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Copyright
Copyright © 2018 by Judea Pearl and Dana Mackenzie
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First Edition: May 2018
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Library of Congress Cataloging-in-Publication Data
Names: Pearl, Judea, author. | Mackenzie, Dana, author.
Title:  The  book  of  why  :  the  new  science  of  cause  and  effect  /  Judea  Pearl  and  Dana
Description:  New  York  :  Basic  Books,  [2018]  |  Includes  bibliographical  references  and
Mackenzie.
index.
Identifiers: LCCN 2017056458 (print) | LCCN 2018005510 (ebook) | ISBN 9780465097616
(ebook)  |  ISBN  9780465097609  (hardcover)  |  ISBN  046509760X  (hardcover)  |  ISBN
0465097618 (ebook)
Subjects: LCSH: Causation. | Inference.
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Classification: LCC Q175.32.C38 (ebook) | LCC Q175.32.C38 P43 2018 (print) | DDC 501—
dc23
LC record available at https://lccn.loc.gov/2017056458
ISBNs: 978-0-465-09760-9 (hardcover); 978-0-465-09761-6 (ebook)
E3-20180417-JV-PC
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CONTENTS
Cover
Title Page
Copyright
Dedication
Preface
INTRODUCTION Mind over Data
CHAPTER 1 The Ladder of Causation
CHAPTER 2 From Buccaneers to Guinea Pigs: The Genesis of Causal Inference
CHAPTER 3 From Evidence to Causes: Reverend Bayes Meets Mr. Holmes
CHAPTER 4 Confounding and Deconfounding: Or, Slaying the Lurking Variable
CHAPTER 5 The Smoke-Filled Debate: Clearing the Air
CHAPTER 6 Paradoxes Galore!
CHAPTER 7 Beyond Adjustment: The Conquest of Mount Intervention
CHAPTER 8 Counterfactuals: Mining Worlds That Could Have Been
CHAPTER 9 Mediation: The Search for a Mechanism
CHAPTER 10 Big Data, Artificial Intelligence, and the Big Questions
Acknowledgments
About the Authors
Also by Judea Pearl
Notes
Bibliography
Index
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PREFACE
ALMOST  two  decades  ago,  when  I  wrote  the  preface  to  my  book  Causality  (2000),  I
made  a  rather  daring  remark  that  friends  advised  me  to  tone  down.  “Causality  has
undergone a major transformation,” I wrote, “from a concept shrouded in mystery into a
mathematical object with well-defined semantics and well-founded logic. Paradoxes and
controversies have been resolved, slippery concepts have been explicated, and practical
problems relying on causal information that long were regarded as either metaphysical or
unmanageable can now be solved using elementary mathematics. Put simply, causality
has been mathematized.”
Reading this passage today, I feel I was somewhat shortsighted. What I described as a
“transformation” turned out to be a “revolution” that has changed the thinking in many of
the  sciences.  Many  now  call  it  “the  Causal  Revolution,”  and  the  excitement  that  it  has
generated in research circles is spilling over to education and applications. I believe the
time is ripe to share it with a broader audience.
This  book  strives  to  fulfill  a  three-pronged  mission:  first,  to  lay  before  you  in
nonmathematical language the intellectual content of the Causal Revolution and how it is
affecting  our  lives  as  well  as  our  future;  second,  to  share  with  you  some  of  the  heroic
journeys, both successful and failed, that scientists have embarked on when confronted
by critical cause-effect questions.
Finally, returning the Causal Revolution to its womb in artificial intelligence, I aim to
describe to you how robots can be constructed that learn to communicate in our mother
tongue—the language of cause and effect. This new generation of robots should explain to
us why things happened, why they responded the way they did, and why nature operates
one way and not another. More ambitiously, they should also teach us about ourselves:
why our mind clicks the way it does and what it means to think rationally about cause and
effect, credit and regret, intent and responsibility.
When I write equations, I have a very clear idea of who my readers are. Not so when I
write  for  the  general  public—an  entirely  new  adventure  for  me.  Strange,  but  this  new
experience has been one of the most rewarding educational trips of my life. The need to
shape  ideas  in  your  language,  to  guess  your  background,  your  questions,  and  your
reactions, did more to sharpen my understanding of causality than all the equations I have
written prior to writing this book.
For this I will forever be grateful to you. I hope you are as excited as I am to see the
results.
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Judea Pearl
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Los Angeles, October 2017
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INTRODUCTION: MIND OVER DATA
Every science that has thriven has thriven upon its own symbols.
—AUGUSTUS DE MORGAN (1864)
THIS book tells the story of a science that has changed the way we distinguish facts from
fiction and yet has remained under the radar of the general public. The consequences of
the new science are already impacting crucial facets of our lives and have the potential to
affect more, from the development of new drugs to the control of economic policies, from
education  and  robotics  to  gun  control  and  global  warming.  Remarkably,  despite  the
diversity  and  apparent  incommensurability  of  these  problem  areas,  the  new  science
embraces  them  all  under  a  unified  framework  that  was  practically  nonexistent  two
decades ago.
The new science does not have a fancy name: I call it simply “causal inference,” as do
many of my colleagues. Nor is it particularly high-tech. The ideal technology that causal
inference  strives  to  emulate  resides  within  our  own  minds.  Some  tens  of  thousands  of
years  ago,  humans  began  to  realize  that  certain  things  cause  other  things  and  that
tinkering with the former can change the latter. No other species grasps this, certainly not
to the extent that we do. From this discovery came organized societies, then towns and
cities, and eventually the science- and technology-based civilization we enjoy today. All
because we asked a simple question: Why?
Causal  inference  is  all  about  taking  this  question  seriously.  It  posits  that  the  human
brain is the most advanced tool ever devised for managing causes and effects. Our brains
store an incredible amount of causal knowledge which, supplemented by data, we could
harness to answer some of the most pressing questions of our time. More ambitiously,
once  we  really  understand  the  logic  behind  causal  thinking,  we  could  emulate  it  on
modern computers and create an “artificial scientist.” This smart robot would discover yet
unknown  phenomena,  find  explanations  to  pending  scientific  dilemmas,  design  new
experiments, and continually extract more causal knowledge from the environment.
But before we can venture to speculate on such futuristic developments, it is important
to understand the achievements that causal inference has tallied thus far. We will explore
the way that it has transformed the thinking of scientists in almost every data-informed
discipline and how it is about to change our lives.
The new science addresses seemingly straightforward questions like these:
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