Propositional Logic
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Chapter 5: Methods of Proof for Boolean Logic
Chapter 5: Methods of Proof for Boolean Logic § 5.1 Valid inference steps Conjunction elimination Sometimes called simplification. From a conjunction, infer any of the conjuncts. • From P ∧ Q, infer P (or infer Q). Conjunction introduction Sometimes called conjunction. From a pair of sentences, infer their conjunction. • From P and Q, infer P ∧ Q. § 5.2 Proof by cases This is another valid inference step (it will form the rule of disjunction elimination in our formal deductive system and in Fitch), but it is also a powerful proof strategy. In a proof by cases, one begins with a disjunction (as a premise, or as an intermediate conclusion already proved). One then shows that a certain consequence may be deduced from each of the disjuncts taken separately. One concludes that that same sentence is a consequence of the entire disjunction. • From P ∨ Q, and from the fact that S follows from P and S also follows from Q, infer S. The general proof strategy looks like this: if you have a disjunction, then you know that at least one of the disjuncts is true—you just don’t know which one. So you consider the individual “cases” (i.e., disjuncts), one at a time. You assume the first disjunct, and then derive your conclusion from it. You repeat this process for each disjunct. So it doesn’t matter which disjunct is true—you get the same conclusion in any case. Hence you may infer that it follows from the entire disjunction. In practice, this method of proof requires the use of “subproofs”—we will take these up in the next chapter when we look at formal proofs. -
Inversion by Definitional Reflection and the Admissibility of Logical Rules
THE REVIEW OF SYMBOLIC LOGIC Volume 2, Number 3, September 2009 INVERSION BY DEFINITIONAL REFLECTION AND THE ADMISSIBILITY OF LOGICAL RULES WAGNER DE CAMPOS SANZ Faculdade de Filosofia, Universidade Federal de Goias´ THOMAS PIECHA Wilhelm-Schickard-Institut, Universitat¨ Tubingen¨ Abstract. The inversion principle for logical rules expresses a relationship between introduction and elimination rules for logical constants. Hallnas¨ & Schroeder-Heister (1990, 1991) proposed the principle of definitional reflection, which embodies basic ideas of inversion in the more general context of clausal definitions. For the context of admissibility statements, this has been further elaborated by Schroeder-Heister (2007). Using the framework of definitional reflection and its admis- sibility interpretation, we show that, in the sequent calculus of minimal propositional logic, the left introduction rules are admissible when the right introduction rules are taken as the definitions of the logical constants and vice versa. This generalizes the well-known relationship between introduction and elimination rules in natural deduction to the framework of the sequent calculus. §1. Inversion principle. The idea of inverting logical rules can be found in a well- known remark by Gentzen: “The introductions are so to say the ‘definitions’ of the sym- bols concerned, and the eliminations are ultimately only consequences hereof, what can approximately be expressed as follows: In eliminating a symbol, the formula concerned – of which the outermost symbol is in question – may only ‘be used as that what it means on the ground of the introduction of that symbol’.”1 The inversion principle itself was formulated by Lorenzen (1955) in the general context of rule-based systems and is thus not restricted to logical rules. -
Logical Inference and Its Dynamics
View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by PhilPapers Logical Inference and Its Dynamics Carlotta Pavese 1 Duke University Philosophy Department Abstract This essay advances and develops a dynamic conception of inference rules and uses it to reexamine a long-standing problem about logical inference raised by Lewis Carroll's regress. Keywords: Inference, inference rules, dynamic semantics. 1 Introduction Inferences are linguistic acts with a certain dynamics. In the process of making an inference, we add premises incrementally, and revise contextual assump- tions, often even just provisionally, to make them compatible with the premises. Making an inference is, in this sense, moving from one set of assumptions to another. The goal of an inference is to reach a set of assumptions that supports the conclusion of the inference. This essay argues from such a dynamic conception of inference to a dynamic conception of inference rules (section x2). According to such a dynamic con- ception, inference rules are special sorts of dynamic semantic values. Section x3 develops this general idea into a detailed proposal and section x4 defends it against an outstanding objection. Some of the virtues of the dynamic con- ception of inference rules developed here are then illustrated by showing how it helps us re-think a long-standing puzzle about logical inference, raised by Lewis Carroll [3]'s regress (section x5). 2 From The Dynamics of Inference to A Dynamic Conception of Inference Rules Following a long tradition in philosophy, I will take inferences to be linguistic acts. 2 Inferences are acts in that they are conscious, at person-level, and 1 I'd like to thank Guillermo Del Pinal, Simon Goldstein, Diego Marconi, Ram Neta, Jim Pryor, Alex Rosenberg, Daniel Rothschild, David Sanford, Philippe Schlenker, Walter Sinnott-Armstrong, Seth Yalcin, Jack Woods, and three anonymous referees for helpful suggestions on earlier drafts. -
Introduction to Logic 1
Introduction to logic 1. Logic and Artificial Intelligence: some historical remarks One of the aims of AI is to reproduce human features by means of a computer system. One of these features is reasoning. Reasoning can be viewed as the process of having a knowledge base (KB) and manipulating it to create new knowledge. Reasoning can be seen as comprised of 3 processes: 1. Perceiving stimuli from the environment; 2. Translating the stimuli in components of the KB; 3. Working on the KB to increase it. We are going to deal with how to represent information in the KB and how to reason about it. We use logic as a device to pursue this aim. These notes are an introduction to modern logic, whose origin can be found in George Boole’s and Gottlob Frege’s works in the XIX century. However, logic in general traces back to ancient Greek philosophers that studied under which conditions an argument is valid. An argument is any set of statements – explicit or implicit – one of which is the conclusion (the statement being defended) and the others are the premises (the statements providing the defense). The relationship between the conclusion and the premises is such that the conclusion follows from the premises (Lepore 2003). Modern logic is a powerful tool to represent and reason on knowledge and AI has traditionally adopted it as working tool. Within logic, one research tradition that strongly influenced AI is that started by the German philosopher and mathematician Gottfried Leibniz. His aim was to formulate a Lingua Rationalis to create a universal language based only on rationality, which could solve all the controversies between different parties. -
List of Rules of Inference 1 List of Rules of Inference
List of rules of inference 1 List of rules of inference This is a list of rules of inference, logical laws that relate to mathematical formulae. Introduction Rules of inference are syntactical transform rules which one can use to infer a conclusion from a premise to create an argument. A set of rules can be used to infer any valid conclusion if it is complete, while never inferring an invalid conclusion, if it is sound. A sound and complete set of rules need not include every rule in the following list, as many of the rules are redundant, and can be proven with the other rules. Discharge rules permit inference from a subderivation based on a temporary assumption. Below, the notation indicates such a subderivation from the temporary assumption to . Rules for classical sentential calculus Sentential calculus is also known as propositional calculus. Rules for negations Reductio ad absurdum (or Negation Introduction) Reductio ad absurdum (related to the law of excluded middle) Noncontradiction (or Negation Elimination) Double negation elimination Double negation introduction List of rules of inference 2 Rules for conditionals Deduction theorem (or Conditional Introduction) Modus ponens (or Conditional Elimination) Modus tollens Rules for conjunctions Adjunction (or Conjunction Introduction) Simplification (or Conjunction Elimination) Rules for disjunctions Addition (or Disjunction Introduction) Separation of Cases (or Disjunction Elimination) Disjunctive syllogism List of rules of inference 3 Rules for biconditionals Biconditional introduction Biconditional Elimination Rules of classical predicate calculus In the following rules, is exactly like except for having the term everywhere has the free variable . Universal Introduction (or Universal Generalization) Restriction 1: does not occur in . -
Beginning Logic MATH 10130 Spring 2019 University of Notre Dame Contents
Beginning Logic MATH 10130 Spring 2019 University of Notre Dame Contents 0.1 What is logic? What will we do in this course? . .1 I Propositional Logic 5 1 Beginning propositional logic 6 1.1 Symbols . .6 1.2 Well-formed formulas . .7 1.3 Basic translation . .8 1.4 Implications . 11 1.5 Nuances with negations . 12 2 Proofs 13 2.1 Introduction to proofs . 13 2.2 The first rules of inference . 14 2.3 Rule for implications . 20 2.4 Rules for conjunctions . 24 2.5 Rules for disjunctions . 28 2.6 Proof by contradiction . 32 2.7 Bi-conditional statements . 35 2.8 More examples of proofs . 36 2.9 Some useful proofs . 39 3 Truth 48 3.1 Definition of Truth . 48 3.2 Truth tables . 49 3.3 Analysis of arguments using truth tables . 53 4 Soundness and Completeness 56 4.1 Two notions of validity . 56 4.2 Examples: proofs vs. truth tables . 57 4.3 More examples . 63 4.4 Verifying Soundness . 70 4.5 Explanation of Completeness . 72 i II Predicate Logic 74 5 Beginning predicate logic 75 5.1 Shortcomings of propositional logic . 75 5.2 Symbols of predicate logic . 76 5.3 Definition of well-formed formula . 77 5.4 Translation . 82 5.5 Learning the idioms for predicate logic . 84 5.6 Free variables and sentences . 86 6 Proofs 91 6.1 Basic proofs . 91 6.2 Rules for universal quantifiers . 93 6.3 Rules for existential quantifiers . 98 6.4 Rules for equality . 102 7 Structures and Truth 106 7.1 Languages and structures . -
Derivations in the Sentential Calculus Having Already Two Methods For
Derivations in the Sentential Calculus Having already two methods for showing an SC argument to be valid, the method of truth tables and search-for-counterexamples, we now want to introduce a third, in which we show an argument to be valid by producing aprooJ which is a sequence of sentences formed according to certain rules that guarantee that any sentence that the rules permit you to write down is a logical consequence of a specified set of premisses. It may seem a little perverse to introduce a new technique when we have two already, but the actual situation is even worse than this, for the new procedure is, in a crucial respect, vastly inferior to the ones we have already. The old methods were decisionprocedures, which enabled us to test whether a given argument was valid. If the argument was valid, the procedure would show its validity, whereas is the argument was invalid, the procedure would show its invalidity. The new method is only aproofprocedure: If an argument is valid, the method will enable to show that it is valid, but the method won't provide us any way of showing that an invalid argument is invalid. If an argument is valid, we can show it's valid by providing a proof, but the fact that we haven't been able to find a proof for a given argument doesn't show that the argument is invalid; maybe we just haven't looked hard enough. The new method is a giant step backward. The reason for taking it will only appear when we start work on the predicate calculus. -
Sentential Logic Primer
Sentential Logic Primer Richard Grandy Daniel Osherson Rice University1 Princeton University2 July 23, 2004 [email protected] [email protected] ii Copyright This work is copyrighted by Richard Grandy and Daniel Osherson. Use of the text is authorized, in part or its entirety, for non-commercial non-profit purposes. Electronic or paper copies may not be sold except at the cost of copy- ing. Appropriate citation to the original should be included in all copies of any portion. iii Preface Students often study logic on the assumption that it provides a normative guide to reasoning in English. In particular, they are taught to associate con- nectives like “and” with counterparts in Sentential Logic. English conditionals go over to formulas with → as principal connective. The well-known difficul- ties that arise from such translation are not emphasized. The result is the conviction that ordinary reasoning is faulty when discordant with the usual representation in standard logic. Psychologists are particularly susceptible to this attitude. The present book is an introduction to Sentential Logic that attempts to situate the formalism within the larger theory of rational inference carried out in natural language. After presentation of Sentential Logic, we consider its mapping onto English, notably, constructions involving “if . then . .” Our goal is to deepen appreciation of the issues surrounding such constructions. We make the book available, for free, on line (at least for now). Please be respectful of the integrity of the text. Large portions should not be incorporated into other works without permission. Feedback will be greatly appreciated. Errors, obscurity, or other defects can be brought to our attention via [email protected] or [email protected]. -
An Introduction to Formal Logic
forall x: Calgary An Introduction to Formal Logic By P. D. Magnus Tim Button with additions by J. Robert Loftis Robert Trueman remixed and revised by Aaron Thomas-Bolduc Richard Zach Fall 2021 This book is based on forallx: Cambridge, by Tim Button (University College Lon- don), used under a CC BY 4.0 license, which is based in turn on forallx, by P.D. Magnus (University at Albany, State University of New York), used under a CC BY 4.0 li- cense, and was remixed, revised, & expanded by Aaron Thomas-Bolduc & Richard Zach (University of Calgary). It includes additional material from forallx by P.D. Magnus and Metatheory by Tim Button, used under a CC BY 4.0 license, from forallx: Lorain County Remix, by Cathal Woods and J. Robert Loftis, and from A Modal Logic Primer by Robert Trueman, used with permission. This work is licensed under a Creative Commons Attribution 4.0 license. You are free to copy and redistribute the material in any medium or format, and remix, transform, and build upon the material for any purpose, even commercially, under the following terms: ⊲ You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use. ⊲ You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits. The LATEX source for this book is available on GitHub and PDFs at forallx.openlogicproject.org. -
Rules of Inference
Appendix I: Rules of Inference What follows are graphic representations for each of the 15 Rules of Inference, followed by a brief discussion, that will be used in this course. This supplement to the material in the Manual Part II, is intended to provide an alternative explanation of what each rule requires and what results from its application, in a more condensed, visual way that may be more easily accessible for some learners. Each rule is presented as an “input-output” machine. The “input(s)” represent the type of statement to which you can apply a given rule, or the type of statement or statements required to be “in play” in order to apply the rule. These statements could be premises that are given at the outset of the argument, derived statements or even assumptions. In the diagram the input is presented as a type of statement. Any substitution instance of this type can be a suitable input. The “output” is the result, the inference, or the conclusion you can draw forward when the rule is validly applied to the input(s). The output statement would be the statement you add to your left-hand derivation column, and justify with the rule you are applying. Within each box is an explanation of what the rule does to any given input to achieve the output. Each rule works on a main logical operator, or with a quantifier. Each rule is unique, reflecting the specific meaning of the logical sign (quantifier or operator) addressed by the rule. Quantifier rules Universal Elimination E Input 1) ELIMINATE the universal (x) Bx quantifer Output 2) Change the Ba variable (x, y, or z) to an individual Discussion: You can use this rule on ANY universally quantified statement, at any time in a proof. -
Forall X: an Introduction to Formal Logic 1.30
University at Albany, State University of New York Scholars Archive Philosophy Faculty Books Philosophy 12-27-2014 Forall x: An introduction to formal logic 1.30 P.D. Magnus University at Albany, State University of New York, [email protected] Follow this and additional works at: https://scholarsarchive.library.albany.edu/ cas_philosophy_scholar_books Part of the Logic and Foundations of Mathematics Commons Recommended Citation Magnus, P.D., "Forall x: An introduction to formal logic 1.30" (2014). Philosophy Faculty Books. 3. https://scholarsarchive.library.albany.edu/cas_philosophy_scholar_books/3 This Book is brought to you for free and open access by the Philosophy at Scholars Archive. It has been accepted for inclusion in Philosophy Faculty Books by an authorized administrator of Scholars Archive. For more information, please contact [email protected]. forallx An Introduction to Formal Logic P.D. Magnus University at Albany, State University of New York fecundity.com/logic, version 1.30 [141227] This book is offered under a Creative Commons license. (Attribution-ShareAlike 3.0) The author would like to thank the people who made this project possible. Notable among these are Cristyn Magnus, who read many early drafts; Aaron Schiller, who was an early adopter and provided considerable, helpful feedback; and Bin Kang, Craig Erb, Nathan Carter, Wes McMichael, Selva Samuel, Dave Krueger, Brandon Lee, Toan Tran, and the students of Introduction to Logic, who detected various errors in previous versions of the book. c 2005{2014 by P.D. Magnus. Some rights reserved. You are free to copy this book, to distribute it, to display it, and to make derivative works, under the following conditions: (a) Attribution. -
Form and Content: an Introduction to Formal Logic
Connecticut College Digital Commons @ Connecticut College Open Educational Resources 2020 Form and Content: An Introduction to Formal Logic Derek D. Turner Connecticut College, [email protected] Follow this and additional works at: https://digitalcommons.conncoll.edu/oer Recommended Citation Turner, Derek D., "Form and Content: An Introduction to Formal Logic" (2020). Open Educational Resources. 1. https://digitalcommons.conncoll.edu/oer/1 This Book is brought to you for free and open access by Digital Commons @ Connecticut College. It has been accepted for inclusion in Open Educational Resources by an authorized administrator of Digital Commons @ Connecticut College. For more information, please contact [email protected]. The views expressed in this paper are solely those of the author. Form & Content Form and Content An Introduction to Formal Logic Derek Turner Connecticut College 2020 Susanne K. Langer. This bust is in Shain Library at Connecticut College, New London, CT. Photo by the author. 1 Form & Content ©2020 Derek D. Turner This work is published in 2020 under a Creative Commons Attribution- NonCommercial-NoDerivatives 4.0 International License. You may share this text in any format or medium. You may not use it for commercial purposes. If you share it, you must give appropriate credit. If you remix, transform, add to, or modify the text in any way, you may not then redistribute the modified text. 2 Form & Content A Note to Students For many years, I had relied on one of the standard popular logic textbooks for my introductory formal logic course at Connecticut College. It is a perfectly good book, used in countless logic classes across the country.