From Descartes to Locke

Consciousness Knowledge

Science Reality

Brains in Vats

What is the point?

• The point of the “brain in a vat” story is notto convince us that we might actually bebrains in vats,

• But to force us to look at sense experienceas a whole.

• We justify many beliefs on the basis ofsense experience.

• But how can we justify our confidence insense experience itself?

The Problem: I cannot use sense experience to justify sense experience.

Descartes’ Skeptical Challenge

René Descartes

• Meditations on First Philosophy– In which are demonstrated the existence of God

and the distinction between the human soul andbody

• Descartes believes that before we philosophize aboutthe nature of reality (i.e., before we can dometaphysics), we must first philosophize about whatwe can know about the nature of reality (i.e., we mustask epistemological questions first.

The First Meditation

• Some years ago I was struck by how many falsethings I had believed, and by how doubtful was thestructure of beliefs that I had based on them. Irealized that if I wanted to establish anything in thesciences that was stable and likely to last, I needed—just once in my life—to demolish everythingcompletely and start again from the foundations. .... [T]oday .... I will devote myself, sincerely and without holding back, to demolishing my opinions.

Descartes seeks an

Epistemic Foundation

• Knowledge claims that are absolutelycertain.– (like the axioms of geometry.)

• From such claims, all other truths can bederived.

Method of Doubt:

• A proposed method for discoveringtruths that are absolutely certain.

–Withhold belief (for or against) from …

• everything that is even possibly false– i.e., everything that is doubtable.

Descartes applies the Method of Doubt to...

—sources of beliefs, rather than to individual beliefs.

So, he will try to show that we shouldn’t trust

our senses.

Descartes’ Goal

• His goal here is not, in the end, to argue thatour senses never provide knowledge– He will spend the rest of the Meditations trying

to argue that they do,• His goal here is to show us that the senses

do not provide certainty– The senses cannot themselves justify our belief

in them,– Rather, belief in them must be supported by

reason.

Deceived by the Senses

• “Whatever I have accepted until now as mosttrue has come to me through my senses. Butoccasionally I have found that they havedeceived me, and it is unwise to trustcompletely those who have deceived us evenonce.”

• What follows are various arguments forquestioning sense experience.

The “Malicious Demon”

• There could be a “malicious demon” whodirectly causes my sense experiences, eventhough there is no external world.– A being powerful enough to directly cause my

mental states, but not, like God, all good.– This “Malicious Demon” thought experiment

functions much like the “Brain in a Vat” story.• So, I should not trust any of my sense

experiences.

If there is a “Malicious Demon”

• Then I cannot trust anything I know throughthe senses. As far as I know, there is nothingin the world but me and the malicious demon.

• But, must there be a “malicious demon?” Isthe possibility of such a being my only reasonfor not trusting my senses?

Second Meditation

• I will suppose, then, that everything I see isfictitious. .... So what remains true? Perhaps just the one fact that nothing is certain! Still, how do I know that there isn’t something ... a God [or some other being, like the “Malicious Demon”] who gives me the thoughts I am now having? But why do I think this, since I might myself be the author of these thoughts? – But then doesn’t it follow that I am, at least, something?

The Self-Deception Argument:

1) It is possible that I myself am thecause of my own experiences, and so that I (seem to) “see” objects, even though no objects exist.

2) So, I should not trust my senseexperiences.

“I am, I exist.”

• This is the phrase Descartes uses in theMeditations. But he wrote another parallelbook piece called “Discourse on Method.”– In that piece, he made the same point this way:

• I think therefore I am.– Or, as it is stated in the original Latin;

–Cogito ergo sum.

Certainty At Last!

• I cannot doubt that I exist.– If I doubt my existence, I prove it, as I

must exist in order to doubt.

• “I am, I exist, is necessarily true eachtime that I pronounce it or mentallyconceive it.”

Descartes’ Epistemic Foundation:

• From this “foundation,” Descartes seeks tojustify all the rest of his beliefs about thenature of reality.

• (In the next chapter, we will return toaccount of mind and body.)

• What I really know is my own mind and myown conscious states. On this basis,Descartes tries to establish knowledge of aworld outside his own mind.

Is anyone out there?

• If one accepts the “destructive” part of Descartes—his undermining of sense experience, but

• Rejects the “constructive” part—where he arguesfor an “external” world—one is left with

• Solipsism: The view that as far as I know, I (ormy consciousness) am the only thing that exists.– `To be clear, Descartes rejects this view. But some

people argue this is where his position leads.

Descartes’ Account of Consciousness of the World

Med. 3, Paragraph 3

I previously accepted as perfectly certain and evident many things ...—the earth, sky, stars, and everything else that I took in through the senses—but in those cases what I perceived clearly were merely the ideas or thoughts of those things that came into my mind .... But I used also to believe that my ideas came from things outside that resembled them in all respects. .... [This] was false; or anyway if it was true it was not thanks to the strength of my perceptions.

Med. 3, Paragraph 6

“When ideas are considered solely in themselves and not taken to be connected to anything else, they can’t be false; for whether it is a goat that I am imagining or a chimera, either way it is true that I do imagine it. .... All that is left—the only kind of thought where I must watch out for mistakes—are judgments. And the mistake they most commonly involve is to judge that my ideas resemble things outside me.’

What I really knew vs. what I thought I knew

• I know that my ideas (or “sensations”) exist–Whether of “the earth,” “goats.” or mere

“chimera” (i.e., non-existent beings).– I know these ideas (“mental contents”) exist

because I directly (Immediately) perceive them.• But I simply assume that my ideas “come

from” things outside me, and that they“resemble” those things “in all respects.”– This is what makes “mistakes” possible.

What do I see?

What do I immediately perceive? “Normal” Sense Experience “Brain in a Vat” Experience

Descartes’ Analysis of Sense Experience

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What do I know? • I know that I exist.– I know that I am a “thinking thing,” a “mind.”

• i.e., the subject of conscious experiences.– Med. 2 and 6 argue that this “mind” is non-material.

• I know I have ideas or sensations “in” my mind.– These “mental contents” are what I “directly” or

“immediately” perceive.

• I “judge” (i.e., infer) that these mental contents arecaused by things that exist outside my mind, andthat my ideas “resemble them.”– This is what Med. 4-6 attempt to prove.

Descartes’ (Locke’s too) Theory of Perception: The mind perceives ideas which are caused by and

represent real objects.

Mind’s Eye Idea Object

Idea Object

Mind

Descartes, Locke, Berkeley

• All three accept (without much argument) thatwhat we directly or immediately know are only“ideas” or other “mental contents.”

• Descartes argues (in Med. 3-6) that there is aworld outside our mind.

• Locke accepts (without argument) that there issuch a world, but claims that our sensations do notalways resemble it.

• Berkeley argues that there is no world outsidemind (yours, mine, and God’s).

Terminology

• Empiricism:– All knowledge ultimately

rests upon senseexperience.

– Our justification forclaiming we knowsomething must alwaysend up with somethingwe perceive with oursenses.• “Seeing is believing.”

• Rationalism:– Not all knowledge

ultimately rests uponsense experience.

– At least some (maybeall!) knowledge can bejustified withoutappealing to senseperception.• E.g., 2+2=4.