Table Of ContentOther books by Peter Kreeft from St. Augustine's Press
The Philosophy of Jesus
Jesus-Shock
The Sea Within: Waves and the Meaning of All Things
I Surf Therefore I Am
If Einstein Had Been a Surfer
Socrates' Children: Ancient
Socrates' Children: Medieval
Socrates' Children: Modern
Philosophy 101 by Socrates
Socrates Meets Descartes
Socrates Meets Freud
Socrates Meets Hume
Socrates Meets Kant
Socrates Meets Kierkegaard
Socrates Meets Machiavelli
Socrates Meets Marx
Socrates Meets Sartre
Sumrna Philosophica
Socrates 'Students
The Platonic Tradition
Socratic Logic
Edition 3.1
by Peter Kreeft
Edited by Trent Dougherty
A LOGIC TEXT USING
SOCRATIC METHOD,
PLATONIC QUESTIONS, &
ARISTOTELIAN PRINCIPLES
Modeling Socratcs as the ideal teacher for the beginner
and Socratic method as the ideal method
Introducing philosophical issues along with logic
by being philosophical about logic and logical about philosophy
Presenting a complete system of classical Aristotelian logic,
the logic of ordinary language and of the four language arts,
reading, writing, listening, and speaking
©
ST AUGUSTINE'S PRESS
South Bend, Indiana
Copyright © 2004, 2005, 2008, 2010, 2014 by Peter Kreeft
All rights reserved. No part of this book may be reproduced,
stored in a retrieval system, or transmitted, in any form or by any
means, electronic, mechanical, photocopying, recording, or otherwise,
without the prior permission of St. Augustine's Press.
Manufactured in the United States of America
3 4 5 6 20 19 18 17 16 15 14
Library of Congress Cataloging in Publication Data
Kreeft, Peter.
Socratic logic:
a logic text using Socratic method, Platonic questions & Aristotelian
principles / by Peter Kreeft; edited by Trent Dougherty. - Ed. 3.1.
p. cm.
Previously published: 3rd ed. c2008.
Includes bibliographical references and index.
ISBN 978-1-58731-808-5 (hardcover: alk. paper) 1. Logic.
I. Dougherty, Trent. II. Title.
BC108.K67 2010
160 - dc22 2010032937
ooThe paper used in this publication meets the minimum requirements
of the American National Standard for Information Sciences - Permanence
of Paper for Printed Materials, ANSI Z39.48-1984.
ST. AUGUSTINE'S PRESS
www. staugust ine. net
Contents
PREFACE ix
INTRODUCTION 1
1. What good is logic? 1
2. Seventeen ways this book is different 9
3. The two logics (P)# 15
4. All of logic in two pages: an overview (B)* 26
5. The three acts of the mind (B) 28
I. THE FIRST ACT OF THE MIND: UNDERSTANDING 35
1. Understanding: the thing that distinguishes man from
both beast and computer (P) 35
2. Concepts, terms and words (P) 40
3. The "problem of universals" (P) 41
4. The extension and comprehension of terms 43
II. TERMS 47
1. Classifying terms 47
2. Categories (B) 54
3. Predicables (B) 56
4. Division and Outlining (B) 62
III. MATERIAL FALLACIES 68
1. Fallacies of language 71
2. Fallacies of diversion 80
3. Fallacies of oversimplification 86
4. Fallacies of argumentation 92
(cid:149) "P" = "philosophical"; "B" = "basic." See p. 13, last paragraph.
VI SOCRATIC LOGIC
5. Inductivc fallacies 100
6. Procedural fallacies 104
7. Metaphysical fallacies 109
8. Short Story: "Love Is a Fallacy" 114
IV. DEFINITION 123
1. The nature of definition (B) 123
2. The rules of definition (B) 124
3. The kinds of definition 124
4. The limits of definition 129
V. THE SECOND ACT OF THE MIND: JUDGMENT 138
1. Judgments, propositions, and sentences 138
2. What is truth? (P) 143
3. The four kinds of categorical propositions (B) 145
4. Logical form (B) 147
5. Euler's circles (B) 152
6. Tricky propositions 153
7. The distribution of terms 163
VI. CHANGING PROPOSITIONS 166
1. Immediate inference 166
2. Conversion (B) 167
3. Obversion (B) 170
4. Contraposition 171
VII. CONTRADICTION 173
1. What is contradiction? (B) 173
2. The Square of Opposition (B) 174
3. Existential import (P) 179
4. Tricky propositions on the Square 181
5. Some practical uses of the Square of Opposition 183
VIII. THE THIRD ACT OF THE MIND: REASONING 186
1. What does "reason" mean? (P) 186
2. The ultimate foundations of the syllogism (P) 187
3. How to detect arguments 190
4. Arguments vs. explanations 193
5. Truth and validity 194
Contents vii
IX. DIFFERENT KINDS OF ARGUMENTS 200
1. Three meanings of "because" 200
2. The four causes (P) 202
3. A classification of arguments 205
4. Simple argument maps (B) 206
5. Deductive and inductive reasoning (B) 210
6. Combining induction and deduction: Socratic method (P) 211
X. SYLLOGISMS 215
1. The structure and strategy of the syllogism (B) 215
2. The skeptic's objection to the syllogism (P) 219
3. The empiricist's objection to the syllogism (P) 222
4. Demonstrative syllogisms 230
5. How to construct convincing syllogisms (B) 232
XI. CHECKING SYLLOGISMS FOR VALIDITY 237
1. By Euler's Circles (B) 237
2. By Aristotle's six rules (B) 242
3. "Barbara Celarent": mood and figure 257
4. Venn Diagrams 258
XII. MORE DIFFICULT SYLLOGISMS 264
1. Enthymemes: abbreviated syllogisms (B) 264
2. Sorites: chain syllogisms 275
3. Epicheiremas: multiple syllogisms (B) 279
4. Complex argument maps 282
XIII. COMPOUND SYLLOGISMS 289
1. Hypothetical syllogisms (B) 289
2. "Reductio ad absurdum " arguments 294
3. The practical syllogism: arguing about means and ends 296
4. Disjunctive syllogisms (B) 301
5. Conjunctive syllogisms (B) 303
6. Dilemmas (B) 306
XIV. INDUCTION 313
1. What is induction? 313
2. Generalization 315
3. Causal arguments: Mill's methods 319
viii
SOCRATIC LOG1C
4. Scientific hypotheses 325
5. Statistical probability 328
6. Arguments by analogy 329
7. A fortiori and a minore arguments 335
XV. SOME PRACTICAL APPLICATIONS OF LOGIC 342
1. How to write a logical essay 342
2. How to write a Socratic dialogue 344
3. How to have a Socratic debate 348
4. How to use Socratic method on difficult people 350
5. How to read a book Somatically 355
XVI. SOME PHILOSOPHICAL APPLICATIONS OF LOGIC 358
1. Logic and theology (P) 358
2. Logic and metaphysics (P) 359
3. Logic and cosmology (P) 360
4. Logic and philosophical anthropology (P) 361
5. Logic and epistemology (P) 362
6. Logic and ethics (P) 362
APPENDIX: PROBLEMS WITH MATHEMATICAL LOGIC 364
1. Basic modern logic 364
2. The paradoxes of material implication 366
3. Responses to the paradoxes of material implication 367
ANSWERS TO EVEN-NUMBERED EXERCISES 370
INDEX OF PRINCIPAL NAMES 400
Preface
This book is a dinosaur.
Once upon a time in Middle-Earth, two things were different: (1) most stu-
dents learned "the old logic," and (2) they could think, read, write, organize, and
argue much better than they can today. If you believe these two things are not
connected, you probably believe storks bring babies.
It is time to turn back the clock. Contrary to the cliche, you can turn back
the clock, and you should, whenever it is keeping bad time. (I learned that, and
thousands of other very logical paradoxes, from G.K. Chesterton, the 20th-cen-
tury Socrates.)
As I write this, it is the last Sunday of October, and we have just turned back
our clocks from daylight savings time to standard time. This is a parable for what
I am convinced we must do in logic. The prevailing symbolic/mathematical logic
is a logic that a computer can do; it is artificial, like daylight savings time. It is
very useful where there is already much intelligence (in the minds of geniuses,
especially in science), just as daylight savings time is very useful in the summer
when there is a plenitude of sunlight. But as the sunlight of clear thinking, writ-
ing, reading, and debating decreases in our society, it is time to make progress
by turning back the clock from "daylight savings time" to real time, real lan-
guage, real people, and the real world. The old Socratic-Platonic-Aristotelian
logic is simply more effective than the new symbolic logic in helping ordinary
people in dealing with those four precious things.
This text differs from nearly all other logic texts in print in the three ways
suggested by the subtitle. It does this by apprenticing itself to the first three great
philosophers in history, Socrates, Plato, and Aristotle. (Do we have better ones
today?)
(1) No other logic text explicitly sets out to train little Socrateses.
(2) No other logic text in print is so explicitly philosophical in a classical,
Platonic way.
(3) And only two or three other, shorter, formal logic texts bypass mathe-
matical and symbolic logic for the "Aristotelian" logic of real people.
SOCRATIC LOGIC
X
real inquiry, and real conversations. (The only other alternative to sym-
bolic logic available today is "informal logic" or "rhetoric." This is use-
ful, but less exact and less philosophical.)
Introduction
Section 1. What good is logic?
This section will give you 13 good reasons why you should study logic.1
1. Order. You may be wondering, "What can I do with logic?" The answer
is that logic can do something with you. Logic builds the mental habit of
thinking in an orderly way. A course in logic will do this for you even if you
forget every detail in it (which you won't, by the way), just as learning Latin will
make you more habitually aware of the structure of language even if you forget
every particular Latin word and rule.
No course is more practical than logic, for no matter what you are thinking
about, you are thinking, and logic orders and clarifies your thinking. No matter
what your thought's content, it will be clearer when it has a more logical form.
The principles of thinking logically can be applied to all thinking and to every
field.
Logic studies the forms or structures of thought. Thought has form and
structure too, just as the material universe does. Thought is not like a blank
screen, that receives its form only from the world that appears on it, as a movie
screen receives a movie. This book will show you the basic forms (structures)
and the basic laws (rules) of thought, just as a course in physics or chemistry
shows you the basic forms and laws of matter.
2. Power. Logic has power: the power of proof and thus persuasion. Any
power can be either rightly used or abused. This power of logic is rightly used to
win the truth and defeat error; it is wrongly used to win the argument and defeat
1 Making numbered lists like this is the first and simplest way we learn to order "the
buzzing, blooming confusion" that is our world. Children, "primitive" peoples, and
David Letterman love to make lists. Thus we find "twelve-step programs," "the Ten
Commandments," "the Seven Wonders of the World" "the Five Pillars of Islam," "the
Four Noble Truths," and "the Three Things More Miserable Than a Wet Chicken." To
make a list is to classify many things under one general category, and at the same time to
distinguish these things by assigning them different numbers.
