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.
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
