Plato and Aristotle: Defining Rules
for 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 sensory
experience, experiment or observation
Allegory of the cave
Fire
Statues
Chained prisoners
Eternal forms
(mind)
Sensory experience (body)
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--a
literal 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’s
geometrical 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 ONLY
uniform, 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 rotates
CW around DD’ and
outer sphere CCW
around CC’, at same
speeds, planet moves
from 1 to 2 to 3 … to 8,
tracing a ‘hippopede’
or horse fetter
Full Eudoxen planetary model
A
B
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; sensory
experience 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
fire
hot
dry
earth
air
wet
cold
water
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
requires
continuously acting
movers

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 violating
Aristotle’s physics by moving the Earth