Epistemology

Epistemology is the study of knowledge. Epistemologists concern
themselves with a number of tasks, which we might sort into two
categories.

First, we must determine the nature of knowledge; that is, what does
it mean to say that someone knows, or fails to know, something? This
is a matter of understanding what knowledge is, and how to distinguish
between cases in which someone knows something and cases in which
someone does not know something. While there is some general agreement
about some aspects of this issue, we shall see that this question is
much more difficult than one might imagine.

Second, we must determine the extent of human knowledge; that is, how
much do we, or can we, know? How can we use our reason, our senses,
the testimony of others, and other resources to acquire knowledge? Are
there limits to what we can know? For instance, are some things
unknowable? Is it possible that we do not know nearly as much as we
think we do? Should we have a legitimate worry about skepticism, the
view that we do not or cannot know anything at all?

1. Kinds of Knowledge

The term "epistemology" comes from the Greek "episteme," meaning
"knowledge," and "logos," meaning, roughly, "study, or science, of."
"Logos" is the root of all terms ending in "-ology" – such as
psychology, anthropology – and of "logic," and has many other related
meanings.

The word "knowledge" and its cognates are used in a variety of ways.
One common use of the word "know" is as an expression of psychological
conviction. For instance, we might hear someone say, "I just knew it
wouldn't rain, but then it did." While this may an appropriate usage,
philosophers tend to use the word "know" in a factive sense, so that
one cannot know something that is not the case. (This point is
discussed at greater length in section 2b below.)

Even if we restrict ourselves to factive usages, there are still
multiple senses of "knowledge," and so we need to distinguish between
them. One kind of knowledge is procedural knowledge, sometimes called
competence or "know-how;" for example, one can know how to ride a
bicycle, or one can know how to drive from Washington, D.C. to New
York. Another kind of knowledge is acquaintance knowledge or
familiarity; for instance, one can know the department chairperson, or
one can know Philadelphia.

Epistemologists typically do not focus on procedural or acquaintance
knowledge, however, instead preferring to focus on propositional
knowledge. A proposition is something which can be expressed by a
declarative sentence, and which purports to describe a fact or a state
of affairs, such as "Dogs are mammals," "2+2=7," "It is wrong to
murder innocent people for fun." (Note that a proposition may be true
or false; that is, it need not actually express a fact.) Propositional
knowledge, then, can be called knowledge-that; statements of
propositional knowledge (or the lack thereof) are properly expressed
using "that"-clauses, such as "He knows that Houston is in Texas," or
"She does not know that the square root of 81 is 9." In what follows,
we will be concerned only with propositional knowledge.

Propositional knowledge, obviously, encompasses knowledge about a wide
range of matters: scientific knowledge, geographical knowledge,
mathematical knowledge, self-knowledge, and knowledge about any field
of study whatever. Any truth might, in principle, be knowable,
although there might be unknowable truths. One goal of epistemology is
to determine the criteria for knowledge so that we can know what can
or cannot be known, in other words, the study of epistemology
fundamentally includes the study of meta-epistemology (what we can
know about knowledge itself).

We can also distinguish between different types of propositional
knowledge, based on the source of that knowledge. Non-empirical or a
priori knowledge is possible independently of, or prior to, any
experience, and requires only the use of reason; examples include
knowledge of logical truths such as the law of non-contradiction, as
well as knowledge of abstract claims (such as ethical claims or claims
about various conceptual matters). Empirical or a posteriori knowledge
is possible only subsequent, or posterior, to certain sense
experiences (in addition to the use of reason); examples include
knowledge of the color or shape of a physical object or knowledge of
geographical locations. (Some philosophers, called rationalists,
believe that all knowledge is ultimately grounded upon reason; others,
called empiricists, believe that all knowledge is ultimately grounded
upon experience.) A thorough epistemology should, of course, address
all kinds of knowledge, although there might be different standards
for a priori and a posteriori knowledge.

We can also distinguish between individual knowledge and collective
knowledge. Social epistemology is the subfield of epistemology that
addresses the way that groups, institutions, or other collective
bodies might come to acquire knowledge.
2. The Nature of Propositional Knowledge

Having narrowed our focus to propositional knowledge, we must ask
ourselves what, exactly, constitutes knowledge. What does it mean for
someone to know something? What is the difference between someone who
knows something and someone else who does not know it, or between
something one knows and something one does not know? Since the scope
of knowledge is so broad, we need a general characterization of
knowledge, one which is applicable to any kind of proposition
whatsoever. Epistemologists have usually undertaken this task by
seeking a correct and complete analysis of the concept of knowledge,
in other words a set of individually necessary and jointly sufficient
conditions which determine whether someone knows something.
a. Belief

Let us begin with the observation that knowledge is a mental state;
that is, knowledge exists in one's mind, and unthinking things cannot
know anything. Further, knowledge is a specific kind of mental state.
While "that"-clauses can also be used to describe desires and
intentions, these cannot constitute knowledge. Rather, knowledge is a
kind of belief. If one has no beliefs about a particular matter, one
cannot have knowledge about it.

For instance, suppose that I desire that I be given a raise in salary,
and that I intend to do whatever I can to earn one. Suppose further
that I am doubtful as to whether I will indeed be given a raise, due
to the intricacies of the university's budget and such. Given that I
do not believe that I will be given a raise, I cannot be said to know
that I will. Only if I am inclined to believe something can I come to
know it. Similarly, thoughts that an individual has never entertained
are not among his beliefs, and thus cannot be included in his body of
knowledge.

Some beliefs, those which the individual is actively entertaining, are
called occurrent beliefs. The majority of an individual's beliefs are
non-occurrent; these are beliefs that the individual has in the
background but is not entertaining at a particular time.
Correspondingly, most of our knowledge is non-occurrent, or
background, knowledge; only a small amount of one's knowledge is ever
actively on one's mind.
b. Truth

Knowledge, then, requires belief. Of course, not all beliefs
constitute knowledge. Belief is necessary but not sufficient for
knowledge. We are all sometimes mistaken in what we believe; in other
words, while some of our beliefs are true, others are false. As we try
to acquire knowledge, then, we are trying to increase our stock of
true beliefs (while simultaneously minimizing our false beliefs).

We might say that the most typical purpose of beliefs is to describe
or capture the way things actually are; that is, when one forms a
belief, one is seeking a match between one's mind and the world. (We
sometimes, of course, form beliefs for other reasons – to create a
positive attitude, to deceive ourselves, and so forth – but when we
seek knowledge, we are trying to get things right.) And, alas, we
sometimes fail to achieve such a match; some of our beliefs do not
describe the way things actually are.

Note that we are assuming here that there is such a thing as objective
truth, so that it is possible for beliefs to match or to fail to match
with reality. That is, in order for someone to know something, there
must be something one knows about. Recall that we are discussing
knowledge in the factive sense; if there are no facts of the matter,
then there's nothing to know (or to fail to know). This assumption is
not universally accepted – in particular, it is not shared by some
proponents of relativism – but it will not be defended here. However,
we can say that truth is a condition of knowledge; that is, if a
belief is not true, it cannot constitute knowledge. Accordingly, if
there is no such thing as truth, then there can be no knowledge. Even
if there is such a thing as truth, if there is a domain in which there
are no truths, then there can be no knowledge within that domain. (For
example, if beauty is in the eye of the beholder, then a belief that
something is beautiful cannot be true or false, and thus cannot
constitute knowledge.)
c. Justification

Knowledge, then, requires factual belief. However, this does not
suffice to capture the nature of knowledge. Just as knowledge requires
successfully achieving the objective of true belief, it also requires
success with regard to the formation of that belief. In other words,
not all true beliefs constitute knowledge; only true beliefs arrived
at in the right way constitute knowledge.

What, then, is the right way of arriving at beliefs? In addition to
truth, what other properties must a belief have in order to constitute
knowledge? We might begin by noting that sound reasoning and solid
evidence seem to be the way to acquire knowledge. By contrast, a lucky
guess cannot constitute knowledge. Similarly, misinformation and
faulty reasoning do not seem like a recipe for knowledge, even if they
happen to lead to a true belief. A belief is said to be justified if
it is obtained in the right way. While justification seems, at first
glance, to be a matter of a belief's being based on evidence and
reasoning rather than on luck or misinformation, we shall see that
there is much disagreement regarding how to spell out the details.

The requirement that knowledge involve justification does not
necessarily mean that knowledge requires absolute certainty, however.
Humans are fallible beings, and fallibilism is the view that it is
possible to have knowledge even when one's true belief might have
turned out to be false. Between beliefs which were necessarily true
and those which are true solely by luck lies a spectrum of beliefs
with regard to which we had some defeasible reason to believe that
they would be true. For instance, if I heard the weatherman say that
there is a 90% chance of rain, and as a result I formed the belief
that it would rain, then my true belief that it would rain was not
true purely by luck. Even though there was some chance that my belief
might have been false, there was a sufficient basis for that belief
for it to constitute knowledge. This basis is referred to as the
justification for that belief. We can then say that, to constitute
knowledge, a belief must be both true and justified.

Note that because of luck, a belief can be unjustified yet true; and
because of human fallibility, a belief can be justified yet false. In
other words, truth and justification are two independent conditions of
beliefs. The fact that a belief is true does not tell us whether or
not it is justified; that depends on how the belief was arrived at.
So, two people might hold the same true belief, but for different
reasons, so that one of them is justified and the other is
unjustified. Similarly, the fact that a belief is justified does not
tell us whether it's true or false. Of course, a justified belief will
presumably be more likely to be true than to be false, and justified
beliefs will presumably be more likely or more probable to be true
than unjustified beliefs. (As we will see in section 3 below, the
exact nature of the relationship between truth and justification is
contentious.)
d. The Gettier Problem

For some time, the justified true belief (JTB) account was widely
agreed to capture the nature of knowledge. However, in 1963, Edmund
Gettier published a short but widely influential article which has
shaped much subsequent work in epistemology. Gettier provided two
examples in which someone had a true and justified belief, but in
which we seem to want to deny that the individual has knowledge,
because luck still seems to play a role in his belief having turned
out to be true.

Consider an example. Suppose that the clock on campus (which keeps
accurate time and is well maintained) stopped working at 11:56pm last
night, and has yet to be repaired. On my way to my noon class, exactly
twelve hours later, I glance at the clock and form the belief that the
time is 11:56. My belief is true, of course, since the time is indeed
11:56. And my belief is justified, as I have no reason to doubt that
the clock is working, and I cannot be blamed for basing beliefs about
the time on what the clock says. Nonetheless, it seems evident that I
do not know that the time is 11:56. After all, if I had walked past
the clock a bit earlier or a bit later, I would have ended up with a
false belief rather than a true one.

This example and others like it, while perhaps somewhat far-fetched,
seem to show that it is possible for justified true belief to fail to
constitute knowledge. To put it another way, the justification
condition was meant to ensure that knowledge was based on solid
evidence rather than on luck or misinformation, but Gettier-type
examples seem to show that justified true belief can still involve
luck and thus fall short of knowledge. This problem is referred to as
"the Gettier problem." To solve this problem, we must either show that
all instances of justified true belief do indeed constitute knowledge,
or alternatively refine our analysis of knowledge.
i. The No-False-Belief Condition

We might think that there is a simple and straightforward solution to
the Gettier problem. Note that my reasoning was tacitly based on my
belief that the clock is working properly, and that this belief is
false. This seems to explain what has gone wrong in this example.
Accordingly, we might revise our analysis of knowledge by insisting
that to constitute knowledge, a belief must be true and justified and
must be formed without relying on any false beliefs. In other words,
we might say, justification, truth, and belief are all necessary for
knowledge, but they are not jointly sufficient for knowledge; there is
a fourth condition – namely, that no false beliefs be essentially
involved in the reasoning that led to the belief – which is also
necessary.

Unfortunately, this will not suffice; we can modify the example so
that my belief is justified and true, and is not based on any false
beliefs, but still falls short of knowledge. Suppose, for instance,
that I do not have any beliefs about the clock's current state, but
merely the more general belief that the clock usually is in working
order. This belief, which is true, would suffice to justify my belief
that the time is now 11:56; of course, it still seems evident that I
do not know the time.
ii. The No-Defeaters Condition

However, the no-false-belief condition does not seem to be completely
misguided; perhaps we can add some other condition to justification
and truth to yield a correct characterization of knowledge. Note that,
even if I didn't actively form the belief that the clock is currently
working properly, it seems to be implicit in my reasoning, and the
fact that it is false is surely relevant to the problem. After all, if
I were asked, at the time that I looked at the clock, whether it is
working properly, I would have said that it is. Conversely, if I
believed that the clock wasn't working properly, I wouldn't be
justified in forming a belief about the time based on what the clock
says.

In other words, the proposition that the clock is working properly
right now meets the following conditions: it is a false proposition, I
do not realize that it is a false proposition, and if I had realized
that it is a false proposition, my justification for my belief that it
is 11:56 would have been undercut or defeated. If we call propositions
such as this "defeaters," then we can say that to constitute
knowledge, a belief must be true and justified, and there must not be
any defeaters to the justification of that belief. Many
epistemologists believe this analysis to be correct.
iii. Causal Accounts of Knowledge

Rather than modifying the JTB account of knowledge by adding a fourth
condition, some epistemologists see the Gettier problem as reason to
seek a substantially different alternative. We have noted that
knowledge should not involve luck, and that Gettier-type examples are
those in which luck plays some role in the formation of a justified
true belief. In typical instances of knowledge, the factors
responsible for the justification of a belief are also responsible for
its truth. For example, when the clock is working properly, my belief
is both true and justified because it's based on the clock, which
accurately displays the time. But one feature that all Gettier-type
examples have in common is the lack of a clear connection between the
truth and the justification of the belief in question. For example, my
belief that the time is 11:56 is justified because it's based on the
clock, but it's true because I happened to walk by at just the right
moment. So, we might insist that to constitute knowledge, a belief
must be both true and justified, and its truth and justification must
be connected somehow.

This notion of a connection between the truth and the justification of
a belief turns out to be difficult to formulate precisely, but causal
accounts of knowledge seek to capture the spirit of this proposal by
more significantly altering the analysis of knowledge. Such accounts
maintain that in order for someone to know a proposition, there must
be a causal connection between his belief in that proposition and the
fact that the proposition encapsulates. This retains the truth
condition, since a proposition must be true in order for it to
encapsulate a fact. However, it appears to be incompatible with
fallibilism, since it does not allow for the possibility that a belief
be justified yet false. (Strictly speaking, causal accounts of
knowledge make no reference to justification, although we might
attempt to reformulate fallibilism in somewhat modified terms in order
to state this observation.)

While causal accounts of knowledge are no longer thought to be
correct, they have engendered reliabilist theories of knowledge, which
shall be discussed in section 3b below.
3. The Nature of Justification

One reason that the Gettier problem is so problematic is that neither
Gettier nor anyone who preceded him has offered a sufficiently clear
and accurate analysis of justification. We have said that
justification is a matter of a belief's having been formed in the
right way, but we have yet to say what that amounts to. We must now
consider this matter more closely.

We have noted that the goal of our belief-forming practices is to
obtain truth while avoiding error, and that justification is the
feature of beliefs which are formed in such a way as to best pursue
this goal. If we think, then, of the goal of our belief-forming
practices as an attempt to establish a match between one's mind and
the world, and if we also think of the application or withholding of
the justification condition as an evaluation of whether this match was
arrived at in the right way, then there seem to be two obvious
approaches to construing justification: namely, in terms of the
believer's mind, or in terms of the world.
a. Internalism

Belief is a mental state, and belief-formation is a mental process.
Accordingly, one might reason, whether or not a belief is justified –
whether, that is, it is formed in the right way – can be determined by
examining the thought-processes of the believer during its formation.
Such a view, which maintains that justification depends solely on
factors internal to the believer's mind, is called internalism. (The
term "internalism" has different meanings in other contexts; here, it
will be used strictly to refer to this type of view about epistemic
justification.)

According to internalism, the only factors that are relevant to the
determination of whether a belief is justified are the believer's
other mental states. After all, an internalist will argue, only an
individual's mental states – her beliefs about the world, her sensory
inputs (for example, her sense data) and her beliefs about the
relations between her various beliefs – can determine what new beliefs
she will form, so only an individual's mental states can determine
whether any particular belief is justified. In particular, in order to
be justified, a belief must be appropriately based upon or supported
by other mental states.

This raises the question of what constitutes the basing or support
relation between a belief and one's other mental states. We might want
to say that, in order for belief A to be appropriately based on belief
B (or beliefs B1 and B2, or B1, B2, and…Bn), the truth of B must
suffice to establish the truth of A, in other words, B must entail A.
(We shall consider the relationship between beliefs and sensory inputs
below.) However, if we want to allow for our fallibility, we must
instead say that the truth of B would give one good reason to believe
that A is also true (by making it likely or probable that A is true).
An elaboration of what counts as a good reason for belief,
accordingly, is an essential part of any internalist account of
justification.

However, there is an additional condition that we must add: belief B
must itself be justified, since unjustified beliefs cannot confer
justification on other beliefs. Because belief B be must also be
justified, must there be some justified belief C upon which B is
based? If so, C must itself be justified, and it may derive its
justification from some further justified belief, D. This chain of
beliefs deriving their justification from other beliefs may continue
forever, leading us in an infinite regress. While the idea of an
infinite regress might seem troubling, the primary ways of avoiding
such a regress may have their own problems as well. This raises the
"regress problem," which begins from observing that there are only
four possibilities as to the structure of one's justified beliefs:

1. The series of justified beliefs, each based upon the other,
continues infinitely.
2. The series of justified beliefs circles back to its beginning (A
is based on B, B on C, C on D, and D on A).
3. The series of justified beliefs begins with an unjustified belief.
4. The series of justified beliefs begins with a belief which is
justified, but not by virtue of being based on another justified
belief.

These alternatives seem to exhaust the possibilities. That is, if one
has any justified beliefs, one of these four possibilities must
describe the relationships between those beliefs. As such, a complete
internalist account of justification must decide among the four.
i. Foundationalism

Let us, then, consider each of the four possibilities mentioned above.
Alternative 1 seems unacceptable because the human mind can contain
only finitely many beliefs, and any thought-process that leads to the
formation of a new belief must have some starting point. Alternative 2
seems no better, since circular reasoning appears to be fallacious.
And alternative 3 has already been ruled out, since it renders the
second belief in the series (and, thus, all subsequent beliefs)
unjustified. That leaves alternative 4, which must, by process of
elimination, be correct.

This line of reasoning, which is typically known as the regress
argument, leads to the conclusion that there are two different kinds
of justified beliefs: those which begin a series of justified beliefs,
and those which are based on other justified beliefs. The former,
called basic beliefs, are able to confer justification on other,
non-basic beliefs, without themselves having their justification
conferred upon them by other beliefs. As such, there is an
asymmetrical relationship between basic and non-basic beliefs. Such a
view of the structure of justified belief is known as
"foundationalism. " In general, foundationalism entails that there is
an asymmetrical relationship between any two beliefs: if A is based on
B, then B cannot be based on A.

Accordingly, it follows that at least some beliefs (namely basic
beliefs) are justified in some way other than by way of a relation to
other beliefs. Basic beliefs must be self-justified, or must derive
their justification from some non-doxastic source such as sensory
inputs; the exact source of the justification of basic beliefs needs
to be explained by any complete foundationalist account of
justification.
ii. Coherentism

Internalists might be dissatisfied with foundationalism, since it
allows for the possibility of beliefs that are justified without being
based upon other beliefs. Since it was our solution to the regress
problem that led us to foundationalism, and since none of the
alternatives seem palatable, we might look for a flaw in the problem
itself. Note that the problem is based on a pivotal but hitherto
unstated assumption: namely, that justification is linear in fashion.
That is, the statement of the regress problem assumes that the basing
relation parallels a logical argument, with one belief being based on
one or more other beliefs in an asymmetrical fashion.

So, an internalist who finds foundationalism to be problematic might
deny this assumption, maintaining instead that justification is the
result of a holistic relationship among beliefs. That is, one might
maintain that beliefs derive their justification by inclusion in a set
of beliefs which cohere with one another as a whole; a proponent of
such a view is called a coherentist.

A coherentist, then, sees justification as a relation of mutual
support among many beliefs, rather than a series of asymmetrical
beliefs. A belief derives its justification, according to coherentism,
not by being based on one or more other beliefs, but by virtue of its
membership in a set of beliefs that all fit together in the right way.
(The coherentist needs to specify what constitutes coherence, of
course. It must be something more than logical consistency, since two
unrelated beliefs may be consistent. Rather, there must be some
positive support relationship – for instance, some sort of explanatory
relationship – between the members of a coherent set in order for the
beliefs to be individually justified.)

Coherentism is vulnerable to the "isolation objection". It seems
possible for a set of beliefs to be coherent, but for all of those
beliefs to be isolated from reality. Consider, for instance, a work of
fiction. All of the statements in the work of fiction might form a
coherent set, but presumably believing all and only the statements in
a work of fiction will not render one justified. Indeed, any form of
internalism seems vulnerable to this objection, and thus a complete
internalist account of justification must address it. Recall that
justification requires a match between one's mind and the world, and
an inordinate emphasis on the relations between the beliefs in one's
mind seems to ignore the question of whether those beliefs match up
with the way things actually are.
b. Externalism

Accordingly, one might think that focusing solely on factors internal
to the believer's mind will inevitably lead to a mistaken account of
justification. The alternative, then, is that at least some factors
external to the believer's mind determine whether or not she is
justified. A proponent of such a view is called an externalist.

According to externalism, the only way to avoid the isolation
objection and ensure that knowledge does not include luck is to
consider some factors other than the individual's other beliefs. Which
factors, then, should be considered? The most prominent version of
externalism, called reliabilism, suggests that we consider the source
of a belief. Beliefs can be formed as a result of many different
sources, such as sense experience, reason, testimony, memory. More
precisely, we might specify which sense was used, who provided the
testimony, what sort of reasoning is used, or how recent the relevant
memory is. For every belief, we can indicate the cognitive process
that led to its formation. In its simplest and most straightforward
form, reliabilism maintains that whether or not a belief is justified
depends upon whether that process is a reliable source of true
beliefs. Since we are seeking a match between our mind and the world,
justified beliefs are those which result from processes which
regularly achieve such a match. So, for example, using vision to
determine the color of an object which is well-lit and relatively near
is a reliable belief-forming process for a person with normal vision,
but not for a color-blind person. Forming beliefs on the basis of the
testimony of an expert is likely to yield true beliefs, but forming
beliefs on the basis of the testimony of compulsive liars is not. In
general, if a belief is the result of a cognitive process which
reliably (most of the time – we still want to leave room for human
fallibility) leads to true beliefs, then that belief is justified.

The foregoing suggests one immediate challenge for reliabilism. The
formation of a belief is a one-time event, but the reliability of the
process depends upon the long-term performance of that process. (This
can include counterfactual as well as actual events. For instance, a
coin which is flipped only once and lands on heads nonetheless has a
50% chance of landing on tails, even though its actual performance has
yielded heads 100% of the time.) And this requires that we specify
which process is being used, so that we can evaluate its performance
in other instances. However, cognitive process can be described in
more or less general terms: for example, the same belief-forming
process might be variously described as sense experience, vision,
vision by a normally-sighted person, vision by a normally-sighted
person in daylight, vision by a normally-sighted person in daylight
while looking at a tree, vision by a normally-sighted person in
daylight while looking at an elm tree, and so forth. The "generality
problem" notes that some of these descriptions might specify a
reliable process but others might specify an unreliable process, so
that we cannot know whether a belief is justified or unjustified
unless we know the appropriate level of generality to use in
describing the process.

Even if the generality problem can be solved, another problem remains
for externalism. Keith Lehrer presents this problem by way of his
example of Mr. Truetemp. Truetemp has, unbeknownst to him, had a
tempucomp – a device which accurately reads the temperature and causes
a spontaneous belief about that temperature – implanted in his brain.
As a result, he has many true beliefs about the temperature, but he
does not know why he has them or what their source is. Lehrer argues
that, although Truetemp's belief-forming process is reliable, his
ignorance of the tempucomp renders his temperature-beliefs
unjustified, and thus that a reliable cognitive process cannot yield
justification unless the believer is aware of the fact that the
process is reliable. In other words, the mere fact that the process is
reliable does not suffice, Lehrer concludes, to justify any beliefs
which are formed via that process.
4. The Extent of Human Knowledge
a. Sources of Knowledge

Given the above characterization of knowledge, there are many ways
that one might come to know something. Knowledge of empirical facts
about the physical world will necessarily involve perception, in other
words, the use of the senses. Science, with its collection of data and
conducting of experiments, is the paradigm of empirical knowledge.
However, much of our more mundane knowledge comes from the senses, as
we look, listen, smell, touch, and taste the various objects in our
environments.

But all knowledge requires some amount of reasoning. Data collected by
scientists must be analyzed before knowledge is yielded, and we draw
inferences based on what our senses tell us. And knowledge of abstract
or non-empirical facts will exclusively rely upon reasoning. In
particular, intuition is often believed to be a sort of direct access
to knowledge of the a priori.

Once knowledge is obtained, it can be sustained and passed on to
others. Memory allows us to know something that we knew in the past,
even, perhaps, if we no longer remember the original justification.
Knowledge can also be transmitted from one individual to another via
testimony; that is, my justification for a particular belief could
amount to the fact that some trusted source has told me that it is
true.
b. Skepticism

In addition to the nature of knowledge, epistemologists concern
themselves with the question of the extent of human knowledge: how
much do we, or can we, know? Whatever turns out to be the correct
account of the nature of knowledge, there remains the matter of
whether we actually have any knowledge. It has been suggested that we
do not, or cannot, know anything, or at least that we do not know as
much as we think we do. Such a view is called skepticism.

We can distinguish between a number of different varieties of
skepticism. First, one might be a skeptic only with regard to certain
domains, such as mathematics, morality, or the external world (this is
the most well-known variety of skepticism). Such a skeptic is a local
skeptic, as contrasted with a global skeptic, who maintains that we
cannot know anything at all. Also, since knowledge requires that our
beliefs be both true and justified, a skeptic might maintain that none
of our beliefs are true or that none of them are justified (the latter
is much more common than the former).

While it is quite easy to challenge any claim to knowledge by glibly
asking, "How do you know?", this does not suffice to show that
skepticism is an important position. Like any philosophical stance,
skepticism must be supported by an argument. Many arguments have been
offered in defense of skepticism, and many responses to those
arguments have been offered in return. Here, we shall consider two of
the most prominent arguments in support of skepticism about the
external world.
c. Cartesian Skepticism

In the first of his Meditations, René Descartes offers an argument in
support of skepticism, which he then attempts to refute in the later
Meditations. The argument notes that some of our perceptions are
inaccurate. Our senses can trick us; we sometimes mistake a dream for
a waking experience, and it is possible that an evil demon is
systematically deceiving us. (The modern version of the evil demon
scenario is that you are a brain-in-a-vat; because scientists have
removed your brain from your skull, connected it to a sophisticated
computer, and immersed it in a vat of preservative fluid.) The
computer produces what seem to be genuine sense experiences, and also
responds to your brain's output to make it seem that you are able to
move about in your environment as you did when your brain was still in
your body. While this scenario may seem far-fetched, we must admit
that it is at least possible.)

As a result, some of our beliefs will be false. In order to be
justified in believing what we do, we must have some way to
distinguish between those beliefs which are true (or, at least, are
likely to be true) and those which are not. But just as there are no
signs that will allow us to distinguish between waking and dreaming,
there are no signs that will allow us to distinguish between beliefs
that are accurate and beliefs which are the result of the machinations
of an evil demon. This indistinguishability between trustworthy and
untrustworthy belief, the argument goes, renders all of our beliefs
unjustified, and thus we cannot know anything. A satisfactory response
to this argument, then, must show either that we are indeed able to
distinguish between true and false beliefs, or that we need not be
able to make such a distinction.
d. Humean Skepticism

According to the indistinguishability skeptic, my senses can tell me
how things appear, but not how they actually are. We need to use
reason to construct an argument that leads us from beliefs about how
things appear to (justified) beliefs about how they are. But even if
we are able to trust our perceptions, so that we know that they are
accurate, David Hume argues that the specter of skepticism remains.
Note that we only perceive a very small part of the universe at any
given moment, although we think that we have knowledge of the world
beyond that which we are currently perceiving. It follows, then, that
the senses alone cannot account for this knowledge, and that reason
must supplement the senses in some way in order to account for any
such knowledge. However, Hume argues, reason is incapable of providing
justification for any belief about the external world beyond the scope
of our current sense perceptions. Let us consider two such possible
arguments and Hume's critique of them.
i. Numerical vs. Qualitative Identity

We typically believe that the external world is, for the most part,
stable. For instance, I believe that my car is parked where I left it
this morning, even though I am not currently looking at it. If I were
to go peek out the window right now and see my car, I might form the
belief that my car has been in the same space all day. What is the
basis for this belief? If asked to make my reasoning explicit, I might
proceed as follows:

I have had two sense-experiences of my car: one this morning and
one just now.
The two sense-experiences were (more or less) identical.
Therefore, it is likely that the objects that caused them are identical.
Therefore, a single object – my car – has been in that parking
space all day.

Similar reasoning would undergird all of our beliefs about the
persistence of the external world and all of the objects we perceive.
But are these beliefs justified? Hume thinks not, since the above
argument (and all arguments like it) contains an equivocation. In
particular, the first occurrence of "identical" refers to qualitative
identity. The two sense-experiences are not one and the same, but are
distinct; when we say that they are identical we mean that one is
similar to the other in all of its qualities or properties. But the
second occurrence of "identical" refers to numerical identity. When we
say that the objects that caused the two sense-experiences are
identical, we mean that there is one object, rather than two, that is
responsible for both of them. This equivocation, Hume argues, renders
the argument fallacious; accordingly, we need another argument to
support our belief that objects persist even when we are not observing
them.
ii. Hume's Skepticism about Induction

Suppose that a satisfactory argument could be found in support of our
beliefs in the persistence of physical objects. This would provide us
with knowledge that the objects that we have observed have persisted
even when we were not observing them. But in addition to believing
that these objects have persisted up until now, we believe that they
will persist in the future; we also believe that objects we have never
observed similarly have persisted and will persist. In other words, we
expect the future to be roughly like the past, and the parts of the
universe that we have not observed to be roughly like the parts that
we have observed. For example, I believe that my car will persist into
the future. What is the basis for this belief? If asked to make my
reasoning explicit, I might proceed as follows:

My car has always persisted in the past.
Nature is roughly uniform across time and space (and thus the
future will be roughly like the past).
Therefore, my car will persist in the future.

Similar reasoning would undergird all of our beliefs about the future
and about the unobserved. Are such beliefs justified? Again, Hume
thinks not, since the above argument, and all arguments like it,
contain an unsupported premise, namely the second premise, which might
be called the Principle of the Uniformity of Nature (PUN). Why should
we believe this principle to be true? Hume insists that we provide
some reason in support of this belief. Because the above argument is
an inductive rather than a deductive argument, the problem of showing
that it is a good argument is typically referred to as the "problem of
induction." We might think that there is a simple and straightforward
solution to the problem of induction, and that we can indeed provide
support for our belief that PUN is true. Such an argument would
proceed as follows:

PUN has always been true in the past.
Nature is roughly uniform across time and space (and thus the
future will be roughly like the past).
Therefore, PUN will be true in the future.

This argument, however, is circular; its second premise is PUN itself!
Accordingly, we need another argument to support our belief that PUN
is true, and thus to justify our inductive arguments about the future
and the unobserved.
5. Conclusion

The study of knowledge is one of the most fundamental aspects of
philosophical inquiry. Any claim to knowledge must be evaluated to
determine whether or not it indeed constitutes knowledge. Such an
evaluation essentially requires an understanding of what knowledge is
and how much knowledge is possible. While this entry provides on
overview of the important issues, it of course leaves the most basic
questions unanswered; epistemology will continue to be an area of
philosophical discussion as long as these questions remain.
6. References and Further Reading

* Alston, William P., 1989. Epistemic Justification: Essays in the
Theory of Knowledge. Ithaca, NY: Cornell University Press.
* Armstrong, David, 1973. Belief, Truth, and Knowledge. Cambridge:
Cambridge University Press.
o A defense of reliabilism.
* BonJour, Laurence, 1985. The Structure of Empirical Knowledge.
Cambridge, MA: Harvard University Press.
o A defense of coherentism.
* Chisholm, Roderick, 1966. Theory of Knowledge, Englewood Cliffs,
NJ: Prentice-Hall.
* Chisholm, Roderick, 1977. Theory of Knowledge, 2nd edition.
Englewood Cliffs, NJ: Prentice-Hall.
* Chisholm, Roderick, 1989. Theory of Knowledge, 3rd edition.
Englewood Cliffs, NJ: Prentice-Hall.
o Chisholm was one of the first authors to provide a
systematic analysis of knowledge. His account of justification is
foundationalist.
* Descartes, Rene, 1641. Meditations on First Philosophy.
Reprinted in The Philosophical Writings of Descartes (3 volumes).
Cottingham, Stoothoff and Murdoch, trans. Cambridge: Cambridge
University Press.
o Descartes presents an infallibilist version of
foundationalism, and attempts to refute skepticism.
* Dancy, Jonathan and Ernest Sosa (eds.), 1993. A Companion to
Epistemology. Oxford: Blackwell.
* DeRose, Keith, 1995. "Solving the Skeptical Problem"
Philosophical Review, 104, pp. 1-52.
* DeRose Keith and Ted Warfield (eds.), 1999. Skepticism: A
Contemporary Reader. Oxford: Oxford University Press.
* Feldman, Richard and Earl Conee, 1985. "Evidentialism."
Philosophical Studies, 48, pp. 15-34.
o The authors present and defend an (internalist) account of
justification according to which a belief is justified or unjustified
in virtue of the believer's evidence.
* Gettier, Edmund, 1963. "Is Justified True Belief Knowledge?"
Analysis, 23, pp. 121-123.
o In which the Gettier problem is introduced.
* Goldman, Alvin, 1976. "A Causal Theory of Knowing." Journal of
Philosophy, 64, pp. 357-372.
* Goldman, Alvin, 1986. Epistemology and Cognition. Cambridge:
Harvard University Press.
o Perhaps the most important defense of reliabilism.
* Haack, Susan, 1991. "A Foundherentist Theory of Empirical
Justification," In Theory of Knowledge: Classical and Contemporary
Sources (3rd ed.), Pojman, Louis (ed.), Belmont, CA: Wadsworth.
o An attempt to combine coherentism and foundationalism into
an internalist account of justification which is superior to either of
the two.
* Hume, David, 1739. A Treatise on Human Nature. Oxford: Oxford
University Press.
* Hume, David, 1751. An Enquiry Concerning Human Understanding.
Indianapolis: Hackett.
* Lehrer, Keith, 2000. Theory of Knowledge (2nd ed.). Boulder, CO: Westview.
o A defense of coherentism. This is also where we find the
Truetemp example.
* Lehrer, Keith and Stewart Cohen, 1983. "Justification, Truth,
and Coherence." Synthese, 55, pp. 191-207.
* Lewis, David, 1996. "Elusive Knowledge" Australasian Journal of
Philosophy, 74, pp. 549-567.
* Locke, John, 1689. An Essay Concerning Human Understanding.
Oxford: Clarendon.
* Plato, Meno and Theaetetus. In Complete Works. J. Cooper, ed.
Indianapolis: Hackett.
o Plato presents and defends a version of the JTB analysis
of knowledge.
* Pollock, John and Joseph Cruz, 1999. Contemporary Theories of
Knowledge (2nd ed.). Lanham, MD: Rowman and Littlefield.
o A defense of non-doxastic foundationalism, in which the
basic states are percepts rather than beliefs.
* Russell, Bertrand, 1912. Problems of Philosophy.
o Russell presents a Gettier-type example, which was largely
overlooked for many years.