PHILOSOPHY 21:
HISTORY OF MODERN EUROPEAN PHILOSOPHY
FALL 2005
HANDOUT #1
ARISTOTELIAN NATURAL PHILOSOPHY
The fundamental concepts in
Aristotelian natural philosophy are the concepts of Species, Substance, and Essence.
Species
A species is a kind. Plants and animals --
including the species of animal ”human being” -- are
natural kinds. Aristotle was, in effect,
the creator of the discipline of Taxonomy:
the science of classifying things.
Our system of taxonomy in biology – classifying organisms into species,
genus, kingdom – developed from his treatises On the Parts of Animals, On the Motion of Animals, On Plants.
However, Aristotle held that
there are also non-living species, particular varieties of stone or metal, and
the “eelements” – earth, air, fire, and water.
(Mendeleev’s ‘Periodic Table of the Elements,’ which
deduces the properties of elements by classfying them correctly, is a very
Aristotelian kind of enterprise, even though it was not developed until 1869.)
Aristotle extended his system
of taxonomy to non-natural objects as well, classifying even things like
philosophical theories and works of literature into species and genus. Aristotle hypothesized that the entire
universe was an ordered system of such kinds.
That’s what makes the universe intelligible.
Substance
Every
individual existing thing must be of some species; it must be some particular kind of
thing. The individuals within each
species are “substances.” So, in
Aristotelian terminology, the individual human beings Socrates, Plato, and
George W. Bush,
individual horses like Seabiscuit, are substances. So are individual all individual animals and
plants. Even individual artifacts like
boats, tools, and buildings are substances.
Essence
Each species of substance has certain characteristic
properties. Most bird species, for
instance, have the property of being
capable of flight;
all bird species have the property of being oviparous (laying eggs).
Certain other animals have the property of being viviparous (bearing live young). Some of the properties found in a species are
necessary to it in the sense that they are properties anything of that speices must have in order to be of that
kind. That collection of necessary
properties is the Essence associated
with that species. Thus the essential properties associated with the
kind “human being” are: being a mammal,
being capable of rational thought, being capable of speech.
Since any individual must be
an individual of a certain kind every substance has the essence
that goes with that species. So, all the
properties of that essence are properties that the individual substance must
have.
Now, individual substances have many, many
properties, and not all of them will be part of the essence. The substance George W. Bush, for example, is
of the species “human being”; thus he must
be a mammal. He couldn’t be a fish. But George W. Bush also has the properties of
being a Republican, and being President of the United States. These are not properties he must have in order
to be the identical substance he is: Mr. Bush could have been a Democrat, or a
Baathist, and still have been the same individual substance he is.
In Aristotle’s view, the
universe is composed of individual substances ordered in a hierarchical structure
of lower and higher-level species.
To understand what a kind of
thing is is to be able to provide a “Statement of the Essence.” The fact that individual substances fall into
kinds was the
most important and revealing fact about
them. It is this orderly structure that
makes the world intelligible – a cosmos
rather than a chaos.
Aristotelian Scientific Method
Aristotelian science takes
the form of determining what essences there are. For instance, it was a discovery that sea
urchins were a kind of animal (rather
than a plant), and that dolphins were mammals, not fish.
An Aristotelian scientific
explanation of a phenomenon then takes the form of deducing the occurrence of
the phenomenon from the essence of the object in which the phenomenon occurs. These demonstrations take the form of logical
syllogisms like the following:
Major Premise All human beings are capable of rational
thought.
(“Being capable of rational thought” is part of the human essence)
Minor Premise All beings capable of rational thought are beings capable of laughing.
(Some further property that necessarily follows from possession of the
first property)
Conclusion All human beings are capable of laughing.
Another example:
(An explanation of why thunder is loud)
Major Premise Thunder
is the quenching of fire in cloud.
(This is a statement of the
essence of thunder.)
Minor Premise Whenever there is a quenching of fire there is a sound.
Conclusion Whenever
there is thunder there is a sound.
AN EXAMPLE:
ARISTOTLE’S EXPLANATION OF MOTION
A contrast between
Aristotelian science and modern science on some particular point would be
helpful for understanding what made the science that originated in the 1600's
so new and so different. We will compare and contrast Aristotle’s
explanation of motion with the explanation put forward by Galileo in the
1600's.
Aristotelian science is not,
as it was for Galileo (and is for us), a matter of finding mathematical
formulas describing how things behave.
Rather it is a qualitative enterprise of distinguishing, among the
properties of substances, which are necessary properties and which are
not. It’s the enterprise of finding a
statement of a thing’s essence.
Aristotle’s theory of motion
and space is found in the treatise entitled Physics
(“On Natural Bodies”). “Natural body” is a very high-level species: it includes all of the kinds of plants,
animals, and non-living substances.
Aristotle did not conceive
of space as we do -- as an empty and neutral container of physical bodies -- but as the expanse of the aggregate of physical substances, bodies, in the
universe. At no point in the
universe is there space where there is no substance.
People take what exists to be body, and
hold that while every body is in (some) place, (a) void is (a) place in which
there is no body. . . .
Void must, if it exists, be place
deprived of body...
It is plain on this showing (that) void
does not exist[1].
No voids or vacuums exist because a void would, by
the statement of its essence (“place where there is no substance”)
be nothing.
But
if objects are to move relative to each other wouldn't the space between them
have to be empty?
Aristotle says no: far from being required for objects to move,
a void would make the motion of objects impossible.
Indeed, as against those who say that a
void is necessary if there is to be movement, reflection shows the very
opposite: were there a...void, there
would not be a single movement.
Every body. .
.has weight or lightness. Hence, by a
syllogism, what has nothing heavy or light in it is void.[2]
All movement is either “natural” movement or
‘violent’ movement. ‘Natural’ movement
is a body's going where its essence makes it tend to go. ‘Violent’ movement is movement of an object
against its essential nature. Bodies
whose essential nature includes a composition of earth tend to go 'downward'
(toward the center of the universe).
Bodies composed of fire tend to go 'upward'.
Just as the earth is
stationary...because of the uniformity of its medium, so, too, in a void there
would have to be complete stability.[3]
The motion of freely-falling bodies occurs because
An object A
which is “heavy” (that is,
it contains a large quantity of earth)
is “above” (that is, further
from the center of the universe than)
an object B, which is “lighter” (contains less earth).
Object A
will thus tend to displace B in order
to get “below” it; and
B will tend to displace A to get “above” it.
Thus Aristotle’s distinction
between natural and violent motion is an instance of our modern distinction
between the motion of a freely-falling body and “projectile” motion.
However, if an object A, however light or heavy,
were suspended in a void ( which would lack the properties either of lightness or heaviness), then A would have no tendency to move naturally in any direction. There could be no “freely-falling” body in a
void.
On the other hand when an
object is placed in violent motion, as when a baseball (whose natural motion is
downward) is thrown into the air, there must be an impelling force on the ball
to make it move against its natural direction.
Furthermore there must be something continuing to force the ball to move
in the unnatural direction during the entire time it is moving that way. So Aristotle has some explaining to do: why
does the ball continue to rise after it leaves the player’s hand?
Aristotle explains the fact
that the baseball continues to rise even after it is separated from the
impelling force of the player's hand thus:
Some explain this phenomenon by mutual
replacement. Another explanation may be
that the air which has been pushed pushes projectiles with a motion more
vigorous than their motion to their resident place. But
none of these things can happen in a void.[4]
In normal projectile motion we observe a tendency
for the ball to rise more and more slowly, then stop rising, and finally resume
its natural motion downward until it hits the ground. Aristotle explains this normal case of
projectile motion by the impelling force of the violently displaced air
continuing to move the ball upward, but
The power to move something else wears
away gradually to the extent that it decreases at each successive stage.[5]
However, if a
baseball were thrown into a void, it would neither slow down nor keep rising.
It would not rise because no impelling force could be transmitted to it through
(or by) the void; and
it would not fall for the same reason that objects will not have their natural
motion in a void. So there could not be
a body in projectile motion in a void either.
Aristotle's theory of motion
can be conceived proportionally. Since a
void is impossible, all motion is motion through a medium.
The medium explains different
velocities because of its resistance....Thus a body will move through a medium
such as water in a given time, and through a thinner medium such as air in a
different time, namely in proportion to the density of the medium....Hence, if
the air is twice as thin as the water, the body willl move through water in
twice the time it takes to move through air.[6]
This formulation suggests a law (which Aristotle
does not state as an equation, of course):
The velocity of a body is a function of the impelling force and the
resisting force:
V = I/M
where 'I' is a measure of the
impelling force and 'M' a measure of the resistance (or density) of the
medium.
Notice that 'V', the measure
of velocity, is stated as a quantity consisting of a particular distance in a particular
time. The comparison of velocities is thus
quite crude: if the 'velocity' of one
body (on this conception) is twice the velocity of another we can say only that
it will cover twice the distance in a given unit of time or that it will take
half the time to cover a given unit of distance. We can say no more than that.
This proportional law of
motion provides an additional reason why a void is impossible. A void would make this law of motion
incoherent. Since a void would provide
no resistance to the body's motion, the value for 'M' would be zero.
Intuitively, one would think that the fact that the body was moving
against no resistance would make its velocity very high. But Aristotle, correctly, points out that it would make
application of the proportional law of motion impossible, since it would make the denominator of the ratio equal
zero. (Try dividing a number by 0 and
see what your calculator does.)
NOTES