Philosophy 226f: Philosophy of Science

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Philosophy 226f Philosophy of Science Prof.
Robert DiSalle (rdisalle_at_uwo.ca) Talbot College
408, 519-661-2111 x85763 Course Website
http//instruct.uwo.ca/philosophy/226f/ New
Link! Some advice about the essays
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The maximum angular elongation of the planets
with respect to the sun Mercury 28 degrees
Venus 46 degrees All other planets 180 degrees
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Angular elongation of Venus and Mercury according
to Copernicus
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The order of the planets according to Ptolemy
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The order of the planets according to Copernicus
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Given a big enough baseline relative to the
object you are looking at, parallax ought to be
detectable. If the heliocentric model is correct,
why dont we observe stellar parallax?
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Copernican astronomy in infinite space
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Assumptions common to Ptolemy and Copernicus
The basic empirical facts Relative positions
of stars on the celestial sphere (directions
and angular separations) positions of the
planets relative to the positions of the stars
the earth is a sphere that is (effectively)
infinitely small in comparison with the size of
the heavens Basic motions daily revolution of
the celestial sphere, and day to day
displacements of the sun, moon, and planets
relative to the fixed stars The basic technique
of astronomy to give approximate predictions of
planetary motions by circular motions and
combinations of circular motions
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The theoretical heritage of Aristotle Basic
physical assumption All natural change is
teleological, or directed toward a certain end
(telos). Local motion is just another kind of
natural change. Natural motion expresses the
essence or form of the moving body. No mechanical
causal explanation is required of its
persistence. Eg. It is in the nature of the
celestial spheres to rotate on their
axes. Violent motion Results from interference
by some agent with the natural state of things.
Always requires some causal agency to maintain
it.
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Scientific arguments against the rotation of the
earth Given the size of the earths orbit, we
ought to observe stellar parallax. Why dont
we? Copernicus The sphere of fixed stars is
immensely larger than previously thought. Why do
all celestial bodies seem to orbit the earth? Why
does the earth appear to be the unique centre of
all celestial and terrestrial motion? Copernicus
Venus and Mercury seem to follow the motion of
the sun, and so suggest that the sun is at least
another centre of motion.
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If the earth is rotating, there must be a
powerful centrifugal force. Why are no effects
noticeable? Copernicus the earths motion is
natural and uniform, and natural motions never
result in violent effects. Rotation is the
natural motion that accords with the earths
form. Why dont projectiles and falling bodies
fall behind the earths motion? Copernicus
bodies share in the natural motion of the
earth, and so they keep up with its
rotation What makes bodies fall toward the
earth, if it is not in the center? Copernicus
Bodies naturally seek to unite with other bodies
of the same nature. Parts of earth naturally
return to earth.
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Galileo, by Tintoretto
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Galileos telescope
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What Galileo saw on the moon
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More views of the moon
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Galileos conflict with the Church Can questions
about the natural world be settled by the Bible?
Or must they be settled by the best empirical and
mathematical methods of science? Are we free to
investigate nature, or must we be constrained by
religious authority? The book of God The history
of God and his creation, written in a language
for general human comprehension The book of
Nature The natural world itself, revealing its
laws in the phenomena-- --but written in the
language of mathematics.
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Philosophy is written in this grand book, the
universe, which stands continually open to our
gaze. But this book cannot be understood unless
one first learns to comprehend the language and
read the letters in which it is composed. It is
written in the language of mathematics, and its
characters are triangles, circles, and other
geometric figures without which it is humanly
impossible to understand a single word of it
without these, one wanders about in a dark
labyrinth. (Galileo)
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Galileos Abjuration before the Inquisition, 1633
because I have been enjoined, by this Holy
Office, altogether to abandon the false opinion
which maintains that the sun is the center and
immovable, and forbidden to hold, defend, or
teach, the said false doctrine in any manner
therefore, with a sincere heart and unfeigned
faith, I abjure, curse, and detest the said
errors and heresies, and generally every other
error and sect contrary to the said Holy Church
and I swear that I will never more in future say,
or assert anything, verbally nor in writing,
which may give rise to a similar suspicion of me
but that if I shall know any heretic, or any one
suspected of heresy, I will denounce him to this
Holy Office, or to the Inquisitor and Ordinary of
the place in which I may be.                      
                             
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Galileo on science and Scripture I think it
would be the part of wisdom not to allow any one
to apply passages of Scripture in such a way as
to force them to support as true any conclusions
concerning nature, the contrary of which may
afterwards be revealed by the evidence of our
senses, or by actual demonstration. Who will set
bounds to man's understanding ? Who can assure us
that every thing that can be known in the world
is known already ?
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Galileo on science and Scripture (continued) . .
. I am inclined to think that Holy Scripture is
intended to convince men of those truths which
are necessary for their salvation, and which
being far above man's understanding cannot be
made credible by any learning, or by any other
means than revelation. But that the same God who
has endowed us with senses, reason, and
understanding, does not permit us to use them,
and desires to acquaint us in another way with
such knowledge as we are in a position to acquire
for ourselves by means of those faculties that,
it seems to me I am not bound to believe
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Galileo vs. the Aristotelians on method Science
can never appeal to the authority of any text, no
matter how great the philosopher who wrote
it. Aristotle himself did not accept the
authority of anyone, but judged for himself the
opinions of his predecessors. In matters of
natural science, Aristotle himself never appealed
to authority, but relied on the evidence of his
senses. If Aristotle were alive in Galileos
time, he would not believe what he wrote in 350
BCE, but would revise his views on the basis of
all the accumulated evidence.
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Galileo vs. the Aristotelians on the natural
world The earth is not unique in the universe,
and the celestial bodies are made of the same
basic stuff. Falling to the earth is not a simple
and natural motion, but a compound of two
motions, horizontal and vertical. Because motions
can be combined, we cannot tell whether the earth
is moving by observing bodies falling on
it. Speed of falling doesnt depend on the weight
of the body, but is the same for all bodies.
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Galileos new approach to physics Aristotle
Natural motions are always simple, and in accord
with the essential nature of the body. Galileo
Different natural motions may be combined in the
same body. Example A body may fall to the earth
while simultaneously continuing in horizontal
motion. Aristotle The natural tendency of any
heavy body is toward the centre of the
universe. Galileo The natural tendency of any
heavy body is to continue moving uniformly,
unless compelled to change its motion by gravity
or any other external influence.
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Composition of uniform and accelerated motion
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Aristotle Forces cause bodies to move. Galileo
Forces cause bodies to accelerate. Aristotle
The speed of a falling body is proportional to
its weight. Galileo All bodies naturally fall
at the same speed, which is uniformly
accelerated. (Acceleration9.8m/sec/sec, i.e. a
body increases its velocity by 9.8 meters per
second, every second.) Observed differences in
rate of fall are explained by the density of the
medium. In a vacuum, a stone and a feather would
fall at the same rate.
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The relativity of motion (Galilean
relativity) Because any motion of a body may be
combined with a uniform motion, a uniform motion
of an entire system of bodies will not be
noticeable from within the system. Since forces
are known by the accelerations they cause, they
will have the same effects in a moving system as
in a system at rest. Example The motions of
bodies in a smoothly-moving ship are not
noticeably influenced by the motion of the ship,
in which they all share. Example The motions
of bodies on and around the earth are combined
with the (almost) uniform motion of the earths
rotation. Thus the rotation is unnoticed.
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Why dropping a stone from a stationary tower is
like dropping a stone from the mast of a moving
ship
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The lesson of falling bodies We must rely on our
senses, but it is a mistake to take what they
tell us at face value. The stone falls straight
from the top of the tower to the bottom that
is not a statement of fact! It is an
interpretation of what we see. If the earth is at
rest, the stone falls in a straight line. But if
the earth is moving, the stone is falling in a
parabola. Science requires the combination of
observation with mathematical reasoning.
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What did Galileo really explain? Why do bodies
accelerate as they approach the
earth? Aristotle Because they are approaching
that place in the universe that is appropriate to
their natures Galileo Because a falling body
accelerates uniformly, following the law that
distance is proportional to the square of the
time (D ? T2)
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Why do projectiles follow a parabolic path?
Aristotelians Because initially, the violent
motion is dominant finally, the natural motion
of the body toward the center of the universe
dominates in between, there is a struggle
between the violent and the natural tendencies.
(Not precisely a parabola, or any other curve.
But who said science is precise?) Galileo
Because natural and forced motions are smoothly
and simply combined the bodys natural
horizontal motion is proportional to time (DH
?T) and its vertical motion due to gravity is
uniformly accelerated and therefore proportional
to the square of the time (DV ? T2). Therefore
vertical motion is proportional to the square of
the horizontal motion (DV ? DH2). But X ? Y2 is,
in general, the equation for a parabola.
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Projectile motion in a parabolic trajectory
DV
DH
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In general, why do bodies in motion tend to
persist in motion? Aristotle As a matter of
fact, they dont tend to persist the little
persistence that they do manage can be explained
by the medium. Galileo Assume that they do
persist in motion then explain what forces are
cause them to speed up, slow down, or change
direction. What keeps the planets in
motion? Aristotle Planets are carried by
spheres, each of which executes the rotational
motion that is naturally suited to its spherical
form. Galileo Suppose that the planets
naturally persist in their rectilinear motion.
Explain their curved orbits by the forces that
cause them to deviate from the straight path.