Aristotle
divided the theoretical sciences into three groups: physics,
mathematics, and theology. Physics as he understood it was equivalent
to what would now be called “natural philosophy,” or the study of
nature (physis); in this sense it encompasses not only the modern
field of physics but also biology, chemistry, geology, psychology,
and even meteorology. Metaphysics, however, is notably absent from
Aristotle’s classification; indeed, he never uses the word, which
first appears in the posthumous catalog of his writings as a name for
the works listed after the Physics. He does, however, recognize the
branch of philosophy now called metaphysics: he calls it “first
philosophy” and defines it as the discipline that studies “being
as being.”
Aristotle’s
contributions to the physical sciences are less impressive than his
researches in the life sciences. In works such as On Generation and
Corruption and On the Heavens, he presented a world-picture that
included many features inherited from his pre-Socratic predecessors.
From Empedocles (c. 490–430 bce) he adopted the view that the
universe is ultimately composed of different combinations of the four
fundamental elements of earth, water, air, and fire. Each element is
characterized by the possession of a unique pair of the four
elementary qualities of heat, cold, wetness, and dryness: earth is
cold and dry, water is cold and wet, air is hot and wet, and fire is
hot and dry. Each element has a natural place in an ordered cosmos,
and each has an innate tendency to move toward this natural place.
Thus, earthy solids naturally fall, while fire, unless prevented,
rises ever higher. Other motions of the elements are possible but are
“violent.” (A relic of Aristotle’s distinction is preserved in
the modern-day contrast between natural and violent death.)
Aristotle’s
vision of the cosmos also owes much to Plato’s dialogue Timaeus. As
in that work, the Earth is at the centre of the universe, and around
it the Moon, the Sun, and the other planets revolve in a succession
of concentric crystalline spheres. The heavenly bodies are not
compounds of the four terrestrial elements but are made up of a
superior fifth element, or “quintessence.” In addition, the
heavenly bodies have souls, or supernatural intellects, which guide
them in their travels through the cosmos.
Even
the best of Aristotle’s scientific work has now only a historical
interest. The abiding value of treatises such as the Physics lies not
in their particular scientific assertions but in their philosophical
analyses of some of the concepts that pervade the physics of
different eras—concepts such as place, time, causation, and
determinism.
Place
Every
body appears to be in some place, and every body (at least in
principle) can move from one place to another. The same place can be
occupied at different times by different bodies, as a flask can
contain first wine and then air. So a place cannot be identical to
the body that occupies it. What, then, is place? According to
Aristotle, the place of a thing is the first motionless boundary of
whatever body is containing it. Thus, the place of a pint of wine is
the inner surface of the flask containing it—provided the flask is
stationary. But suppose the flask is in motion, perhaps on a punt
floating down a river. Then the wine will be moving too, from place
to place, and its place must be given by specifying its position
relative to the motionless river banks.
As
is clear from this example, for Aristotle a thing is not only in the
place defined by its immediate container but also in whatever
contains that container. Thus, all human beings are not only on the
Earth but also in the universe; the universe is the place that is
common to everything. But the universe itself is not in a place at
all, since it has no container outside it. Thus, it is clear that
place as described by Aristotle is quite different from space as
conceived by Isaac Newton (1643–1727)—as an infinite extension or
cosmic grid (see cosmos). Newtonian space would exist whether or not
the material universe had been created. For Aristotle, if there were
no bodies, there would be no place. Aristotle does, however, allow
for the existence of a vacuum, or “void,” but only if it is
contained by actually existing bodies.
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