Skepticism, Rationalism & Empiricism

Three Theories of Knowledge

What is Knowledge?True, Justified, BeliefNeeds to be ALL 3:

It can be true but not justified– you guessed correctly

Justification can be weak or strong…

Plato’s Tripartite Theory of Knowledge Meets Gettier’s ProblemAccording to Plato:A person S knows

proposition P if & only if:

P is trueS believes PS is justified in

believing P

Gettier offers examples where:

Someone forms a belief that is true & justified

BUT does not qualify as knowing what they think they know

Holding a truth can sometimes be luck!

What do we know?Knowledge = justified true belief (Plato)The task:When are our beliefs justified?How are our beliefs justified?

Skepticism

• Beliefs can’t be justified

• We can have beliefs, but no knowledge

Rationalism

• Beliefs can be justified

• Justification comes from reason.

• Math & logic most reliable sources of knowledge

Empiricism

• Beliefs can be justified

• Justification comes from the evidence we receive from our senses

Descartes & SkepticismWanted to prevent

skepticism from undermining all claims to knowledge

Product of Reformation & Scientific Revolution

Is he a skeptic? He does not want to

be one…

I noticed that while I was trying to think every thing false, it was necessary that I, who was thinking this, was something. And observing that this truth, “I am thinking, therefore I exist” was so firm and sure that all the most extravagant suppositions of the skeptics were incapable of shaking it…”

The Brain in the Vat & the Evil Demon in Your Mind

What if you are just a brain in a vat, connected to a computer that causes you to think that you are living a ‘normal’ life? (Hilary Putnam, 1981)

Is everything you think about the world wrong?

Descartes’ malin génie puts wrong ideas in your head… how do you know what is true or real?

Descartes says: cogito ergo sum

Putnam says the scenario is illogical because the envatted brain is invisible & indescribable from within…

Convinced?

SkepticismGenuine Knowledge is not

possibleAll we have are beliefsGlobal skepticism = no

knowledge of any kind is possible

Local skepticism = there are some things we cannot know

Some ways of acquiring beliefs cannot bring knowledge (dreams, psychics, astrology)

The computer is so clever that it can even seem to the victim that he is sitting and reading these very words about the amusing but quite absurd supposition that there is an evil scientist who removes people’s brains from their bodies and places them in a vat of nutrients.

~Hilary Putnam, Reason, Truth & History 1981

Descartes’ MethodThe Method of DoubtReject any belief that

can be doubtedIf an evil-demon is

deceiving you, any of your ‘truths’ could be wrong

What is left?Your mind!

The evil demon…will never bring it about that I am nothing so long as I think that I am something.. I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind.

Can we Rebuild?Knock down the

faulty foundation, but then what?

Descartes concludes that the things we see very clearly & distinctly are true

Why?God

Cartesian CircleUse clear & distinct

ideas to prove existence of God

God gives us clear & distinct ideas

We use those ideas to prove the existence of God

God gives us clear & distinct ideas…..

Descartes’ LimitsInfers too much from

cogitoThinking is going on,

but who is doing the thinking?

Limited to the first person

Demon could trick me into thinking you are thinking

Limited to present tense

EmpiricistsBritish response to

Continental Philosophers

John LockeGeorge BerkeleyDavid Hume

EmpiricismJustification for knowledge

comes from our sensesBasic empirical beliefs are

directly derived from experience

They are therefore justified & count as evidence

Example of a basic empirical belief: I see a red phone & so I believe there is a

red phone in front of me

Inferential BeliefsThere are beliefs

about things that are NOT directly observable

Example: dinosaurs once existed

Can see them or touch them now

Inferential beliefs: bones, archaeological remains

Justification of Inferential Beliefs If you can’t confirm or

falsify with evidence …then you should be

skeptical of this beliefExamples:

All men are created equal

God wrote the BibleEmpiricists tend to be

skeptical about religion &moral knowledge

John Locke Locke says:Human

understanding is like a closet wholly shut from light, with only some little openings left, to let in external visible resemblances, or ideas of things without.

Primary and Secondary QualitiesSound skeptical?Locke has a

solution…Our perception

might be unreliable, but it depends on what type of qualities we are perceiving…

Take the red tomato…It’s redness is a

product of the interaction of various factors (texture, surface, our sensory system, environment at the time)

These properties don’t belong to the tomato

Therefore secondary qualities

Primary QualitiesThe tomato has some

true qualities: size & shape

These do not depend on the conditions under which it is observed

Does not even require an observer

These are primary qualities

Locke thought primary qualities were proof against the skeptics doubts…

George Berkeley, critic of LockeAccepts

representational model of perception

i.e.- seen objects are ideas

But, he asks, how could you check on all this if you are in Locke’s closet?

Berkeley’s ImmaterialismReality consists in

the ideas or sensations themselves

We are fully & properly connected with these!

Skepticism averted!But… wait, is

anything real?Berkeley says: to

exist is to be perceived

Berkeley & GodDo things cease to

exist the moment we stop looking at them?

Berkeley says: Everything is

conceived all the time in the mind of God

So, the continual existence of the world is assured

All the choir of heaven and furniture of earth - in a word, all those bodies which compose the frame of the world - have not any subsistence without a mind.

RationalismAgree with

Empiricists that knowledge is based on basic beliefs

BUT justification does not come from experience

Justification comes from Pure Reason

Math & RationalismWe cannot see, feel

or touch numbersSo if we have a

knowledge of numbers

This knowledge cannot be based on experience

Knowledge of numbers is based on Reason

Rational propositionsRationalists say…When we think about

certain propositions we can quickly see that they are true

These propositions have something self-evident about them

Simple reflection shows that these propositions (beliefs) are likely to be true

I believe that, if a triangle could speak, it would say... that God is eminently triangular, while a circle would say that the divine nature is eminently circular…~ Baruch Spinoza

You be the Rationalist… True? Or False?If one multiplies any

natural number by 2 then the resulting number is even

The income of the average worker in the US is higher than the income of the average worker in Europe

Every state must have some form of government

Every event has a cause.

Sugar is sweetAll human beings have

the same fundamental rights

All cats are animalsIf any nation should

ever use nuclear weapons again, then millions of people will die

Types of Rationalist JustificationA prioriMeans from the formerUsed to refer to a

justification that can take place prior to consulting empirical knowledge

You can know something a priori if you can know it without 1st seeing, touching or hearing anything in particular

Examples:All red cars are colored

carsAll triangles have 3 sides

A priori doesn’t mean no experience necessary…Just not any

particular experience

Example:In order to know that

tigers are animals…I might have to look

at a picture or see some

BUT I don’t have to see any particular tiger

A posteriori knowledgeAssociated with

EmpiricistsMeans from the

latterRequires that we

refer to specific experiences of the world

Example:There are 9 planets

in the solar systemDr. Ernst’s bike is

green

True or False?I can know a priori

that all bachelors are not married

A fully committed empiricist holds that all our knowledge is justified a posteriori.

It is impossible to know a priori whether New York has more inhabitants than Mexico City.

I can know a priori that there is life on other planets.

I can know a priori that if somebody is hot to death then somebody must have been the shooter.

I can know a priori that all swans are white.

All of mathematics is based on a priori reasoning.