locomotion. Aristotle also adapts Plato’s demiurge (Timaios) and his Forms into a dynamic
conception of the good. Natural, like artificial, change is purposive, and tends regularly and
through its own agency toward some end, its perfected form. Aristotle’s four causes (formal,
final, efficient and material) are thus accounted for: the form, the good, and the agent are
identified and made immanent in each material substance. The substrate-form response to
Parmenide ̄s is melded with the final cause to generate the potentiality-actuality distinction:
change is the fulfillment of potentiality. The changer is actually, what the changed thing is
potentially.
The first half of the Physics discusses the existence and definition of the fundamental
principles concerning change: infinity (material stuff is a continuum infinitely divisible, but
its extent is finite: Aristotle uses this principle throughout the physical treatises), place (the
innermost boundary of the containing thing, having direction up and down: useful in cos-
mology and biology), void (does not exist: antiperistasis, mutual replacement of material,
explains locomotion), time (the measure of motion, a continuum on which the now is a
point: used throughout the physical works).
The second half of the Physics applies these principles (along with continuity and contact)
to prove facts about motion. Aristotle uses the infinite to demonstrate that Z’ para-
doxes of motion and time are misdirected: motion, time and magnitude are all continuous
and infinitely divisible. The theorems culminate in a set of arguments for the first unmoved
mover. All alteration, growth and decay, and terrestrial locomotion occur between some
terminal contraries. All such motion and change come to an end. Yet change and motion
are eternal: for how, if it ceased, would it ever get going again? Moreover, there cannot be
an infinite series of moved movers, and so there must be eventually an unmoved mover,
which is the source of the other finite movements. This unmoved mover will have no
magnitude and will cause movement by desire (Met. 12).
On the Heavens (de Caelo) uses the conclusions of the Physics as principles and studies
locomotion in its specific kinds: the natural motions of the elements, the heavy (earth and
water), the light (air and fire) and the fifth element (aithe ̄r) whose natural motion is
circular. Again Aristotle’s method is philosophical and deductive, constantly engaging the
theories of Empedokle ̄s, Anaxagoras, De ̄mokritos and Plato. He discusses the roles of the
elements as heavy and light. He avoids mathematical methods: his frequent use of inverse
proportion (among speed, distance, weight, resistance) is intended merely to prove the
impossibility of situations involving zero and infinity, and not to establish finite mathemat-
ical relations. He is keenly aware how tenuous his conclusions are where direct knowledge
is impossible.
The world is a unique, single, finite, uncreated and everlasting thing, comprising all
matter. The earth is spherical, about 50,000 miles in circumference (2.14 [298a15–17]). It is
unmoving and heavy things, like earth, tend to its center, which is identical with the center
of the universe. Light things, like fire, have an opposite natural tendency to seek the per-
iphery. Such is the world below the level of the moon. The heavens are occupied by a finite
but much greater amount of an element – aithe ̄r – whose natural motion is circular, and
which cannot transmute into other elements. The animate planets and stars are fixed into,
and made of the same stuff as, the spheres that revolve regularly from the right (south pole
is up). In Metaphysics 12, Aristotle adapts the homocentric model of E and K
to explain the complex motions of the planets.
Generation and Corruption considers differences among changes (alteration, growth, and
generation and corruption) and the lower four elements. Change among contrary powers
ARISTOTLE