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LANGUAGE,
PROOF AND
LOGIC
JON BARWISE & JOHN ETCHEMENDY
In collaboration with
Gerard Allwein
Dave Barker-Plummer
Albert Liu
7
7
SEVEN BRIDGES PRESS
NEW YORK • LONDON
Library of Congress Cataloging-in-Publication Data
Barwise, Jon.
Language, proof and logic / Jon Barwise and John Etchemendy ;
in collaboration with Gerard Allwein, Dave Barker-Plummer, and
Albert Liu.
p. cm.
ISBN 1-889119-08-3 (pbk. : alk. paper)
I. Etchemendy, John, 1952- II. Allwein, Gerard, 1956-
III. Barker-Plummer, Dave. IV. Liu, Albert, 1966- V. Title.
IN PROCESS
99-41113
CIP
Copyright © 1999
CSLI Publications
Center for the Study of Language and Information
Leland Stanford Junior University
03 02 01 00 99 5 4 3 2 1
Acknowledgements
Our primary debt of gratitude goes to our three main collaborators on this
project: Gerry Allwein, Dave Barker-Plummer, and Albert Liu. They have
worked with us in designing the entire package, developing and implementing
the software, and teaching from and refining the text. Without their intelli-
gence, dedication, and hard work, LPL would neither exist nor have most of
its other good properties.
In addition to the five of us, many people have contributed directly and in-
directly to the creation of the package. First, over two dozen programmers have
worked on predecessors of the software included with the package, both earlier
versions of Tarski’s World and the program Hyperproof, some of whose code
has been incorporated into Fitch. We want especially to mention Christopher
Fuselier, Mark Greaves, Mike Lenz, Eric Ly, and Rick Wong, whose outstand-
ing contributions to the earlier programs provided the foundation of the new
software. Second, we thank several people who have helped with the develop-
ment of the new software in essential ways: Rick Sanders, Rachel Farber, Jon
Russell Barwise, Alex Lau, Brad Dolin, Thomas Robertson, Larry Lemmon,
and Daniel Chai. Their contributions have improved the package in a host of
ways.
Prerelease versions of LPL have been tested at several colleges and uni-
versities. In addition, other colleagues have provided excellent advice that we
have tried to incorporate into the final package. We thank Selmer Bringsjord,
Renssalaer Polytechnic Institute; Tom Burke, University of South Carolina;
Robin Cooper, Gothenburg University; James Derden, Humboldt State Uni-
versity; Josh Dever, SUNY Albany; Avrom Faderman, University of Rochester;
James Garson, University of Houston; Ted Hodgson, Montana State Univer-
sity; John Justice, Randolph-Macon Women’s College; Ralph Kennedy, Wake
Forest University; Michael O’Rourke, University of Idaho; Greg Ray, Univer-
sity of Florida; Cindy Stern, California State University, Northridge; Richard
Tieszen, San Jose State University; Saul Traiger, Occidental College; and Lyle
Zynda, Indiana University at South Bend. We are particularly grateful to John
Justice, Ralph Kennedy, and their students (as well as the students at Stan-
ford and Indiana University), for their patience with early versions of the
software and for their extensive comments and suggestions.
We would also like to thank Stanford’s Center for the Study of Language
and Information and Indiana University’s College of Arts and Sciences for
iii
iv / Acknowledgements
their financial support of the project. Finally, we are grateful to our two
publishers, Dikran Karagueuzian of CSLI Publications and Clay Glad of Seven
Bridges Press, for their skill and enthusiasm about LPL.
Acknowledgements
Contents
Acknowledgements iii
Introduction 1
The special role of logic in rational inquiry . . . . . . . . . . . . . . 1
Why learn an artificial language? . . . . . . . . . . . . . . . . . . . . 2
Consequence and proof . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Instructions about homework exercises (essential! ) . . . . . . . . . . 5
To the instructor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Web address . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
I Propositional Logic 17
1 Atomic Sentences 19
1.1 Individual constants . . . . . . . . . . . . . . . . . . . . . . . . 19
1.2 Predicate symbols . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.3 Atomic sentences . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.4 General first-order languages . . . . . . . . . . . . . . . . . . . 28
1.5 Function symbols (optional ) . . . . . . . . . . . . . . . . . . . . 31
1.6 The first-order language of set theory (optional) . . . . . . . . 37
1.7 The first-order language of arithmetic (optional) . . . . . . . . 38
1.8 Alternative notation (optional) . . . . . . . . . . . . . . . . . . 40
2 The Logic of Atomic Sentences 41
2.1 Valid and sound arguments . . . . . . . . . . . . . . . . . . . . 41
2.2 Methods of proof . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.3 Formal proofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
2.4 Constructing proofs in Fitch . . . . . . . . . . . . . . . . . . . . 58
2.5 Demonstrating nonconsequence . . . . . . . . . . . . . . . . . . 63
2.6 Alternative notation (optional) . . . . . . . . . . . . . . . . . . 66
3 The Boolean Connectives 67
3.1 Negation symbol: ¬ . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.2 Conjunction symbol: ∧ . . . . . . . . . . . . . . . . . . . . . . . 71
3.3 Disjunction symbol: ∨ . . . . . . . . . . . . . . . . . . . . . . . 74
3.4 Remarks about the game . . . . . . . . . . . . . . . . . . . . . 77
v
vi / Contents
3.5 Ambiguity and parentheses . . . . . . . . . . . . . . . . . . . . 79
3.6 Equivalent ways of saying things . . . . . . . . . . . . . . . . . 82
3.7 Translation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
3.8 Alternative notation (optional) . . . . . . . . . . . . . . . . . . 89
4 The Logic of Boolean Connectives 93
4.1 Tautologies and logical truth . . . . . . . . . . . . . . . . . . . 94
4.2 Logical and tautological equivalence . . . . . . . . . . . . . . . 106
4.3 Logical and tautological consequence . . . . . . . . . . . . . . . 110
4.4 Tautological consequence in Fitch . . . . . . . . . . . . . . . . . 114
4.5 Pushing negation around (optional) . . . . . . . . . . . . . . . 117
4.6 Conjunctive and disjunctive normal forms (optional) . . . . . . 121
5 Methods of Proof for Boolean Logic 127
5.1 Valid inference steps . . . . . . . . . . . . . . . . . . . . . . . . 128
5.2 Proof by cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
5.3 Indirect proof: proof by contradiction . . . . . . . . . . . . . . . 136
5.4 Arguments with inconsistent premises (optional ) . . . . . . . . 140
6 Formal Proofs and Boolean Logic 142
6.1 Conjunction rules . . . . . . . . . . . . . . . . . . . . . . . . . . 143
6.2 Disjunction rules . . . . . . . . . . . . . . . . . . . . . . . . . . 148
6.3 Negation rules . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
6.4 The proper use of subproofs . . . . . . . . . . . . . . . . . . . . 163
6.5 Strategy and tactics . . . . . . . . . . . . . . . . . . . . . . . . 167
6.6 Proofs without premises (optional) . . . . . . . . . . . . . . . . 173
7 Conditionals 176
7.1 Material conditional symbol: → . . . . . . . . . . . . . . . . . . 178
7.2 Biconditional symbol: ↔ . . . . . . . . . . . . . . . . . . . . . . 181
7.3 Conversational implicature . . . . . . . . . . . . . . . . . . . . 187
7.4 Truth-functional completeness (optional) . . . . . . . . . . . . . 190
7.5 Alternative notation (optional) . . . . . . . . . . . . . . . . . . 196
8 The Logic of Conditionals 198
8.1 Informal methods of proof . . . . . . . . . . . . . . . . . . . . . 198
8.2 Formal rules of proof for → and ↔ . . . . . . . . . . . . . . . . 206
8.3 Soundness and completeness (optional) . . . . . . . . . . . . . . 214
8.4 Valid arguments: some review exercises . . . . . . . . . . . . . . 222
Contents
Contents / vii
II Quantifiers 225
9 Introduction to Quantification 227
9.1 Variables and atomic wffs . . . . . . . . . . . . . . . . . . . . . 228
9.2 The quantifier symbols: ∀, ∃ . . . . . . . . . . . . . . . . . . . . 230
9.3 Wffs and sentences . . . . . . . . . . . . . . . . . . . . . . . . . 231
9.4 Semantics for the quantifiers . . . . . . . . . . . . . . . . . . . . 234
9.5 The four Aristotelian forms . . . . . . . . . . . . . . . . . . . . 239
9.6 Translating complex noun phrases . . . . . . . . . . . . . . . . 243
9.7 Quantifiers and function symbols (optional) . . . . . . . . . . . 251
9.8 Alternative notation (optional) . . . . . . . . . . . . . . . . . . 255
10 The Logic of Quantifiers 257
10.1 Tautologies and quantification . . . . . . . . . . . . . . . . . . . 257
10.2 First-order validity and consequence . . . . . . . . . . . . . . . 266
10.3 First-order equivalence and DeMorgan’s laws . . . . . . . . . . 275
10.4 Other quantifier equivalences (optional ) . . . . . . . . . . . . . 280
10.5 The axiomatic method (optional) . . . . . . . . . . . . . . . . . 283
11 Multiple Quantifiers 289
11.1 Multiple uses of a single quantifier . . . . . . . . . . . . . . . . 289
11.2 Mixed quantifiers . . . . . . . . . . . . . . . . . . . . . . . . . . 293
11.3 The step-by-step method of translation . . . . . . . . . . . . . . 298
11.4 Paraphrasing English . . . . . . . . . . . . . . . . . . . . . . . . 300
11.5 Ambiguity and context sensitivity . . . . . . . . . . . . . . . . 304
11.6 Translations using function symbols (optional) . . . . . . . . . 308
11.7 Prenex form (optional ) . . . . . . . . . . . . . . . . . . . . . . . 311
11.8 Some extra translation problems . . . . . . . . . . . . . . . . . 315
12 Methods of Proof for Quantifiers 319
12.1 Valid quantifier steps . . . . . . . . . . . . . . . . . . . . . . . . 319
12.2 The method of existential instantiation . . . . . . . . . . . . . . 322
12.3 The method of general conditional proof . . . . . . . . . . . . . 323
12.4 Proofs involving mixed quantifiers . . . . . . . . . . . . . . . . 329
12.5 Axiomatizing shape (optional ) . . . . . . . . . . . . . . . . . . 338
13 Formal Proofs and Quantifiers 342
13.1 Universal quantifier rules . . . . . . . . . . . . . . . . . . . . . 342
13.2 Existential quantifier rules . . . . . . . . . . . . . . . . . . . . . 347
13.3 Strategy and tactics . . . . . . . . . . . . . . . . . . . . . . . . 352
13.4 Soundness and completeness (optional) . . . . . . . . . . . . . . 361
Contents
viii / Contents
13.5 Some review exercises (optional) . . . . . . . . . . . . . . . . . 361
14 More about Quantification (optional) 364
14.1 Numerical quantification . . . . . . . . . . . . . . . . . . . . . . 366
14.2 Proving numerical claims . . . . . . . . . . . . . . . . . . . . . 374
14.3 The, both, and neither . . . . . . . . . . . . . . . . . . . . . . . 379
14.4 Adding other determiners to fol . . . . . . . . . . . . . . . . . 383
14.5 The logic of generalized quantification . . . . . . . . . . . . . . 389
14.6 Other expressive limitations of first-order logic . . . . . . . . . 397
III Applications and Metatheory 403
15 First-order Set Theory 405
15.1 Naive set theory . . . . . . . . . . . . . . . . . . . . . . . . . . 406
15.2 Singletons, the empty set, subsets . . . . . . . . . . . . . . . . . 412
15.3 Intersection and union . . . . . . . . . . . . . . . . . . . . . . . 415
15.4 Sets of sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419
15.5 Modeling relations in set theory . . . . . . . . . . . . . . . . . . 422
15.6 Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427
15.7 The powerset of a set (optional) . . . . . . . . . . . . . . . . . 429
15.8 Russell’s Paradox (optional) . . . . . . . . . . . . . . . . . . . . 432
15.9 Zermelo Frankel set theory zfc (optional) . . . . . . . . . . . . 433
16 Mathematical Induction 442
16.1 Inductive definitions and inductive proofs . . . . . . . . . . . . 443
16.2 Inductive definitions in set theory . . . . . . . . . . . . . . . . . 451
16.3 Induction on the natural numbers . . . . . . . . . . . . . . . . . 453
16.4 Axiomatizing the natural numbers (optional ) . . . . . . . . . . 456
16.5 Proving programs correct (optional ) . . . . . . . . . . . . . . . 458
17 Advanced Topics in Propositional Logic 468
17.1 Truth assignments and truth tables . . . . . . . . . . . . . . . . 468
17.2 Completeness for propositional logic . . . . . . . . . . . . . . . 470
17.3 Horn sentences (optional) . . . . . . . . . . . . . . . . . . . . . 479
17.4 Resolution (optional) . . . . . . . . . . . . . . . . . . . . . . . . 488
18 Advanced Topics in FOL 495
18.1 First-order structures . . . . . . . . . . . . . . . . . . . . . . . . 495
18.2 Truth and satisfaction, revisited . . . . . . . . . . . . . . . . . . 500
18.3 Soundness for fol . . . . . . . . . . . . . . . . . . . . . . . . . 509
Contents
Contents / ix
18.4 The completeness of the shape axioms (optional ) . . . . . . . . 512
18.5 Skolemization (optional) . . . . . . . . . . . . . . . . . . . . . . 514
18.6 Unification of terms (optional ) . . . . . . . . . . . . . . . . . . 516
18.7 Resolution, revisited (optional) . . . . . . . . . . . . . . . . . . 519
19 Completeness and Incompleteness 526
19.1 The Completeness Theorem for fol . . . . . . . . . . . . . . . 527
19.2 Adding witnessing constants . . . . . . . . . . . . . . . . . . . . 529
19.3 The Henkin theory . . . . . . . . . . . . . . . . . . . . . . . . . 531
19.4 The Elimination Theorem . . . . . . . . . . . . . . . . . . . . . 534
19.5 The Henkin Construction . . . . . . . . . . . . . . . . . . . . . 540
19.6 The L¨owenheim-Skolem Theorem . . . . . . . . . . . . . . . . . 546
19.7 The Compactness Theorem . . . . . . . . . . . . . . . . . . . . 548
19.8 The G¨odel Incompleteness Theorem . . . . . . . . . . . . . . . 552
Summary of Formal Proof Rules 557
Propositional rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557
First-order rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559
Inference Procedures (Con Rules) . . . . . . . . . . . . . . . . . . . 561
Glossary 562
General Index 573
Exercise Files Index 585
Contents
[...]... intelligence logic and computer science Why learn an artificial language? 4 / Introduction artificial languages logic and ordinary language logical basis of Prolog a bit in Part III of this book Fol serves as the prototypical example of what is known as an artificial language These are languages that were designed for special purposes, and are contrasted with so-called natural languages, languages like English and. .. artificial language that is precise yet rich enough to program computers was inspired by this language In addition, all extant programming languages borrow some notions from one or another dialect of fol Finally, there are so-called logic programming languages, like Prolog, whose programs are sequences of sentences in a certain dialect of fol We will discuss the logic and philosophy logic and artificial... rational inquiry, like medicine and finance, it is also a valuable tool for understanding the principles of rationality underlying these disciplines as well The language goes by various names: the lower predicate calculus, the functional calculus, the language of first-order logic, and fol The last of Why learn an artificial language? / 3 these is pronounced ef–oh–el, not fall, and is the name we will use... there both are and are not nine planets The importance of logic has been recognized since antiquity After all, no 1 logic and rational inquiry logic and convention 2 / Introduction laws of logic goals of the book science can be any more certain than its weakest link If there is something arbitrary about logic, then the same must hold of all rational inquiry Thus it becomes crucial to understand just what... what the laws of logic are, and even more important, why they are laws of logic These are the questions that one takes up when one studies logic itself To study logic is to use the methods of rational inquiry on rationality itself Over the past century the study of logic has undergone rapid and important advances Spurred on by logical problems in that most deductive of disciplines, mathematics, it developed... put the language to more sophisticated uses As we said earlier, besides teaching the language fol, we also discuss basic methods of proof and how to use them In this regard, too, our approach is somewhat unusual We emphasize both informal and formal methods of proof We first discuss and analyze informal reasoning methods, the kind used in everyday life, and then formalize these using a Fitch-style natural... subject-predicate sentence It consists of the subject Max followed by the predicate likes Claire In fol, by contrast, we view this as a claim involving two “logical subjects,” the names Max and Claire, and a 1 There is, however, a variant of first-order logic called free logic in which this assumption is relaxed In free logic, there can be individual constants without referents This yields a language. .. fol In this book, we will explore methods of proof how we can prove that one claim is a logical consequence of another and also methods for showing that a claim is not a consequence of others In addition to the language fol itself, these two methods, the method of proof and the method of counterexample, form the principal subject matter of this book proof and counterexample Essential instructions about... particularly, we have two main aims The first is to help you learn a new language, the language of first-order logic The second is to help you learn about the notion of logical consequence, and about how one goes about establishing whether some claim is or is not a logical consequence of other accepted claims While there is much more to logic than we can even hint at in this book, or than any one person... predicates like Cube, Larger, and Between Some examples of atomic sentences in this language are Cube(b), Larger(c, f), and Between(b, c, d) These sentences say, respectively, that b is a cube, that c is larger than f , and that b is between c and d Later in this chapter, we will look at the atomic sentences used in two other versions of fol, the first-order languages of set theory and arithmetic In the next . Cataloging-in-Publication Data Barwise, Jon. Language, proof and logic / Jon Barwise and John Etchemendy ; in collaboration with Gerard Allwein, Dave Barker-Plummer, and Albert Liu. p. cm. ISBN 1-8 8911 9-0 8-3 . cm. ISBN 1-8 8911 9-0 8-3 (pbk. : alk. paper) I. Etchemendy, John, 195 2- II. Allwein, Gerard, 195 6- III. Barker-Plummer, Dave. IV. Liu, Albert, 196 6- V. Title. IN PROCESS 9 9-4 1113 CIP Copyright. The Logic of Boolean Connectives 93 4.1 Tautologies and logical truth . . . . . . . . . . . . . . . . . . . 94 4.2 Logical and tautological equivalence . . . . . . . . . . . . . . . 106 4.3 Logical
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