Aristotle

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Aristotle ( Greek thought) Because the ideas
of the Greeks of antiquity concerning much of
physics is essentially wrong according to our
modern understanding, we dont want to spend too
much time on them, but it is worthwhile
understanding why it was natural for them to hold
their views. We have to be careful, and often
skeptical, of our own views in physics, even
today - especially with what has happened in the
last century (20th century) in physics. We will
see that we often have to make major adjustments
in our views of what constitutes the basic laws
of nature, and these laws and rules often do
not jive with our intuitive everyday experiences.
We have to get used to new not-so-intuitive
ideas. In addition, the views of Aristotle were
so influential that they more or less controlled
all intellectual activities in what we might
refer to as physics for 2000 years, and as a
result hindered its development. He was quite an
influential character.
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Imaginative Background To comprehend the views
of the Greeks concerning physics (or of any age,
for that matter), we must understand their
imaginative background (our world paradigm). In
addition to our formal backgrounds (usually
acquired by education), every one of us has a
much simpler interpretation of the world, which
we are often unaware of and which will affect the
way we try to understand our physical
surroundings. We may even unconsciously conceal
this view (even from ourselves), but it takes
training to achieve a sophistication, and
objectiveness, in order to deal intuitively and
effectively with many of the more abstract
(formal) concepts of physical theory. The
imaginative background of Aristotle was quite
different than our imaginative background
nowadays. Why is this? Well, the environment was
very different.
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Purposeful motivation For example, there were no
machines present (at least not in our present
understanding), and locomotion was achieved on
foot, or by animals (and slaves performed the
laborious duties that machines do today) that
is, it always included some purposeful
motivation, usually of a living being. It was
not easy to imagine flying objects (planes) or
moving trains which are propelled by physical
principles, where is easy to separate our
thoughts from any purposeful implementation of
the motion. The flying objects were birds, and
they could easily be associated with magical
properties, and a purpose. Our environment
today is full of examples in our material
environment of machines and objects that seem to
be propelled by lifeless technology. Nowadays we
do not image that there is a horse in our car in
spite of our measure of power - horse power.
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What influence does this have on the conceptual
imaginativeness of the observer, even an
interested scientist (we use the word in a
broad sense)? There are two types of motion
important for the Greeks of antiquity (1) the
movement of animals, and (2) the movement of the
heavenly bodies. To the Greeks, except for a
few (e.g. Democritus and Archimedes), it was not
natural for them to imagine a purely mechanical
view of the world when considering motion. We can
see the analogy between animals and machines, but
to the Greeks, the peculiarity of animals is that
they can move of themselves. This suggested
to them, especially to Aristotle, that this was a
mode of movement common to everything that moves.
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Heavenly bodies
In contrast to animal movement (which was
assumed to be part of all movement of objects on
earth), the heavenly bodies moved very regularly.
Therefore, assuming that movement requires some
purpose, it was very natural for them to assume
that the regular movement of the heavenly bodies
was due to the will of some divine being(s)
(Gods) who was responsible for this geometric
simplicity and orderly movement. This is
certainly a simplification, but it is probable
that such beliefs formed part of what we have
termed as their imaginative background, and
influenced their physical picture of the world.
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AristotleBorn 384 BC in Stagirus, Macedonia,
Greece Died 322 BC in
Chalcis, Euboea, Greece

• Aristotle, more than any other thinker,
  determined the orientation and the content of
  Western intellectual history. He was the
  author of a philosophical and scientific system
  that through the centuries became the support
  and vehicle for both medieval Christian and
  Islamic scholastic thought until the end of the
  17th century, Western culture was Aristotelian.
  And, even after the intellectual revolutions of
  centuries to follow, Aristotelian concepts and
  ideas remained embedded in Western thinking.
• Encyclopaedia Britannica
• Find out more at http//www-history.mcs.st-andrew
  s.ac.uk/history/Mathematicians/Aristotle.html

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Aristotle

• The anecdotes related of him reveal him as a
  kindly, affectionate character, and they show
  barely any trace of the self-importance that some
  scholars think they can detect in his works. His
  will, which has been preserved, exhibits the same
  kindly traits he makes references to his happy
  family life and takes solicitous care of his
  children, as well as his servants.
• Encyclopaedia Britannica
• He was a bit of a dandy, wearing rings on his
  fingers and cutting his hair fashionably short.
  He suffered from poor digestion, and is said to
  have been spindle-shanked. He was a good speaker,
  lucid in his lectures, persuasive in
  conversation and he had a mordant wit. His
  enemies, who were numerous, made him out to be
  arrogant and overbearing. ... As a man he was, I
  suspect, admirable rather than amiable.
• J Barnes, Aristotle (Oxford, 1982)

This is a detail from the fresco The School of
Athens by Raphael
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PHUSIS physics The word for physics, in
Aristotle, is the science of what the Greeks
called phusis, which translates as nature.
But the meaning is very different than what we
mean now. Phusis was related to growth and has
a teleological meaning some sort of a purpose.
That is, it is the sake, or the end, for which a
thing exists. This nature is in the form, rather
than in the matter. It involves a potential
becoming, and things have some sort of a
potential nature, which is an internal principle
for existence.
I eat therefore, I walk, hunt, kill, ....
Obvious
I open close that is my natural potential
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PHUSIS physics This view probably comes from
observations in biology, and considering the
imaginative surroundings of the people of this
time, it is not surprising that such views would
develop. This explains the attention given
whether something is natural or unnatural. Even
their word for natural incorporates this idea
of fulfillment of some purpose (such as the
purpose of a seed to become a plant). This
conception of nature which embodies purpose,
creates great problems when we try to develop
what we now call physics. For instance motion
is supposed to be the fulfilling of what
potentially exists. This creates problems right
away, as we will see
Really simple. Its obvious!
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Why do things move?
If something moves from A to B, why does it then
move from B to A? An animal can move an object
from A to B because it has a purpose and this
movement can even be imagined to fulfill the
nature of the animal, or maybe of the object.
The motion of an animal was fulfilling the
nature of the animal, and the motion was
directed by its will. The motion of a lifeless
object is explained with an extension of this
concept. He suggests that the motion of
inanimate objects can be explained by postulating
that things tend to seek their natural place.
What do we mean the natural place? It has to do
with the order of things in the universe water
flows downhill because water want to go down.
Stones move downward more strongly than water
(because stones sink in water). Air goes up
naturally (fire also). He had to make composite
structures to explain things like wood, which
have apparently both earth and air in it. But, as
we will see, in the movement of lifeless objects,
it only creates problems to imagine a purpose, or
end, that belongs to the object which can be used
to describe its motion (or even to describe the
static place where it is). According to
Aristotle, there is also violent motion, which is
motion opposed to natural motion (such as
throwing a stone into the air. The violent motion
brings it up, and the natural motion brings it
down. Nowadays (since Newton) we realize that
gravity brings the stone down, and this is what
Aristotle (and essentially many afterward) would
call natural, although they had no real
conception of gravity as we will explain it.
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Aristotles laws of motion 1.The speed of
falling is proportional to the weight of the
object. Heavier things fall faster. 2.The speed
by which an object falls depends inversely on the
density of the medium it is falling
through. Also he asserted that during violent
motion the speed of the object is directly
proportional to the applied force. We will see
below the fallacy of this. Because of his two
laws of motion Aristotle rejects the void,
because a void would be incompatible with his
notion of how things move. According to
Aristotle, something moves faster the less
resistance it feels (for instance air resistance
or the resistance of water as a medium).
Therefore, it would move infinitely fast in a
vacuum, and this is ridiculous. Note that the
thought of trying to carry out some sort of
experiment, by comparing quantitatively the
movement in different media and then
extrapolating, never occurred to him (or to the
Greeks in general). But we have to give him
credit, he was the first person who thought
quantitatively about movement.
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Aristotles thought experiment heavy things fall
faster
Simple really! Just a little thinking will make
things clear. For 2000 years! - heh-heh -
Big things fall fast Small things fall
slowly. Aristotle said so.
Have they ever thought to do an experiment?
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Aristotles thought experiment about falling in a
medium
I tell you, there can be no void or things
go infinitely fast. Ridiculous!! We dont
need experiments to see that!
Water
Void!!
Air
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Hey Aristotle! Did you see the experiment? Hey!
Comere! Hey - LOOK! Why is that?
A real experiment
Water
Void!!
Air
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Aristotle asserted that during violent motion the
speed of the object is directly proportional to
the applied force.
Constant Velocity
Constant Force
Is this right?
No!
We will see below the fallacy of this (2000
years later!)
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Aristotles view was a geocentric theory of the
universe (were at the center!). There are two
domains to the universe the astronomical
(heavens) and the sublunar (the earth). The
astronomical realm is made of ether (this stuff
is not on the earth), and the earth of earth,
fire, water and air. The natural movement of
objects in the heavens was assumed to take place
on spheres. This was partly because the heavenly
objects were supposed to be divine, and spheres
are perfectly symmetrical shapes, corresponding
to what a God would want. In this way, circular
motion was said to be natural for heavenly
bodies. In actuality, the circular motion
is a continual change in the direction of motion
that requires a force towards the center of the
circle (gravity). Interestingly Aristotle thought
that the heat from the sun was produced by some
sort of friction. This was compatible with
the idea that a void could not exist. The
celestual motion was governed by different rules
than the earthly motions.
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Lets see how this compares to our modern
classical view Aristotle said that things move
at a constant velocity because there is some
action (force) that is keeping the constant
motion (we will not get into the details here).
This is incompatible with the first law of
Newton, that any body will continue in a straight
line with a constant velocity, if left to itself.
We will get to this later, but of course these
early ideas are in accord with what one often
sees everyday, especially at that time.
According to Newtons second law, if a force is
applied to an object, this effects a change in
the velocity (acceleration). You can see the big
difference in the two views. The view of
Aristotle (that a force is required to keep a
constant velocity) involves not only the wrong
objective physical description (which we can
attribute to the inability to understand the
effect of friction) but also involves a deeper
teleological view which ascribes a purposeful
intention for all happenings, including the
movement of rocks and other objects.
Aristotles physics is a very different view
than we have now. But we can se how easy it was
to accept such a view, considering their everyday
environment.
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It is important to remember a very big difference
between Plato and Aristotle Plato thought that
we could arrive at the truth by mathematical
reasoning alone. Aristotle believed that
empirical observations were necessary to
understand what went on in the natural world
(sometimes). We should say that Aristotle was
much more productive, and his ideas much more
useful, in biology. This was Aristotles great
(positive) contribution to science. It was from
biology that both Plato and Aristotle saw the
evidence for a design in nature, which was
controlled by final causes. Each species was to
fit in the right place in the grand scheme of
things.
! only
?. Then, !