Chapter9-11.pdf

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CHAPTER OBJECTIVES

Descartes: Doubt and Certainty

9.1 THE PURSUIT OF KNOWLEDGE
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9.2 PLATO’S RATIONALISM
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9.3 DESCARTES’ DOUBT
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9.4 DESCARTES’ CERTAINTY
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CHAPTER 9

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9.1 THE PURSUIT OF KNOWLEDGE

In René Descartes’ time, the world must have seemed to
many to be turning upside down. Time-honored ideas,
established religious doctrines, and traditional attitudes
were being called into question both by new discoveries
in science and by radically di!erent religious outlooks on
the Continent. “is was the era of Galileo, Copernicus,
Kepler, Bacon, Newton, and Martin Luther—thinkers
who were dismantling the old ideological structures piece
by piece. “ese were unsettling signs that the modern
world—the world we inherited—was being born. Into this
crucible of upheaval and change came the brilliant Des-
cartes (1596–1650)—the inventor of analytic geometry
and founder of modern philosophy. He was determined to
see if there could be any epistemological certainties in an
age of doubt. He hoped that knowledge could be given a
foundation as sturdy as that which buttressed mathemat-
ics. If only knowledge of the world could be as certain as
knowledge of geometry! “us Descartes chose bravely to
wrestle with a very old and di#cult issue: whether it was
possible for anyone to possess knowledge.

But isn’t it obvious that we do in fact know things? Many
philosophers who have looked deeply into the question have thought the answer is not
obvious at all. “ey ask—quite seriously—this question and several others: If you have
knowledge, how did you attain it? And if you possess it, how much do you possess—
that is, what is the extent of your knowledge? Do you know only the contents of your
own mind or only mathematical or logical truths? Do you know that there is a God,
that ordinary physical objects exist, that there is an external world (one existing inde-
pendently of your mind), that unobservable entities such as electrons are real, that other
minds besides your own exist, that events have occurred before the present moment?

Do these questions seem odd, even absurd, to you? Among serious think-
ers, they are neither. Trying to find good answers to these is the main business of
epistemology, the philosophical study of knowledge (see Chapter 1). It is the branch
of philosophy that systematically investigates whether, how, and to what extent we
know things. For well over two thousand years, philosophers have been searching
for answers because both the asking and the answering have theoretical and practical
value. We value knowledge for its own sake, regardless of what we can do with it.
When we are at our best, we crave the light simply because it is the light. But we also
value knowledge because it can guide us to our goals, steer us away from error, and
help us succeed in life, however we define success. Knowledge is power. Whatever
our reply to the epistemological questions, if we take them seriously, they surely will
a!ect how we see the world and what we do in it.

Knowledge comes in di!erent forms, and philosophy is usually concerned with
only one of them. Knowing what something feels like (for example, what influenza

“If you would be a real
seeker after truth, it is
necessary that at least
once in your life you
doubt, as far as possible,
all things.”

—Descartes

epistemology “e
philosophical study of
knowledge.

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feels like) constitutes one form of knowledge. Knowing how to do something (for ex-
ample, how to throw a ball) constitutes another. Knowing that something is the case
(such as knowing that an elm tree grows in the quad) is propositional knowledge—
knowledge of a proposition. A proposition is a statement that is either true or false,
an assertion that something is or is not a fact. “is kind of knowledge has been the
main focus of philosophers.

As we saw in Chapter 4, thinkers going back as far as Plato have said that propo-
sitional knowledge has three necessary and su#cient conditions: to know a proposi-
tion, (1) you must believe it, (2) it must be true, and (3) you must have good reasons
for—be justified in—believing it true. On this traditional account, merely believing
something is not enough; what you believe must be true. But a mere true belief is not
knowledge either, because you can have a true belief and yet not genuinely know. To
have knowledge, your belief must be true, and you must have good reasons to believe
it true. Knowledge, then, is true belief that is justified.

Most philosophers believe we have some knowledge but di!er on its extent. “ey
may insist that we possess knowledge of the existence of an external world, other

propositional knowledge
Knowledge of a
proposition.

THEN AND NOW

The Scientific (and Philosophical) Revolution
At the center of the universe,
the stationary Earth—
the focus of all God’s
attention—reigns as a per-
fect sphere, nested in the
exact center of a series of
other perfect hollow
spheres, like an onion. On
each sphere rests one of the
planets, the sun, the moon,
or stars, every sphere turn-
ing around the Earth
producing the familiar
movements of these heav-
enly bodies. “e spheres are
not only perfect but change-
less and eternal. Beyond the
sphere of the stars is heaven,
the realm of light where
God abides and human
souls go to their reward.

“is view of cosmic re-
ality is roughly what

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minds, physical objects, the past and future, or self-evident truths. But some philoso-
phers embrace skepticism, the view that we lack knowledge in some fundamental
way. “ey hold that many or all of our beliefs are false or unfounded.

Some skeptics argue that we lack knowledge because we have no way of distin-
guishing between beliefs that we think constitute knowledge from beliefs that clearly
do not constitute knowledge. For all we know, we could be hallucinating, dreaming,
in the grips of an illusion, or mistaken for some other reason. How do we know that
we are not hallucinating or dreaming right now? Hallucinations and dreams can
seem as real as our “normal” experience. If we cannot distinguish these two, the
skeptic says, then we cannot have knowledge.

Other skeptics raise doubts about the reliability of what we take to be our normal
sources of knowledge—perception, memory, introspection, and reasoning. We real-
ize that all these sources are fallible, that they sometimes lead us into error. But skep-
tics ask how we know that these sources are not always in error. If all these sources
are suspect, we cannot use one to check another. We cannot, for example, use our
sense of sight to check the reliability of our sense of touch. And if we think one mode

skepticism “e view
that we lack knowledge in
some fundamental way.

millions of people believed and never questioned in ancient and medieval times. It was said
to be derived from unimpeachable authorities—from Aristotle and Christian scripture or
dogma. But the scientific revolution (dating roughly from the late sixteenth through the
eighteenth centuries) upended the medieval worldview and ushered in a host of new ideas
both in society and philosophy. “e combined work of Copernicus (1473–1543), Kepler
(1571–1630), Galileo (1564–1642), and others showed that the Earth rotated around the sun,
not the other way around; that the movements of the heavens could be precisely described
through mathematics; that the old notion of spheres was unfounded; that (thanks to Galileo’s
telescope) the heavenly bodies are not perfect but have obvious imperfections; and that teleo-
logical explanations (those assuming that everything is guided to an end or goal) are inac-
curate and uninformative.

Descartes was influenced by, and influenced, the scientific revolution, which in turn
a!ected his contributions to philosophy. He was the genius who founded analytic geometry
(remember Cartesian coordinates?), advanced the science of optics, wrote treatises on meteo-
rology, and discovered how rainbows work. In philosophy he insisted that a radically di!er-
ent perspective was required. In light of the methods and findings of the “new sciences,” he
saw that the old ideas had to go and that a new mechanistic and mathematical view of the
world had to take its place. In fact, he deemed mathematics a model for human understand-
ing. He declared that the simple mathematical steps used to derive very complicated proofs
“gave me occasion to suppose that all things which fall within the scope of human under-
standing are interconnected in the same way” (Discourse on Method, Part II).

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“”e first precept was
never to accept a thing as
true until I knew it as such
without a single doubt.”

—Descartes

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of perception is more trustworthy than the others, how do we know that? We seem
forced once again into skepticism.

Philosophers distinguish two ways to acquire knowledge: through reason and
through sense experience. “e former is called a priori; it yields knowledge gained
independently of or prior to sense experience. “e latter is known as a posteriori; it
gives us knowledge that depends entirely on sense experience. We can come to know
many propositions a priori—for example, that all bachelors are unmarried, that all
triangles have three sides, that 2 ! 3 ” 5, and that something is either a cat or not
a cat. We need not do a survey of bachelors to see if they really are all unmarried;
we can know this just by thinking about it. And we know that the statement “some-
thing is either a cat or not a cat” is true; it is a simple logical truth—and we know it
without having to observe any cats. It seems that we can also come to know many
propositions a posteriori—for instance, John the bachelor has red hair, that he just
drew a triangle on paper, that he is holding five pencils, and that Tabby the cat is on
the mat. To know these things, we must rely on our senses.

“rough the centuries, philosophers have debated whether our knowledge of
the world is fundamentally a priori or a posteriori (if indeed we have knowledge),
and these arguments continue today in many forms in both philosophy and science.
On one side of this divide are the rationalists, who believe that some or all of our
knowledge about the world is gained independently of sense experience. “at is,
they maintain that some or all of our knowledge is a priori. On the other side are
the empiricists, who contend that our knowledge of the world comes solely from

a priori knowledge
Knowledge gained inde-
pendently of or prior to
sense experience.

a posteriori knowledge
Knowledge that de-
pends entirely on sense
experience.

rationalists “ose who
believe that some or all of
our knowledge about the
world is gained indepen-
dently of sense experience.

empiricists “ose
who believe that our
knowledge of the world
comes solely from sense
experience.

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“I think, therefore I am.”
—Descartes

“I feel, therefore I exist.”
—”omas Je!erson

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sense experience. We acquire knowledge entirely a posteriori. We may come to know
logical and mathematical truths through reason, but we can know nothing about the
world unless we use our senses.

At this point, you may find yourself being more sympathetic to the empiricists
than to the rationalists. After all, you seem to acquire an enormous amount of infor-
mation via your five senses. “rough them, you grasp that the grass is green, the stove
is hot, the music is loud, the lime is tart, and the sugar is sweet. But what can you
know through reason alone? Rationalists like Descartes would say that you can know
a great deal. Without making any empirical observations, mathematicians can not
only discover new mathematical truths but also develop mathematical models that
can accurately describe the empirical world. “ey can, for example, accurately pre-
dict the existence of astronomical objects and their movements without once look-
ing through a telescope. Likewise we know the fundamental truths of logic, without
which reasoning itself would be impossible. We know, for example, that nothing can
both have a property and lack it at the same time, and that for any particular property,
everything either has it or lacks it. “us we know without looking that nothing can
both be a dog and not be a dog at the same time, there are no square circles, and mar-
ried bachelors don’t exist. Some rationalists have gone further and asserted that reason
alone can reveal the most important, basic truths about the world—such as “every
event has a cause” and “the shortest distance between two points is a straight line.”

Many of the greatest thinkers in history have taken the rationalist approach to
knowledge, most notably Plato and Descartes. Descartes, espousing the most influen-
tial rationalist theory, thinks sense experience is an unreliable source of knowledge, so
he looks to reason to give all our knowledge a foundation as firm as that which sup-
ports unshakeable mathematical truths. “rough reason he hopes to defeat skepticism.
His method is first to doubt everything that he cannot be certain of, a process that
leaves him knowing hardly anything. But through reason alone he soon uncovers what
he considers to be self-evident; certain truths from which he derives other indubitable
propositions. In this way he tries to build an edifice of knowledge that, like an inverted
pyramid, rests on one or two rock-solid foundation stones that support all the others.

“e empiricist view of knowledge has been advanced most famously by the Brit-
ish empiricists John Locke (1632–1704), George Berkeley (1685–1753), and David
Hume (1711–1776) (see Chapter 10). “ey want to turn Descartes’ pyramid right
side up, resting all knowledge on a vast foundation of sense data (the content of our
experience) that supports the upper stones. Among these thinkers, Hume probably
has been most influential, arguing for an uncompromising empiricism that leads
to a far-reaching skepticism that not all empiricists have shared. He holds that all
our knowledge (aside from purely logical truths) is derived from sense perceptions
or ideas about those perceptions. Like other empiricists, he believes that the mind is
empty—a blank slate—until experience gives it content. We can have knowledge of
something only if it can be sensed, and any proposition that does not refer to what
can be sensed is meaningless. Guided by this latter empiricist principle, Hume is
driven to skepticism about many things that others have taken for granted, including
the existence of the external world, causation, a continuing self, religious doctrines,
and inductive reasoning. His skepticism arises because he thinks that even though

“All that we see or seem
is but a dream within a
dream.”

—Edgar Allan Poe

empiricism “e view
that our knowledge of the
world comes solely from
sense experience.

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all our knowledge is based on sense experience, we cannot know how the objects of
our sense experience are related. We cannot know, for example, that there is a cause-
and-e!ect relationship between associated objects. To infer such a connection is to
go beyond what our senses tell us. Our notions of causal connections are merely
matters of custom and habit.

9.2 PLATO’S RATIONALISM

“e great-grandfather of modern rationalism is Plato. As noted in Chapter 4, Plato
maintained that sense experience alone could not be the source of knowledge, al-
though many of his contemporaries claimed otherwise. Some of them assumed that
since knowledge must be based on sense experience, and since sense experience can
vary from person to person or culture to culture, relativism must be true. If one
person says a grape is sour, and another says it’s sweet, there must be no objective
fact of the matter, just truth relative to di!erent persons. Other thinkers thought
that since our perceptions are often illusory, distorted, or otherwise mistaken, sense
experience is not a reliable source of knowledge. And since our perceptions are the
only possible route to knowledge, we must not know anything. “us skepticism,
they said, is the proper epistemological attitude. Plato thought that our perceptions
were just as unreliable a guide to genuine knowledge as the relativists and skeptics
assumed. But he argued that since we clearly do have knowledge, we must derive it
from a reliable source—and that source has to be reason.

Plato deduced that we must be able to acquire knowledge because we can iden-
tify false beliefs, and we obviously possess knowledge because we can grasp, through
reason, mathematical, conceptual, and logical truths. We know that 2 ! 5 ” 7;
that a triangle has three sides; and that if A is larger than B, and B is larger than C,
then A is larger than C. Plato pointed out that these truths are objective: they are
true regardless of what we think. We do not invent them out of our imaginations;
we discover them. No matter how hard we try we cannot make 2 ! 5 ” 9. Plato
reasoned that if such truths are objective, they must also be about real things. “ey
must refer to an independently existing, immaterial reality that is beyond sense ex-
perience. In addition, he insisted, these truths must also be immutable and eternal,
existing in the immaterial realm unchanged for all time. Only through our powers
of reason can we reach beyond the physical world to take hold of real knowledge of
fundamental truths. Sense experience, in contrast, can yield only transitory, ever-
changing information—mere opinion that is vastly inferior to everlasting truths.

So for Plato, reality comprises two worlds: the fleeting world of the physical ac-
cessed through sense experience; and the eternal, nonphysical, changeless world of
genuine knowledge accessed only through reason. In spelling out the contents of the
latter, Plato articulates his notion of the Forms. As discussed earlier, the Forms (also
called Ideas) are perfect conceptual models for every existing thing, residing only in
the eternal world penetrated by reason alone. “ey are the ideals, or standards, that
we can first come to know and then use to assess the notions and objects we encoun-
ter in our lives. “rough reason, we can access the Form of “table” and thus know

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rationalism “e view
that some or all of our
knowledge about the
world is gained indepen-
dently of sense experience.

“”ere is thus, in all
philosophy derived from
Descartes, a tendency
to subjectivism, and
to regarding matter as
something only knowable,
if at all, by inference from
what is known of mind.”

—Bertrand Russell

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the ideal template of “table.” With this knowledge we can understand the essence
of a table and use this understanding to make judgments about all physical tables.
Likewise when we access the Form “courage,” we know what the ideal of courage

DETAILS

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Innate Ideas
Rationalists like Plato and Des-
cartes typically accept the doc-
trine of innate ideas—the notion
that we are born with some gen-
eral ideas already in our minds.
Ideas such as God, freedom, the
laws of logic, the truths of math-
ematics, moral principles, and
causality (specifically, that every
event has cause) are supposed to
be hidden in our minds until we
are cognitively mature enough
to see that they are self-evident.
Rationalists believe that the
theory of innate ideas is the
best explanation of why we can
know some concepts a priori—
that is, without ever experienc-
ing them. We understand the
idea of perfection, for example,
even though we have never en-
countered a perfect object.

Most philosophers reject the doctrine, but in contemporary life, advocates of the view are
not hard to find. “e linguist Noam Chomsky, for instance, argues that children come into the
world possessing a universal grammar that enables them to learn a first language—an achieve-
ment that Chomsky thinks would not be possible without some innate grammatical ideas.

Critics of innate ideas ask how we can possess knowledge that we are not aware of.
Innate ideas are not learned; the mind is supposed to somehow always have them. But if we
are unaware of them, how can they be or become knowledge?

Empiricists admit that our minds do possess innate cognitive capacities to learn certain
concepts, but they maintain that having these capacities is a long way from having innate
knowledge.

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is and can use this knowledge to appraise a particular instance of courage. As Plato
sees it, the truly real world is the world of the Forms—the domain of the perfect
and everlasting. With knowledge of the really real, we can understand the “less real”
realm of the imperfect and transitory.

How is it, Plato asks, that we seem to have knowledge of the Forms, however
dimly, even though our senses can tell us nothing about them? Our sense experience
can acquaint us only with material objects, but the Forms are not material. It’s as if
these universals were already in our minds waiting to be uncovered. Plato’s answer—
and the answer of most other rationalists—is the doctrine of innate knowledge: at
birth our minds come equipped with conceptual content, ideas about the world that
we can know a priori. In Plato’s version of this view, knowledge of the Forms is pres-
ent at the very beginning of our lives, inscribed in our minds (our immortal souls)
in a previous existence. We are born with this knowledge. Accessing this knowledge
then is a matter of using reason to recall what we previously knew in another life.

Many thinkers reject Plato’s notion of a preexisting state in which we acquire
knowledge, and they are skeptical of the claim that we are born with knowledge. But
the idea of innate knowledge in some form is still attractive to rationalist philoso-
phers, for it would explain how we could possess knowledge without relying on sense
experience. In any case, both rationalists and empiricists must provide an explana-
tion of how we seem to know logical and mathematical truths (and perhaps scientific
truths) that we do not arrive at through our senses.

9.3 DESCARTES’ DOUBT

Oddly enough, Descartes begins his quest for knowledge by first plunging into skep-
ticism. He sees that a great many things he thought he knew appear now to be false.
So he decides to “raze everything to the ground and begin again” from a firm foun-
dation, doubting all beliefs except those that are “certain and indubitable,” beliefs
that cannot possibly be false. Only beliefs that are certain can count as knowledge,
he says. If he has reason to doubt any of them, they are not knowledge. Here he is at
the start of his quest:

“[T]o have recourse to
the veracity of the su-
preme Being, in to
prove the veracity of our
senses, is surely making a
very unexpected circuit.”

—David Hume

“Common sense is the
most fairly distributed
thing in the world, for
each one thinks he is so
well-endowed with it that
even those who are hard-
est to satisfy in all other
matters are not in the
habit of desiring more of
it than they already have.”

—Descartes

“Hamlet did think a
great many things; does it
follow that he existed?”

—Jaako Hintikka

3FO�%FTDBSUFT��Meditations on First Philosophy

…