Plato and Aristotle: Defining Rulesfor Western Cosmology

Raphael, School of Athens, 1510

Last time ... Initial ordering of the heavens

Horizon phenomena for seasonal calendars in N.Europe during Stone and Bronze Ages

Zodiacal constellations for calendars in Babylonia Decans for daily clocks in Egypt

“Physics” of the pre-Socratics Underlying reality is simple, unified Two fundamental questions

– What is the substance of the cosmos?– How is change possible?

Took the gods out, natural ≠ supernatural Introduced debate, criticism, skepticism

Task of today’s lecture Fate of “physics” after -500

Shift from cosmology to politics and ethics(“What is the good life?”)

The gods return Influence of Plato and Aristotle

Timaeus 2d best-seller (after Bible) to 1600 Aristotle provides core curriculum for

universities until 1750 Defined basic conceptual frameworks for Western

tradition

Context in -4c philosophy Socrates (d. -399), the sophist

Shifted attention from physics to politics Itinerant teacher who challenged authority

Plato (d. -347) and his Academy Philosophical community of scholars No fees, no fixed curricula Many religious ceremonies

Aristotle (d. -322) and his Lyceum Studied for 20 years at Plato’s Academy Collaborative research Train political philosophers for state

Plato on “physics” Why study physics?

Practical utility Cultivation of reason

– “Allegory of the Cave” (Republic, VII)

Dualism of form/matter, soul/body Pythagorean origins (geometry is true not in

drawn diagrams but in abstract ideas of line)? Objectively real = unchanging perfect forms Solves problem of change

– Imperfect matter changes, perfect forms do not Elevates reason above empiricism

– Truth arises from philosophical reflection, not sensoryexperience, experiment or observation

Allegory of the cave

Chained prisoners

Statues

Sensory experience (body)Eternal forms(mind)

Fire

Cosmogony in the Timaeus Only coherent non-biblical cosmogony in

Western tradition through 1100 An imagined story of origins, a creation myth

A sensible world cannot be eternal Three explanatory entities

– Mind (demiurge, divine craftsmen, abstract mind--aliteral but limited god?)

– Eternal forms– Recalcitrant matter forces compromises

Rational plan (telos) of geometry– Four roots become Pythagorean solids

Plato’s geometrical atomism

Combines Pythagorean five regular solids & Empedocles’(fl. -450) four “elements” (types of unchanging matter)

Implications of Plato’sgeometrical atomism Only one type of matter (like Thales) Explains change by rearranging triangles of

air/water/fire atoms Mathematization of nature Plenum cosmos, no void or “empty space” Gods return as principle of order

World Soul produces all motion in cosmos Spherical cosmos of uniform motion

Creates problem of “saving the phenomena” (e.g.,retrograde motion of planets) using ONLYuniform, circular motion

Sets rules for doing astronomy for next 1900 years!

Platos’s spherical cosmos

Eudoxus’s nested spheres First “working” model of cosmos following

Plato’s rules, c. -400 Twenty-seven nested spheres for 7 planets

– 1 for fixed stars– 3 each for Sun and Moon– 4 per planet (Mer, Ven, Sun, Moon, Mars, Jup, Sat)

2 for Hippopede, “figure-8” that produces retrograde 1 for annual, 1 for daily motions

“Mechanics” very vague– Saves retrograde motion of planets– Does not save variable brightness of planets

Eudoxan “Hippopede” model

As inner sphere rotatesCW around DD’ andouter sphere CCWaround CC’, at samespeeds, planet movesfrom 1 to 2 to 3 … to 8,tracing a ‘hippopede’or horse fetter

Full Eudoxen planetary model

B

A

Sphere A = daily rotation

Sphere B = period of planet

Spheres C, D = retrograde motions

Aristotle’s critique of Plato Places reality in sensible objects, not

invisible forms Separates objects into:

Properties (color, temperature, weight,etc)Subjects (that which possesses properties)

Reason downplayed; sensoryexperience emphasized

Sought comprehensive philosophy

A’s conceptual frameworks Explain change by 4 causes

Formal, material, moving, final Natural and forced motion Matter of 4 substances combined with 4

qualities Spherical earth at the center of a spherical

cosmos Separate physics for the terrestrial and the

celestial realms Eternal cosmos, no beginning or end of time

Aristotle on matter

earth water

fire airhot

cold

wetdry

Aristotle’s two physics Celestial realm

Perfect, changeless Aetherial spheres

(56 total) Natural motion

=circular Unmoved mover

acts continuously(God’s love)

Terrestrial realm Imperfect,

changeable Fire-air-earth-water Natural motion = up

and down Forced motion

requirescontinuously actingmovers

Aristotle’s cosmos, 1540

“Physics” for Plato & Aristotle Differently value reason and experience Seek coherent, consistent,

comprehensive explanations Brought back the gods (or agency),

demiurge and unmoved mover Defined key conceptual vocabulary Separate “physics” for heavens/earth

Minority Greek cosmologies Heraclides (d. -339)

Earth at center, in daily rotation Sun circles Earth, Venus/Mercury circle Sun;

other planets circle Earth

Aristarchus (d. -230) Sun at center (largest object) All planets circle Sun

Both widely ridiculed for violatingAristotle’s physics by moving the Earth