The Timpson Lyceum · History of Philosophy (2024)

Aristotle's Metaphysics & Physics – Lecture 26 (History of Philosophy)

Overview

Aristotle's metaphysics and physics together form his answer to the two great Presocratic challenges — Heraclitus' flux and Parmenides' denial of change — and his replacement for Plato's transcendent Forms. Where Plato located the real in a separate world of Forms, Aristotle plants form back inside the ordinary things around us: the primary substance (this individual man, this horse) is what most fully exists, and everything else — universals, qualities, relations — depends on substances for its being. The analytical toolkit is a pair of matched distinctions, matter/form and potentiality/actuality, which let him define change without contradiction: in every change some underlying matter takes on a new form, so nothing is created from nothing and nothing perishes into nothing.

The lecture then works outward from small changes to the cosmos as a whole. Every change is fully explained by its four causes (material, formal, efficient, final), and because natural things reliably strive toward their mature form, Aristotle is a universal teleologist: purpose is built into nature itself. His physics — four terrestrial elements plus celestial ether, each with a natural place and natural motion — is empirically motivated but wrong in detail. The arc culminates in the question that opens theology: if the cosmos is eternal and motion never began, what sustains all motion now? The answer is the unmoved mover, pure actuality, later absorbed by Aquinas as an argument for God.

Key Concepts

• Primary substance — The individual, particular thing (this chalk, this person). It is neither predicable of anything nor present-in anything; it exists in its own right, and everything else depends on it. Aristotle's inversion of Plato: universals like "chair" and qualities like "red" are real, but only in the substances that bear them, not in a separate realm.
• The four ontological categories — Every item is either a universal or a particular, and either exists independently or must be "in" something. Primary substances are particular-and-independent; species/genera (Plato's Forms) are universal-and-independent; qualities like red are universal-and-dependent; a specific memory is particular-and-dependent.
• Matter and form (hylomorphism) — Matter is the individuating element (why this marker differs from that one, even if identical in kind); form is the universal, intelligible element (what makes each a marker). The terms are relative: bricks are matter to a house but form to the clay; the same nesting runs bricks → concrete → atoms → protons/neutrons → quarks.
• Prime matter — The limit of that regress: wholly formless matter. Since a thing potentially anything would actually be nothing (nothing can be potentially what it actually is), prime matter is a valid concept that can never actually exist; every real thing is matter already possessing some form.
• Potentiality and actuality — A thing's actual form is what it now is; its potentialities are the forms its matter could take on. An acorn is actually an acorn, potentially an oak. An object's actuality fixes its range of potentialities (an acorn can become an oak but never a maple), which grounds the intelligibility of nature.
• Change resolved — Against Heraclitus (change implies contradiction — the thing both is and is not itself), Aristotle answers: the matter stays the same, the form changes, so no contradiction. Against Parmenides (change requires being to arise from non-being), he answers: no matter is ever created or destroyed; it merely takes on new form. Change is the actualization of a potentiality.
• Change presupposes logic — One cannot use change to attack the laws of logic: asserting that a thing became red requires the law of non-contradiction (red ≠ not-red), and denying that swapping one object for another counts as change requires the law of identity.
• The four causes — To fully explain any change one must give its material cause (what it is made from, e.g. marble), formal cause (the form imposed, e.g. a statue), efficient cause (the agent producing it, e.g. the sculptor), and final cause (the purpose for which, e.g. to honor Plato). All four apply to natural as well as artificial change.
• Teleology and the coincidence of form and final cause — In natural change the formal and final causes converge: the acorn becomes an oak for the sake of being an oak. Aristotle's teleology is immanent — the end lies within the thing, not in some external cosmic plan or divine will.
• Not strict determinism — Nature holds "always or for the most part": occasionally an acorn fails to become an oak, blamed on matter resisting its form. This leaves room some readers exploit for chance and freedom, unlike the exceptionless laws of later physics.
• The five elements and natural place — Four terrestrial elements (earth, water, air, fire), each defined by a pair of qualities (hot/cold, wet/dry), plus celestial ether. Each seeks its natural place: earth at the center, then water, air, fire, and ether beyond. Density is set by the mix of elements.
• Natural vs. violent motion — Terrestrial elements naturally move only up or down toward their place; any side-to-side ("violent") motion needs an external mover. Aristotle's mistaken dynamics — $F = mv$ rather than $F = ma$ — implies a mover must act continuously to sustain motion, and that a true vacuum is impossible (speed of fall would become infinite).
• Celestial ether and the eternal cosmos — The heavens move in perfect circles forever, so the heavenly bodies are made of ether, whose natural motion is circular. The universe is eternal: motion never began, since time is the measure of motion and a motionless (hence timeless) moment is incoherent.
• The unmoved mover — The eternal source of all motion. It must be eternal, itself unmoved, without potentiality (else it would change), hence pure actuality / pure form, non-physical, and perfect. It moves the outermost sphere not by pushing but as an object of desire — the sphere's intelligence, aware of its perfection, imitates it in flawless rotation, and that motion is communicated inward. Aristotle's "God," later taken up by Aquinas.

Worked Example

From the fact of motion to a first, unmoved mover. Aristotle refuses an infinite chain of movers stretching back in time; he argues that present motion cannot be grounded unless the regress of simultaneous movers terminates in something itself unmoved. Formalize the deduction.

1. Empirical premise. Something is now in motion. Let $M(x)$ mean "$x$ is in motion," and take a particular $a$ with $M(a)$ — for instance, a boulder rolling downhill.

2. The mover principle. Nothing moves itself; whatever is in motion is moved by another. Writing $y \mathrel{\triangleright} x$ for "$y$ moves $x$":

$\forall x \, \big( M(x) \Rightarrow \exists y \, (y \mathrel{\triangleright} x \ \wedge\ y \neq x) \big).$

1. Generate the chain. Applying the principle to $a$ gives a mover $a_1 \mathrel{\triangleright} a$. But $a_1$ is itself in motion insofar as it is actively moving, so it too has a mover $a_2 \mathrel{\triangleright} a_1$, and so on, yielding a chain $\cdots \mathrel{\triangleright} a_2 \mathrel{\triangleright} a_1 \mathrel{\triangleright} a_0 = a.$

2. No actual infinite. This is not a chain running backward in time but a chain of movers acting now to ground the motion of $a$. An actually infinite such chain cannot exist: with no first term there is no source from which motive power is derived, and the present motion of $a$ would have no sufficient ground. Formally, the ordering $\mathrel{\triangleright}$ restricted to the present cause of $a$ must be well-founded — it cannot descend forever.

3. Terminate the regress. A well-founded chain has a first element $\mu$ such that $\mu \mathrel{\triangleright} a_{n} \quad\text{and}\quad \neg\,\exists z \,( z \mathrel{\triangleright} \mu ).$ By the mover principle (step 2), anything in motion would require a further mover; since $\mu$ has none, $\neg M(\mu)$: $\mu$ is itself unmoved.

4. Characterize $\mu$. If $\mu$ had any unrealized potentiality it would tend to actualize it and so would move; therefore $\mu$ has no potentiality and is pure actuality. Having no matter, it is non-physical; being fully actual, it is perfect. It moves as the final cause — the beloved object of the outermost sphere's striving — not as a pushed efficient cause.

5. Conclusion. There exists a first, itself-unmoved mover, pure actuality, which grounds all motion in the cosmos. $\blacksquare$

Summary

• Aristotle relocates the real from Plato's separate Forms to primary substances — the individual things around us — on which all universals and qualities depend.
• Matter/form and potentiality/actuality dissolve the Heraclitus–Parmenides problem: in change the underlying matter persists while its form is actualized anew, so nothing arises from or lapses into non-being.
• Every change is fully explained by its four causes, and in natural change the formal and final causes coincide, making Aristotle a universal (but immanent) teleologist.
• His physics — four terrestrial elements plus ether, natural places, natural vs. violent motion, and the impossibility of a vacuum — is a genuinely scientific but empirically mistaken system ($F = mv$, not $F = ma$).
• Because the cosmos and its motion are eternal, present motion demands an eternal, itself-unmoved source: the unmoved mover, pure actuality and non-physical perfection, which moves the heavens as an object of desire and later becomes Aquinas' God.