 Episteme = knowledge
 Logos = science/theory/study of
Philosophical relevance:
1. Philosophy seeks truth, wisdom, and
i. How do we know that we have
obtained the truth?
ii. Need to know the conditions of
2. Seeking knowledge is irresistible to
human beings.
3. Want to understand science.
4. Problem of scepticism.
Different senses of “knowledge”
1. Skill knowledge (“know-how”)
 X knows how to: walk, ride a bike, etc.
 May be unable to explain how it’s done.
2. Knowledge by acquaintance (familiarity)
 I know: John, this city, her voice, etc.
 Perception: I’m aware of this board.
 Animals have these kinds of knowledge.
3. Propositional knowledge (“know-that”)
 S knows that p.
 Propositions: true/false.
 I know that: 2 + 2 = 4, this is a
chalkboard, etc.
 Requires cognitive ability to distinguish
Epistemologists tend to focus on #3. Why?
Historical reason: Science
Science is perhaps the most successful
knowledge-gaining enterprise in history.
Think of the advances made after learning:
 That F = ma
 That E = mc2
 That the atom is composed of parts
 That humans evolved from animals
 Etc.
Understanding how propositional knowledge
is gained will help us to understand science.
Is this a theoretician’s bias?
 Much of science depends on developing
new experimental techniques.
 I.e., “know-how”.
 Have philosophers overlooked the
importance of this?
Historical Reason: Intelligence
Our uniquely human intelligence seems to
be the result of our ability to represent and
understand propositions.
Classic view of the mind:
 Understanding is grasping propositions.
 E.g. knowing what snow is =
representing to oneself all or most of the
propositions about snow (that it is white,
cold, melts at room temperature, etc.).
 Classic AI theory: intelligence =
manipulation of symbols that represent
propositions (“have content”).
 Machines programmed with millions of
 When connected to a robotic body,
couldn’t perform simple of tasks (avoid
an obstacle, pick up an item, etc.).
 Is there more to intelligence than
propositional knowledge?
Nonetheless, propositional knowledge is of
central importance.
Here’s why:
 It seems to be what’s special about
humans (and perhaps higher primates,
I.e. we can form complex conceptions:
 Right/wrong; rational/irrational;
true/false; matter/antimatter; etc.
 We make sophisticated claims about the
 E.g.: “It is irrational to act immorally”;
“Everything is made of matter”.
The ability to form such ideas underlies all
of politics, science, law, philosophy, etc.
So, evaluating these sorts of knowledge
claims seems particularly important to
understanding the human condition.
Propositional knowledge
Key epistemological questions:
1. What is it?
2. Is it possible and if so, how?
Historically a great deal of epistemology
was stimulated by worries over #2:
 Is it possible to know anything?
Descartes thought that investigating
whether knowledge is possible teaches us
what knowledge is.
Descartes: Scepticism to Rationalism
The source of our knowledge appears to be
the senses.
 But often the senses deceive us (e.g.
about what is far away, illusions).
Can I at least be certain that I am standing
in a room, talking, looking at a window, etc?
Descartes: No! You could be dreaming.
 There could be an omnipotent demon
that constantly deceives you so that all
your sense impressions are misleading.
Contemporary version: Maybe you are a
brain in a vat!
 Even what appears most certain might
be misleading.
Radical doubt
1. In order to have knowledge we need
to be able to tell the difference
between a hallucination (deception)
and a perception.
2. It is impossible to distinguish between
a hallucination (deception) and a
veridical perception.
3. We do not know whether any of our
perceptual beliefs are true. We don’t
know whether anything exists at all.
We have “lost the world”.
Certainty: not from the senses
No matter how much I may be deceived,
one thing cannot be doubted:
 That I am having some kind of thought
(even a deceptive one).
 No matter how hard the Demon tries to
deceive me, if it succeeds it follows of
necessity that I exist at least enough to
have thoughts, perceptions, desires, etc.
So it is absolutely certain that I exist as a
thinking thing.
 The senses did not give me this
knowledge (see also: wax example).
 This knowledge came from reason
Rationalism: knowledge is derived from
reason not sensation.
 Reason may use the senses as data.
Wax example
I observe a piece of wax. It is:
 Solid, cold, scented, white.
I approach the fire. The wax changes:
 Liquid, warm, scentless, clear.
Every sensory property has changed.
Descartes: If I know that it is one and the
same piece of wax, this cannot be based on
the senses.
 Even empirical knowledge does not
come from the senses.
Conclusion: Even empirical knowledge is
derived from reason.
Rationalism and knowledge
Descartes: That which cannot be doubted
(“clear and distinct ideas”) must be true.
Descartes’ solution to scepticism:
1. I cannot doubt that an omnipotent God
2. I cannot doubt that God is benevolent.
3. Therefore, I am certain that God
doesn’t deceive me.
So, my senses must be trustworthy.
Well, this seems quite unsatisfying.
 It seems quite possible to doubt God’s
 It certainly seems possible to doubt
God’s benevolence.
But does the empiricist do any better?
Perception and empirical knowledge
Descartes assumes that we can have a full
array of perceptions without any objects to
be perceived.
 What is certain is that we have ideas
(internal) of external objects.
 What’s doubtful is the existence of the
external (physical) world.
This assumes that we don’t directly perceive
the world.
 There is something intermediate
between a perceiver and reality.
 The “idea” of an object is directly
perceived, not the object itself.
Why believe there are such intermediaries?
Hume and empiricism
Hume: All we ever experience are
 These sensations are not of continuous
 E.g.: we don’t see objects when we
close our eyes.
So, our sensations cannot give us the idea
of a permanent, physical reality.
Can we infer that physical reality exists and
causes our sensations?
 This would be unjustified.
 Since we only ever observe sense data,
how can we conclude there is
something else “beyond” them?
What about causation?
But surely objects cause our sensations.
 Hume: causation is only known through
 It applies to sensations: sensation A
always followed by B.
Therefore, we can’t claim it applies to the
 I.e., we can’t know that continuing,
physical objects cause anything!
 Let alone sensations.
We should admit that the only reality is the
reality of sense data (and minds?).
Physical reality is a useful fiction.
 It helps us to organize and predict our
 Physical objects are “phenomena” of
 “Truth” = “accurate predictions”.
Empiricism: all knowledge is derived from
Evaluating empiricism
 Avoids murky talk of unobservable
reality “behind” the sense data.
 Avoids scepticism: only commits to the
existence of sense impressions, and we
are certain that we have those.
1. Lost the world again.
 Reply: No: the only reality is sensory, so
reality is quite intimately known to us.
2. How do physical objects persist when not
 Reply: Objects are bundles of actual
and possible experience.
 The table persists when not perceived
because if you were to look there you
would see a table.
Sub-problem: what’s a “possible
3. Sense data can be indeterminate (how
many leaves on that tree?) but physical
objects are not.
 Possible reply: objects are sometimes
4. Let A, B, and C be slightly different
shades of red.
 I cannot distinguish A from B.
 I cannot distinguish B from C.
 I can distinguish A from C.
This is possible, but how can empiricism
explain it?
Some further thoughts on scepticism
Descartes: I know that p only if p can’t be
But do we need to accept this criterion? Is
absolute certainty required for knowledge?
In particular, on what basis do we assume
that certainty is required?
Common sense and certainty
In fact, our ordinary concept of knowledge is
one that tolerates uncertainty.
If it is 99% certain that p:
 It is possible (1%) that p is false.
 Still, can’t I know that p?
Example: I claim to know that we are safe.
 We are flying.
 Flying is safe: <0.001% of flights crash.
 Therefore, I know that we are safe.
I claim both:
1. I know we are safe.
2. It is possible (though very unlikely) I
am wrong.
Science and certainty
Science never assumes that anything is
known beyond all possibility of being wrong.
 Science proceeds on the assumption
that anything might turn out to be false.
 But science is our best system of
gaining empirical knowledge.
 So its standards ought to be good
enough for everybody.
So the crux of the issue is: is certainty
required for knowledge and if so, why?
 If logical certainty is required, scepticism
is hard to defeat.
Where does this leave us?
We want to understand the nature of
knowledge so it would be nice to determine
whether knowledge is possible.
Problem: scepticism.
Two solutions:
Rationalism: physical reality can be known,
but only via reason.
Empiricism: we can have knowledge of the
world, but the world is not what we think it
To adjudicate this dispute, we need to know
just what knowledge is.
 Fortunately, our investigation into
scepticism has given us some clues