Language, Proof, and Logic - Barwise_ Jon.pdf

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LANGUAGE,
PROOF AND
LOGIC
JON BARWISE & JOHN ETCHEMENDY
In collaboration with
Gerard Allwein
Dave Barker-Plummer
Albert Liu
SEVEN BRIDGES PRESS
NEW YORK • LONDON
879800361.002.png 879800361.003.png 879800361.004.png 879800361.005.png 879800361.001.png
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
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
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 re¯ning 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 ¯ve 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 ¯nal 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 ¯nancial 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 arti¯cial 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 ¯rst-order languages
. . . . . . . . . . . . . . . . . . .
28
1.5
Function symbols (optional ) . . . . . . . . . . . . . . . . . . . .
31
1.6
The ¯rst-order language of set theory (optional )
. . . . . . . .
37
1.7
The ¯rst-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
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