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Plato

The Reality of Ideas

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Plato

427(?) - 348 BCE

Lived about 200 yearsafter Pythagoras.

Plato means thebroad possibly hisnickname.

Son of a wealthyAthens family.

Served in the Athensarmy during thePeloponnesian War.

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Plato and Socrates

Plato was Socrates student.

Almost all we know about Socrates

is from Platos writings. After Socrates execution for

corrupting the young and neglectingthe gods, Plato left Athens in

disgust and travelled widely. In Italy, Plato met the Pythagoreans.

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The Academy

In (ca.) 387 BCE, Plato returned toAthens and established a school forphilosophy, built in a grovededicated to the famous heroAcademos.

The Academy continued until it was

closed in 529 CE, over 900 years.

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Pre-eminence of Mathematics

Though planned as a school forfuture statesmen, Plato had becomeconvinced that the road toknowledge lay in exact reasoning,as in mathematics.

The famous inscription over the

entrance read: Let no one who does not know

geometry enter here.

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Platos Dialogues

Platos works span approximately30 dialogues dramaticconversations with statesmen,citizens, and other recognizablenames from Platos time and earlier.

Socrates is the main interlocutor.

It is hard to tell what are just Socratesown views and what is just Platosvoice.

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Plato on Reality

Most of Platos writings are notabout nature, but his concepts ofreality and knowledge have had aprofound impact.

These are characterized by twowell-known passages from his

dialogue, The Republic.

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The Divided Line

Think ofeverythingthat is,

placed onsingle line,extendingfrom thelowest to

the highestsense ofreality.

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The Divided Line, 2

There aretwo mainsections ofthe line,

representingthose thingsapprehendedby thesenses andthose things

onlyapprehendedby the mind.

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The Divided Line, 3

Each sectioncan also bedivided intotwo

subsections.

At thebottom thedivision isbetweenobjects andmereappearances.

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The Divided Line, 4

In the uppersection, thelower partrepresents

mattersunderstoodby deductivereasoning

Deductionimplies valid

argumentsfrom anassumedstarting place.

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The Divided Line, 5

At the verytop is thepurest formof reality, the

forms.

Understandingthe forms isthe highest

goal ofphilosophy.

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The Divided Line, 6

Knowledge ispossible only ofwhat lies in theIntelligible

World. Opinion is all

that is possiblefor theSensible World.

Therefore trueknowledgedepends entirelyon the mind.

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The Allegory of the Cave

Also in The Republic, Plato explains the route toknowledge and the responsibilities of philosophersthrough an allegory about prisoners in a cave.

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The Allegory of the Cave, 2

Imagine a cave in which prisoners arechained and seated so that they all faceone way, toward a wall.

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The prisoners have been there all theirlives and know nothing of the outsideworld.

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All that the prisoners see are the shadowscast on the wall before them.

This is the lowest segment of the Divided Line.

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Behind the prisoners is a fire, which theycannot see, that casts the shadows on thewall before them.

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Between the fire and the prisoners is a parapet,or walkway, where people are crossing back andforth with strange objects held above their heads.

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Everything the prisoners see or hear isbounced off the wall. They therefore thinkof that as the true reality.

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Now, suppose one of the prisoners isunshackled and led away, up out of thecave and into the world outside.

The prisoner will probably object andwhen outside, will be blinded by the light.

But in time the released prisoner willrealize that it is the world outside that is

real and the world in the cave only one ofillusion.

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If then the prisoner is led back down into the caveand placed in his original position, the otherprisoners would mock him if he told them of theworld outside and think him a fool. And theywould object to anyone else being led away.

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From The Republic:

the prison-house is the world ofsight, the light of the fire is the sun,and the journey upwards [is] theascent of the soul into the intellectualworld.

Complete text in Glimpses of Reality,

chapter 5.

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The prisoner who is released andattains a full understanding of whatis real (the philosopher), has aresponsibility to return to the caveand instruct others in what is real,so that they too may escape into

the world of truth.

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The Duty of the Philosopher

For Plato, the philosopher has aduty to enlighten the uneducated.

Compare this to the Pythagoreans,who sought to prevent any specialknowledge they had from escapingfrom their cult.

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Saving the Phenomena

The key is to show the real causes of thephenomena that are sensed by the

unenlightened. To show how a lower part of the divided line is

accounted for by a higher part.

This is called Saving the Phenomena. (Or,

Saving the Appearances.)

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Saving the Phenomena, 2

Examples would be:

Explaining to the prisoners that theshadows they see are caused by the

fire behind them and the peoplewalking on the parapet.

Explaining that night comes when thesun is no longer visible in the sky.

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The Mysteries of the Cosmos

Accounting for the strange behaviour ofthe heavens provided an excellentproving ground for the philosophers task.

Everyone sees the same phenomena. Some aspects of the heavenly bodies

seemed to defy explanation.

Note the role of science as uncovering the

secrets of nature.

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Empedocles, 492-434 BCE

One more minorphilosopher to mention.

Not a Pre-Socratic, but a

contemporary of Socrates. Came up with the most

popular and acceptedworld view of ancient

times.

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Two Forces

Causes of change:

Two forces: Love and Hate

The universe cycles through stages asLove and Hate counter each other.

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The Two Sphere Universe

Empedoclesdescribed thecosmos as a large

spherical ball, withthe stars all at theedge.

Earth was a

sphere in themiddle.

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The Two Sphere Universe, 2

The Earthremainedstationary in the

centre, and theentire celestialsphere revolvedaround every day,

carrying the sunand the moon withit.

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The Problem of the Planets

The stars allappeared to turnaround the Earth as

one, going fromeast to west, asthough imbedded inthe celestial sphere.

Note: Going thewrong way in theillustration.

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The Problem of the Planets, 2

Exceptions:

A few of the stars meaningheavenly bodies did not stay in the

same place relative to the others.

They moved against the backdrop ofthe celestial sphere.

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The Problem of the Planets, 4

The sun and the moon wereobviously different sorts of heavenlybodies.

But there were five other bodiesthat looked like stars, yet they alsochanged position relative to the

celestial sphere.

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Retrograde motion

Theplanetsnot onlymove

relative tothe fixedstars,theychange

direction.

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Retrograde motion

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The Spheres of Eudoxus

Eudoxus came up with a scheme toaccount for the strange forward andbackward motions of the planets.

He imagined a series of concentricspherical shells for each planet, turningon different axes nested inside eachother.

On the innermost spherical shell wouldbe the only part visible: the planet.

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The Spheres of Eudoxus, 2

The outersphere isaligned northand south andturnssimultaneouslywith thecelestialsphere.

This swings

the planetaround daily.

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The Spheres of Eudoxus, 4

The third andfourth spheresare aligneddifferently foreach planet andproduce theloopingretrogrademotions.

The planet is on

the innermostsphere.

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The Problem of the Planets Solved

The phenomena, i.e. the crazymotion of the planets, are saved.

This means that they are shown to be

nothing more than the combination ofregular motions of regular geometricshapes.

The sphere is a philosophical shape.

Therefore the planetary motions arephilosophical.

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A complex (invisible) system in the sky

Eudoxus required 27 differentconcentric spheres.

3 for each of the sun and moon,

4 for each of the other 5 planets,

and the celestial sphere for the fixedstars.

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On the positive side

Eudoxus goal was to capture theunexplained movements of the planets bymathematics.

This became a model of what a properexplanation of nature should look like.

Note:The sensible world (the visibleplanets) are explained by reference to the

intelligible world (mathematics).

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