Unit 8: Knowledge
Chris Heathwood
Office: Hellems 192
heathwood@colorado.edu
What We’ll Cover in Unit 8
I.
The Nature of Knowledge
A. What is a theory of knowledge?
B. Plato on Knowledge
1. Theaetetus’ Theory of Knowledge
2. Socrates’ Refutation of Theaetetus
3. Plato’s Theory of Knowledge
C. Gettier’s Refutation of Plato
II. Hume’s Problem of Induction
The Three Fundamental
Questions of Philosophy
1. What is there?
(Metaphysics)
2. What should I do?
(Ethics)
3. How can I know?
(Epistemology)
Some Questions in
Epistemology
1. What is knowledge?
2. What is epistemic justification?
3. What are the fundamental sources of
knowledge?
4. What are the limits of human
knowledge?
5. What is the status of skepticism?
The Nature of Knowledge
Our First Question:
What Is Knowledge?
• Putting the question this way makes the
question sound really hard. Here are two
other ways to put it:
– “What is it to know something?”
– “Under what conditions is it true that a person
qualifies as knowing that something is the
case?”
• An answer to this question will be a
theory of knowledge.
What is a theory of
knowledge?
A theory of knowledge is a statement of
the conditions under which a person
knows that something is the case.
It is a statement of this form:
S knows that p if and only if
____S____p____ .
Theories are knowledge
are supposed to reveal the
nature of knowledge.
Further Clarification of the
Question ‘What is Knowledge?’
Three Ways the Word ‘Knows’ Is Used:
• “Bob knows how to ride a bicycle.”
• “Bob knows the president of the U.S.”
• “Bob knows that the earth is round.” 
The theories of knowledge we’re looking at are
about the third kind of knowledge – called
knowledge that, or propositional knowledge.
How Do We Go About
Constructing (and Evaluating)
a Theory of Knowledge?
Analogy: Bachelorhood.
What is bachelorhood?
What is it to be a bachelor?
What are the conditions under which a person
qualifies as a bachelor?
What a “theory of bachelorhood” looks like:
x is a bachelor if and only if _____x_____.
The Socratic Method, or the
Method of Counterexamples
• A generalization is proposed
• We try to come up with a “counterexample”
to it – i.e., a concrete example that
“counters”, or shows false, the generalization
just proposed
• If we do, we have refuted the generalization
(but we might use the counterexample to
help us improve on the generalization just
refuted)
• If we can’t, perhaps the generalization is
true.
What We’ll Cover in Unit 3
I.
The Nature of Knowledge
A. What is a theory of knowledge?
B. Plato on Knowledge
1. Theaetetus’ Theory of Knowledge
2. Socrates’ Refutation of Theaetetus
3. Plato’s Theory of Knowledge
C. Gettier’s Refutation of Plato
II. Hume’s Problem of Induction

Plato on Knowledge
Plato (428-347 BC)
• The best known ancient Greek
philosopher
• Student of Socrates; teacher of Aristotle
• Wrote about 23 philosophical dialogues
• Famous doctrine: the Theory of the
Forms
• Western philosophy “consists of a
series of footnotes to Plato.”
- A. N. Whitehead (1929)
excerpt from the
Theaetetus
by
Plato
transla ted by F.M. C ornfo rd
So crate s :
Well , that is prec isely wh at I am p uz zled abou t. I cann ot
ma ke ou t to my own sa tisfact ion w hat know ledge is . Can
w e an swer t hat question . Wha t do you thin k ?
Socrates:
But the question you were asked, Theaetetus, was not, what
are the objects of knowledge, nor yet now many sorts of
knowledge there are. We did not want to count them, but to
find out what the thing itself – knowledge – is. Is there
nothing to that?
Theaetetus: No, you are quite right. …
Socrates:
Then tell me, what definition can we give with the least risk
of contradicting ourselves?
Theaetetus: The one we tried before, Socrates. I have nothing else to
suggest.
Socrates:
What was that?
Theaetetus: That true belief is knowledge. Surely there can at least be
no mistake in believing what is true and the consequences
are always satisfactory.
Theaetetus’ Theory of
Knowledge
The True Belief Theory:
S knows that p if and only if
(i) S believes that p; and
(ii) p is true.
Socrates’ Argument Against
the True Belief Theory
Soc: You will find a whole profession to prove that true belief is not knowledge.
…
The profession of those paragons of intellect known as orators and
lawyers. There you have men who use their skill to produce conviction,
not by instruction, but by making people believe whatever they want
them to believe. You can hardly imagine teachers so clever as to be
able, in the short time allowed by the clock, to instruct their hearers
thoroughly in the true facts of a case of robbery or other violence which
those hearers had not witnessed. …
… when a jury is rightly convinced of facts which can be known only by
an eyewitness, then, judging by hearsay and accepting a true belief,
they are judging without knowledge, although, if they find the right
verdict, their conviction is correct? …
But if true belief and knowledge were the same thing, the best of
jurymen could never have a correct belief without knowledge. It now
appears that they must be different things.
Socrates’ Argument Against
The True Belief Theory
The Argument
1. If the True Belief Theory is true, then the
jury knows that I committed the crime.
2. But they don’t know I committed the
crime.
3. Therefore, the True Belief Theory is not
true.
Further Counterexamples to the True
Belief Theory of Knowledge:
a. My belief that our football team will
win their next game.
b. Groundhog’s Day example.
Each case shows that true belief is not
sufficient for knowledge.
The Lesson:
a belief that is true
just because of luck does not
qualify as knowledge.
Plato’s Theory of Knowledge
Socrates: So when a man gets a hold of the true notion
of something without an account, his mind
does think truly of it, but he does not know
it, for if one cannot give and receive an
account of a thing, one has no knowledge of
that thing. But when he also has got hold of
an account, all this becomes possible to him
and he is fully equipped with knowledge. …
a true notion with the addition of an account
is knowledge?
Plato’s Theory of Knowledge
The JTB Theory
S knows that p if and only if
(i) S believes that p;
(ii) p is true; and
(iii) S is justified in believing that p.
Comments About the JTB
Theory
a.
b.
c.
How it avoids the counterexamples to the
True Belief Theory
Theory of Justification still needed.
Some possible ways to be justified in
believing something:
i.
ii.
iii.
d.
e.
perception
introspection
memory
iv. testimony
v. induction
vi. deduction
Theory accepted for thousands of years.
Theory no longer accepted today.
What We’ll Cover in Unit 3
I.
The Nature of Knowledge

A. What is a theory of knowledge?

B. Plato on Knowledge
1. Theaetetus’ Theory of Knowledge
2. Socrates’ Refutation of Theaetetus
3. Plato’s Theory of Knowledge
C. Gettier’s Refutation of Plato
II. The Problem of Induction
Gettier’s Refutation of Plato
Edmund Gettier (1927- )
• Not the best known contemporary American
philosopher, but pretty well know.
• Student of his teachers at Cornell; teacher of
me at UMass.
• Wrote just one 3-page paper.
• Famous doctrine: Justified true belief ain’t
knowledge.
• A. N. Whitehead (1929) probably didn’t say
anything about Gettier.
• Really good at badminton.
A Gettier-style Counterexample
• STEP 1. Suppose I see your driver’s
license, an Alaska driver’s license.
This seems to justify me in believing
(1) You are from Alaska.
Note: this assumes that justification does
not entail truth.
(That is, that what justifies me in believing
something need not absolutely guarantee
that that thing is true.)
A Gettier-style Counterexample
• STEP 2. Now suppose that on the basis of
my belief that
(1) You are from Alaska
I come to believe that
(2) Someone in my class is from Alaska.
It seems that I am justified in believing (2).
This is due to the following principle:
If S is justified in believing p, and p entails q,
and S believes q on the basis of S’s belief
that p, then S is justified in believing q.
A Gettier-style Counterexample
• STEP 3. Now suppose that the driver’s
license I saw was in fact a fake ID, and that
(1) You are from Alaska
is in fact false.
(Note: I have a false justified belief in (1).)
(Note also: the JTB Theory thus far implies,
correctly, that I do not know (1).)
A Gettier-style Counterexample
• STEP 4. Finally, suppose that, just by
chance, someone else in the class really is
from Alaska.
In other words, my belief that
(2) Someone in my class is from Alaska
actually turns out to be true.
It is true just by luck.
A Gettier-style Counterexample
• STEP 5. Let’s ask some questions about
this proposition:
(2) Someone in my class is from Alaska.
• FIRST QUESTION: Would you say that I
know (2)?
ANSWER: No.
• SECOND SET OF QUESTIONS:
YES
Is (2) true?
YES
Do I believe (2)?
Am I justified in believing (2)? YES
A Gettier-style Counterexample
• STEP 6: Thus, bringing it all together:
I have a justified true belief in (2), but I
don’t know (2).
In the form of a little argument …
A Gettier-style Argument Against JTB:
1. If the JTB Theory is true, then I know that
someone in our class is from Alaska.
2. But it’s not true that I know that someone
in our class is from Alaska.
3. Therefore, the JTB Theory is not true.
Other Gettier-style Examples
• The Hallucination
• Russell’s Clock
• The Sheep in the Field
A Way to Save the JTB Theory
• Note that what all the examples have in
common: the subject has highly reliable,
but not infallible, evidence for the
proposition believed.
• To say that e is infallible evidence for p
is to say that e entails p.
• Recall that Gettier’s argument assumed
that a person can be justified in
believing something without having
infallible evidence for it.
A Way to Save the JTB Theory
• But consider this thesis about
justification:
Infallibilism: S is justified in believing p
only if S’s evidence for p entails p.
• If Infallibilism is true, then Gettier’s
argument against JTB fails.
• But is Infallibilism true? …