1. Overview — The Study of Physis
For Aristotle (384–322 BC), physics (from physis, nature) was not about equations, but about understanding why things change.
His treatise Physics in eight books asks: What is nature? What is change (kinesis)? What makes motion possible? His answers dominated Western thought for nearly two thousand years.
Key distinction from modern physics: Aristotle explains the world in terms of purposes, qualities, and natural tendencies — not universal mathematical laws, forces, or atoms in a void. A stone falls not because of gravity, but because its nature is to seek its proper place at the center of the cosmos.
Physics in Eight Books
- I–II Principles, nature, and the four causes
- III–IV Change, the infinite, place, void, time
- V–VI Kinds of change and continuity
- VII–VIII Motion, the first mover
Nature, for Aristotle, is an inner principle of motion and rest. Artifacts have no such principle — they are moved from without.
2. Change & The Four Causes
All change is the actualization of potentiality. A block of marble is potentially a statue; the sculptor makes it actually a statue.
Aristotle identifies four kinds of change:
- Substantial — coming-to-be and passing-away (a seed becomes a tree)
- Qualitative — alteration (green to red, hot to cold)
- Quantitative — growth and diminution
- Local — change of place, or locomotion
Interactive: The Four Causes Explorer
To truly know a thing, Aristotle says, we must know its four causes — its whys.
Material Cause
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Formal Cause
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Efficient Cause
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Final Cause
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3. Elements & Natural Place
The sublunary world (below the Moon) is made of four elements, defined by two pairs of contraries: hot/cold and wet/dry.
- Earth — cold & dry — heaviest, moves down
- Water — cold & wet
- Air — hot & wet
- Fire — hot & dry — lightest, moves up
The heavens are made of a perfect fifth element, aether, which moves naturally in circles forever.
Natural Place: Each element seeks its proper layer in the cosmos. Motion is not neutral; it is homecoming. Stop interfering, and things return where they belong.
The Four Qualities
Interactive: Drag to Natural Place
Drag each object. Release it, and watch it seek its natural place in Aristotle's cosmos.
4. Motion: Natural and Violent
Natural motion requires no continuous force — it follows the element's inner nature (stone down, fire up). Violent (forced) motion requires a constant pusher. Remove the pusher, motion stops.
Aristotle's Law of Motion
Speed is proportional to weight and inversely proportional to the resistance of the medium.
No void can exist — in a void, resistance = 0, so speed would be infinite, which is absurd.
Interactive: Falling in Different Media
Projectiles: The Air-Push Theory
Aristotle struggled with thrown objects. His solution: the thrower moves the air, and the air rushes around to keep pushing the arrow from behind. Later thinkers like Philoponus proposed impetus — a quality impressed in the object itself.
Click or tap to launch
5. The Cosmos: Spheres Within Spheres
Two Realms
Sublunary (Earth to Moon): realm of generation, corruption, and the four elements. Things change.
Superlunary (Moon outward): made of aether, perfect, eternal, moving in unceasing circles.
Beyond the outermost sphere is the Unmoved Mover — pure actuality, thinking itself, the final cause that draws all motion as love draws the lover, without itself moving.
6. Time, Place, Void & the Continuum
Place (topos) is not space, but the inner surrounding surface of a container — like a vessel holding water.
Time is “the number of change with respect to before and after.” No change, no time.
Continuum: magnitudes, time, and motion are infinitely divisible — not made of indivisible atoms. Between any two points, there is always another.
Why No Void?
- In a void, no directions (up/down) exist
- Speed would be infinite (v ∝ 1/0)
- No medium to transmit forced motion
- Bodies would not know where to rest
Interactive: Approaching the Void
Drag the slider to reduce the medium's density. Watch Aristotelian speed increase.
7. Legacy — Why It Lasted 2,000 Years
Aristotle's physics was comprehensive, commonsensical, and teleological — it explained the why, not just the how. It fit everyday experience: heavy things do fall faster (in air), flames do rise, rest seems natural.
Medieval Critiques
John Philoponus (6th c.): Dropped weights — they fell at nearly the same rate. Proposed impetus theory.
Avicenna & Averroes: Refined motion theory in the Islamic world.
Jean Buridan (14th c.): Developed impetus as a permanent impressed quality.
Galileo & Newton
Galileo's inclined planes showed acceleration is independent of weight (ignoring air resistance). Newton replaced natural place with inertia and universal gravitation: F=ma, not v∝W/R.
Yet Aristotle's questions — about causation, actuality, and the nature of change — remain alive in philosophy of science.