably naive view, general as well as infallible norms for reasoning. Obviously, few instances of actual reasoning have such prop- erties, and it is therefore not surprising that the naive view has been chal- lenged in many ways. Four kinds of challenges are relevant to the question of where the naive view goes wrong, but each of these challenges is also in- teresting because it allows us to focus on a particular aspect of the relation between deductive logic, reasoning, and logical revision. Challenges to the naive view that classical deductive logic directly yields norms for reason- ing come in two sorts. Two of them are straightforwardly revisionist; they claim that the consequence relation of classical logic is the culprit. The remaining two directly question the naive view about the normative role of logic for reasoning; they do not think that the philosophical notion of en- tailment (however conceived) is as relevant to reasoning as has traditionally been assumed. -
Classifying Material Implications Over Minimal Logic
Classifying Material Implications over Minimal Logic Hannes Diener and Maarten McKubre-Jordens March 28, 2018 Abstract The so-called paradoxes of material implication have motivated the development of many non- classical logics over the years [2–5, 11]. In this note, we investigate some of these paradoxes and classify them, over minimal logic. We provide proofs of equivalence and semantic models separating the paradoxes where appropriate. A number of equivalent groups arise, all of which collapse with unrestricted use of double negation elimination. Interestingly, the principle ex falso quodlibet, and several weaker principles, turn out to be distinguishable, giving perhaps supporting motivation for adopting minimal logic as the ambient logic for reasoning in the possible presence of inconsistency. Keywords: reverse mathematics; minimal logic; ex falso quodlibet; implication; paraconsistent logic; Peirce’s principle. 1 Introduction The project of constructive reverse mathematics [6] has given rise to a wide literature where various the- orems of mathematics and principles of logic have been classified over intuitionistic logic. What is less well-known is that the subtle difference that arises when the principle of explosion, ex falso quodlibet, is dropped from intuitionistic logic (thus giving (Johansson’s) minimal logic) enables the distinction of many more principles. The focus of the present paper are a range of principles known collectively (but not exhaustively) as the paradoxes of material implication; paradoxes because they illustrate that the usual interpretation of formal statements of the form “. → . .” as informal statements of the form “if. then. ” produces counter-intuitive results. Some of these principles were hinted at in [9]. Here we present a carefully worked-out chart, classifying a number of such principles over minimal logic. -
Admissible Rules and Their Complexity
Admissible rules and their complexity Emil Jeřábek [email protected] http://math.cas.cz/~jerabek/ Institute of Mathematics of the Czech Academy of Sciences, Prague 24th Applications of Logic in Philosophy and the Foundations of Mathematics Szklarska Poręba, May 2019 Outline of the talks 1 Logics and admissibility 2 Transitive modal logics 3 Toy model: logics of bounded depth 4 Projective formulas 5 Admissibility in clx logics 6 Problems and complexity classes 7 Complexity of derivability 8 Complexity of admissibility Logics and admissibility 1 Logics and admissibility 2 Transitive modal logics 3 Toy model: logics of bounded depth 4 Projective formulas 5 Admissibility in clx logics 6 Problems and complexity classes 7 Complexity of derivability 8 Complexity of admissibility Propositional logics Propositional logic L: Language: formulas built from atoms x0; x1; x2;::: using a fixed set of finitary connectives Consequence relation: a relation Γ `L ' between sets of formulas and formulas s.t. I ' `L ' I Γ `L ' implies Γ; ∆ `L ' I Γ; ∆ `L ' and 8 2 ∆ Γ `L imply Γ `L ' I Γ `L ' implies σ(Γ) `L σ(') for every substitution σ Emil Jeřábek Admissible rules and their complexity ALPFM 2019, Szklarska Poręba 1:78 Unifiers and admissible rules Γ; ∆: finite sets of formulas L-unifier of Γ: substitution σ s.t. `L σ(') for all ' 2 Γ Single-conclusion rule: Γ = ' Multiple-conclusion rule: Γ = ∆ I Γ = ∆ is L-derivable (or valid) if Γ `L δ for some δ 2 ∆ I Γ = ∆ is L-admissible (written as Γ ∼L ∆) if every L-unifier of Γ also unifies some δ 2 ∆ NB: Γ is L-unifiable -
Mathematical Logic
Proof Theory. Mathematical Logic. From the XIXth century to the 1960s, logic was essentially mathematical. Development of first-order logic (1879-1928): Frege, Hilbert, Bernays, Ackermann. Development of the fundamental axiom systems for mathematics (1880s-1920s): Cantor, Peano, Zermelo, Fraenkel, Skolem, von Neumann. Giuseppe Peano (1858-1932) Core Logic – 2005/06-1ab – p. 2/43 Mathematical Logic. From the XIXth century to the 1960s, logic was essentially mathematical. Development of first-order logic (1879-1928): Frege, Hilbert, Bernays, Ackermann. Development of the fundamental axiom systems for mathematics (1880s-1920s): Cantor, Peano, Zermelo, Fraenkel, Skolem, von Neumann. Traditional four areas of mathematical logic: Proof Theory. Recursion Theory. Model Theory. Set Theory. Core Logic – 2005/06-1ab – p. 2/43 Gödel (1). Kurt Gödel (1906-1978) Studied at the University of Vienna; PhD supervisor Hans Hahn (1879-1934). Thesis (1929): Gödel Completeness Theorem. 1931: “Über formal unentscheidbare Sätze der Principia Mathematica und verwandter Systeme I”. Gödel’s First Incompleteness Theorem and a proof sketch of the Second Incompleteness Theorem. Core Logic – 2005/06-1ab – p. 3/43 Gödel (2). 1935-1940: Gödel proves the consistency of the Axiom of Choice and the Generalized Continuum Hypothesis with the axioms of set theory (solving one half of Hilbert’s 1st Problem). 1940: Emigration to the USA: Princeton. Close friendship to Einstein, Morgenstern and von Neumann. Suffered from severe hypochondria and paranoia. Strong views on the philosophy of mathematics. Core Logic – 2005/06-1ab – p. 4/43 Gödel’s Incompleteness Theorem (1). 1928: At the ICM in Bologna, Hilbert claims that the work of Ackermann and von Neumann constitutes a proof of the consistency of arithmetic. -
Mathematical Logic. Introduction. by Vilnis Detlovs And
1 Version released: May 24, 2017 Introduction to Mathematical Logic Hyper-textbook for students by Vilnis Detlovs, Dr. math., and Karlis Podnieks, Dr. math. University of Latvia This work is licensed under a Creative Commons License and is copyrighted © 2000-2017 by us, Vilnis Detlovs and Karlis Podnieks. Sections 1, 2, 3 of this book represent an extended translation of the corresponding chapters of the book: V. Detlovs, Elements of Mathematical Logic, Riga, University of Latvia, 1964, 252 pp. (in Latvian). With kind permission of Dr. Detlovs. Vilnis Detlovs. Memorial Page In preparation – forever (however, since 2000, used successfully in a real logic course for computer science students). This hyper-textbook contains links to: Wikipedia, the free encyclopedia; MacTutor History of Mathematics archive of the University of St Andrews; MathWorld of Wolfram Research. 2 Table of Contents References..........................................................................................................3 1. Introduction. What Is Logic, Really?.............................................................4 1.1. Total Formalization is Possible!..............................................................5 1.2. Predicate Languages.............................................................................10 1.3. Axioms of Logic: Minimal System, Constructive System and Classical System..........................................................................................................27 1.4. The Flavor of Proving Directly.............................................................40 -
The Deduction Rule and Linear and Near-Linear Proof Simulations
The Deduction Rule and Linear and Near-linear Proof Simulations Maria Luisa Bonet¤ Department of Mathematics University of California, San Diego Samuel R. Buss¤ Department of Mathematics University of California, San Diego Abstract We introduce new proof systems for propositional logic, simple deduction Frege systems, general deduction Frege systems and nested deduction Frege systems, which augment Frege systems with variants of the deduction rule. We give upper bounds on the lengths of proofs in Frege proof systems compared to lengths in these new systems. As applications we give near-linear simulations of the propositional Gentzen sequent calculus and the natural deduction calculus by Frege proofs. The length of a proof is the number of lines (or formulas) in the proof. A general deduction Frege proof system provides at most quadratic speedup over Frege proof systems. A nested deduction Frege proof system provides at most a nearly linear speedup over Frege system where by \nearly linear" is meant the ratio of proof lengths is O(®(n)) where ® is the inverse Ackermann function. A nested deduction Frege system can linearly simulate the propositional sequent calculus, the tree-like general deduction Frege calculus, and the natural deduction calculus. Hence a Frege proof system can simulate all those proof systems with proof lengths bounded by O(n ¢ ®(n)). Also we show ¤Supported in part by NSF Grant DMS-8902480. 1 that a Frege proof of n lines can be transformed into a tree-like Frege proof of O(n log n) lines and of height O(log n). As a corollary of this fact we can prove that natural deduction and sequent calculus tree-like systems simulate Frege systems with proof lengths bounded by O(n log n). -
MOTIVATING HILBERT SYSTEMS Hilbert Systems Are Formal Systems
MOTIVATING HILBERT SYSTEMS Hilbert systems are formal systems that encode the notion of a mathematical proof, which are used in the branch of mathematics known as proof theory. There are other formal systems that proof theorists use, with their own advantages and disadvantages. The advantage of Hilbert systems is that they are simpler than the alternatives in terms of the number of primitive notions they involve. Formal systems for proof theory work by deriving theorems from axioms using rules of inference. The distinctive characteristic of Hilbert systems is that they have very few primitive rules of inference|in fact a Hilbert system with just one primitive rule of inference, modus ponens, suffices to formalize proofs using first- order logic, and first-order logic is sufficient for all mathematical reasoning. Modus ponens is the rule of inference that says that if we have theorems of the forms p ! q and p, where p and q are formulas, then χ is also a theorem. This makes sense given the interpretation of p ! q as meaning \if p is true, then so is q". The simplicity of inference in Hilbert systems is compensated for by a somewhat more complicated set of axioms. For minimal propositional logic, the two axiom schemes below suffice: (1) For every pair of formulas p and q, the formula p ! (q ! p) is an axiom. (2) For every triple of formulas p, q and r, the formula (p ! (q ! r)) ! ((p ! q) ! (p ! r)) is an axiom. One thing that had always bothered me when reading about Hilbert systems was that I couldn't see how people could come up with these axioms other than by a stroke of luck or genius. -
Chapter 9: Initial Theorems About Axiom System
Initial Theorems about Axiom 9 System AS1 1. Theorems in Axiom Systems versus Theorems about Axiom Systems ..................................2 2. Proofs about Axiom Systems ................................................................................................3 3. Initial Examples of Proofs in the Metalanguage about AS1 ..................................................4 4. The Deduction Theorem.......................................................................................................7 5. Using Mathematical Induction to do Proofs about Derivations .............................................8 6. Setting up the Proof of the Deduction Theorem.....................................................................9 7. Informal Proof of the Deduction Theorem..........................................................................10 8. The Lemmas Supporting the Deduction Theorem................................................................11 9. Rules R1 and R2 are Required for any DT-MP-Logic........................................................12 10. The Converse of the Deduction Theorem and Modus Ponens .............................................14 11. Some General Theorems About ......................................................................................15 12. Further Theorems About AS1.............................................................................................16 13. Appendix: Summary of Theorems about AS1.....................................................................18 2 Hardegree, -
An Anti-Locally-Nameless Approach to Formalizing Quantifiers Olivier Laurent
An Anti-Locally-Nameless Approach to Formalizing Quantifiers Olivier Laurent To cite this version: Olivier Laurent. An Anti-Locally-Nameless Approach to Formalizing Quantifiers. Certified Programs and Proofs, Jan 2021, virtual, Denmark. pp.300-312, 10.1145/3437992.3439926. hal-03096253 HAL Id: hal-03096253 https://hal.archives-ouvertes.fr/hal-03096253 Submitted on 4 Jan 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. An Anti-Locally-Nameless Approach to Formalizing Quantifiers Olivier Laurent Univ Lyon, EnsL, UCBL, CNRS, LIP LYON, France [email protected] Abstract all cases have been covered. This works perfectly well for We investigate the possibility of formalizing quantifiers in various propositional systems, but as soon as (first-order) proof theory while avoiding, as far as possible, the use of quantifiers enter the picture, we have to face one ofthe true binding structures, α-equivalence or variable renam- nightmares of formalization: quantifiers are binders... The ings. We propose a solution with two kinds of variables in question of the formalization of binders does not yet have an 1 terms and formulas, as originally done by Gentzen. In this answer which makes perfect consensus . -
Starting the Dismantling of Classical Mathematics
Australasian Journal of Logic Starting the Dismantling of Classical Mathematics Ross T. Brady La Trobe University Melbourne, Australia [email protected] Dedicated to Richard Routley/Sylvan, on the occasion of the 20th anniversary of his untimely death. 1 Introduction Richard Sylvan (n´eRoutley) has been the greatest influence on my career in logic. We met at the University of New England in 1966, when I was a Master's student and he was one of my lecturers in the M.A. course in Formal Logic. He was an inspirational leader, who thought his own thoughts and was not afraid to speak his mind. I hold him in the highest regard. He was very critical of the standard Anglo-American way of doing logic, the so-called classical logic, which can be seen in everything he wrote. One of his many critical comments was: “G¨odel's(First) Theorem would not be provable using a decent logic". This contribution, written to honour him and his works, will examine this point among some others. Hilbert referred to non-constructive set theory based on classical logic as \Cantor's paradise". In this historical setting, the constructive logic and mathematics concerned was that of intuitionism. (The Preface of Mendelson [2010] refers to this.) We wish to start the process of dismantling this classi- cal paradise, and more generally classical mathematics. Our starting point will be the various diagonal-style arguments, where we examine whether the Law of Excluded Middle (LEM) is implicitly used in carrying them out. This will include the proof of G¨odel'sFirst Theorem, and also the proof of the undecidability of Turing's Halting Problem. -
Deontic Logic and Meta-Ethics
Deontic Logic and Meta-Ethics • Deontic Logic as been a field in which quite apart from the questions of antinomies "paradoxes" have played a decisive roles, since the field has been invented. These paradoxes (like Ross' Paradox or the Good Samaritian) are said to show that some intuitively valid logical principles cannot be true in deontic logic, even if the same principles (like deductive closure within a modality) are valid in alethic modal logic. Paraconsistency is not concerned with solutions to these paradoxes. pp ∧¬∧¬pp • There are, however, lots of problems with ex contradictione qoudlibet Manuel Bremer in deontic logic, since systems of norms and regulations, even more so Centre for Logic, Language and if they are historically grown, have a tendency to contain conflicting Information requirements. To restrict the ensuing problems at least a weak paraconsistent logic is needed. • There may be, further on, real antinomies of deontic logic and meta- ethics (understood comprehensively as including also some basic decision and game theory). Inconsistent Obligations • It often may happen that one is confronted with inconsistent obligations. • Consider for example the rule that term papers have to be handed in at the institute's office one week before the session in question combined with the rule that one must not disturb the secretary at the institute's office if she is preparing an institute meeting. What if the date of handing in your paper falls on a day when an institute meeting is pp ∧¬∧¬pp prepared? Entering the office is now obligatory (i.e. it is obligatory that it is the Manuel Bremer case that N.N.