Aristotle’s World

Joshua Lande

Aristotle (384 BC – 322 BC)• Born of a well-to-do

family in the Macedonian• In 384 BC.• At 17 he went to Athens

to study at Plato’s Academy.

• He remained for the next 20 years.

Aristotle’s Physics• Is amazingly plausible,

coherent, and common sense.

• Lacks Mathematical rigor• “Mathematics was

concerned with unchanging objects that lacked reality, while his natural philosophy focused upon changing objects with a reality of their own.”

• Almost fully accepted until the Copernican revolution.

The Qualities

• Four fundamental quantities– Hot, cold, dry, & wet

• Hot & cold are active quantities• Addition or subtraction will change things• Wet & Dry are result of action of active

quantities• Fundamental qualities can be combined in

only 4 ways to create the 4 elements

Material Science

• Four main elements make up the terrestrial world:

• earth, air, water and fire.• Earth=Dry + Cold• Water = Wet + Cold• Fire = Dry + Hot• Air = Hot + Wet• Earth is not the same as

earth!

The Periodic Table of the Elements

• 4 elements roughly correspond to 4 states of matter– Solid, Liquid, Gas,

Plasma.• Heavy material like

rocks and Iron made up mainly of earth

• Lighter objects like wood contain more water and air

This Can Explain Lots of Observations

• Example – Boiling• The addition of heat to

a wet object.• The heat draws the

wetness (water/air) out• The resulting substance

is thicker (more earth)• The separate wetness

(water/air) is lighter so rises

Theoretical Issue

• Ice is dryer and colder than water - contain more earth.

• But earth is attracted downwards more strongly.

• Strangely ice floats in water?

Human Biology• Humans consist of a

complex mixture of all the elements:– Earth gives strength and

weight– Fire provides warmth– Water for blood and bodily

fluids– Air fills lungs and breath of

life.• Each human has a

different composition making us all unique.

The Universe Has a ‘Cosmic Graph Paper’

• In the center of the graph is the geometric center of the universe.

• The center of the universe is where all the radial lines converge.

Natural State

• The natural state of objects is at rest

• This is obvious from terrestrial observations

• All things put in motion eventually stop

Natural Place

• All mater has a natural place it wants to go to

• The natural ordering is Fire, Air, Water, and Earth

• Earth wants to “go down” more than water.

• Lightning is fire in the sky.

Dynamics

• Natural motion caused by objects seeking their natural place

• Motion is straight up / down in radial lines• Explains why rocks sink in water but air

bubbles rise up• Fire rises through air.

Details• Natural motion, proportional to weight.• Explains why rocks fall faster than feathers• The speed of fall depends inversely on the

density it falls through.• Explains water resistance, etc• Because of this, there can be no vacuum• Things would move infinitely fast through it

Why the Earth is Round

• The way for most earthy material to get as close as possible to the center.

• The water rest on surface because it wants to be further from center.

The real world is more complicated.

• Our world isn’t in equilibrium• Water somehow gets pulled up into the

atmosphere for a while (clouds) before eventually falling to earth

• Fire trapped in things like trees eventually released during burning

• Volcanoes occasionally spew earthy and watery and fiery junk into the atmosphere

• It lingers for a while before moving back toward its natural place

Violent Motion

• Only way for an object to move unnatural (in non-rectilinear motion up/down) is violently.

• Violent motion happens when one object acts directly acts upon another causing it to move.

• So motion only happens when a force is involved.

• This explains pushing a boulder up a hill, walking, etc.

• Speed of violent motion proportional to force– If you push something harder it moves faster

Impossibility of Heliocentric Universe

• Imagine a bird taking off and flying• After leaving the ground, it would come to

rest• But rest is moving quickly away from the earth• So the bird would leave the earth.

Projectiles• Hard to explain• Continues to move in the

air instead of immediately falling towards earth

• Explanation is that the air in front of the object became disturbed by the movement

• Air swirls behind it pushing it along.

• This tendency weakens with time

Cosmology• Everything in the sky move

round in circles.• Other than that, it doesn’t

really change.• But our terrestrial world is

always in flux.• Therefore, the heavens

must be made of an entirely different substance.

• This substance is aether, a pure, completely different perfect substance

Aether

• The main difference between aether and matter is that aether’s natural state is circular motion

• Circles are the most perfect geometric shape

• Aether means always running

The Moon• The Moon marks the

boundary between the Earth and the Heavens

• It is mostly aether.• Small amounts of earth

accounted for the visible imperfections on its surface.

• All imperfect celestial objects such as comets, nova, and meteors are sublunar.

Cosmology

• Heavenly objects move in celestial spheres.

• Center is Earth, then moon, then planets, then stars.

• Agreement with rough observations.

Imperfect Planets

• The planets & sun drift relative to the stars

• This is an imperfection• Closer bodies are

increasingly corrupt by terrestrial elements

• E.g. The sun probably contains some fire in it.

Dynamical Connection Between Heavens and Earth

• Rotation of the outer sphere of stars mechanically responsible for all the other motion in the universe.

• The rotation of the stars pulls the planets around• In turn pulls the Sun around• In turn pulls the Moon around• In turn communicates motion to sublunar region.• Plausible– The tides correlates with the motion of the Moon and Sun– So does the daily warming and cooling of the Earth – And yearly progression through the seasons.

Questions?