Aristotle
supposition, there cannot be perpetual increase in the intensity or
velocity of the motion (the increase has an impassable limit), and
therefore there cannot be perpetual relaxation; for both are on the
same footing (s. 5).
It might be urged, indeed, that intensification and relaxation go on
alternately; each proceeding to a certain length, and then giving
place to the other. But this is altogether irrational--nothing better
than a gratuitous fiction. Besides, if there were this alternation,
we may reasonably assume that it could not remain concealed from us;
for contrasting conditions coming in immediate sequence to each other
are more easily discerned by sense. What has been said, then, is
sufficient to prove--That the Heaven or Cosmos is one and only one;
that it is ungenerable and eternal; that its motion is uniform (s.
6).
CH. 7.--Next in order, I have to speak of what are called the Stars
([Greek: tô=n kaloume/nôn a)/strôn]). Of what are they composed?
What is their figure? What are their motions?
It is consistent with the foregoing reasonings, as well as in itself
the most rational doctrine, to conceive each of the stars as composed
of portions of that body in which its current of motion takes place;
that is, of that body, whose nature it is to move in a circle. For
those who affirm the stars to be fire say this because they believe
the upper body to be fire, assuming it as reasonable that each thing
should be composed of the elements in which it is; and I assume the
same also (s. 1). The heat and light of the stars arises from their
friction with the air in their current of motion. If it is the nature
of motion to inflame pieces of wood, and stones, and iron, it is
still more reasonable that what is nearest to fire (that is, air)
should be so inflamed. We see that darts projected are so inflamed,
that their leaden appendages are melted; and, these being thus
inflamed, the air around them must be modified in the same manner.
Now objects like these darts are thus violently heated, because they
are carried along in the medium of the air, which through the shock
given by their motion becomes fire. But each of the upper bodies or
stars is carried round (not in the air, but) in its appropriate
sphere, so that they themselves are not inflamed; while the air which
is under the sphere of the encyclical body becomes of necessity
heated by the rotation of that sphere; and most of all at the point
where the Sun has happened to be fastened in ([Greek: kai\ tau/tê|
ma/lista, ê(=| o( ê(/lios tetu/chêken e)ndedeme/nos]).
Let it then be understood, that the stars are neither composed of
fire, nor are they carried round in the medium of the fire (s. 2).
CH. 8.--It is seen as a fact, that both the stars, and the entire
Heaven, change their place ([Greek: methista/mena]). Now, in this
change, we must assume either that both continue at rest, or that
both are in motion, or that one is at rest, and the other is in
motion. Now it is impossible that both can be at rest, at least if we
assume the earth to be at rest; for the facts which we see would not
have taken place, upon that supposition (s. 1). Either therefore both
are in motion, or one is in motion and the other at rest. Now, if
both are in motion, it is against reason that the stars and the
circles in which they are fastened should have equal velocities of
motion. Each one of them must, be equal in velocity to the circle or
sphere in which it is carried, since all come back round along with
their circles to the same position; so that in one and the same time,
the star has gone round its circle, and the circle has completed its
revolution. It is not reasonable to suppose that the velocities of
the stars and the magnitudes of the circles should be in the same
proportion. Comparing one circle with another, indeed, it is not only
not absurd, but even necessary, that the velocities should be in
proportion to the magnitudes; but it is not reasonable that each of
the stars in these circles should be of such velocity. For, if it be
necessary that what is carried round in the larger circle should have
the greater velocity, the consequence would be that, if the stars in
one circle were transferred to another, their motions would become
accelerated or retarded; which is equivalent to saying that they have
no motion of their own at all, but are carried round by the
revolution of the circles (s. 2). If, on the contrary, it be not
necessary, but a spontaneous coincidence ([Greek: ei)/te a)po\
tau)toma/tou sune/pesen]) that what is carried round in the greater
circle has the greater velocity, neither upon this supposition is it
reasonable that in all the circles without exception the
circumference should be greater, and the motion of the star fastened
in the greater circle quicker, in the same proportion. That this
should happen with one or two of them, might be reasonably expected;
but that it should happen with all alike, savours of fiction.
Moreover chance has no place in matters according to nature; nor is
that which occurs everywhere and belongs to all, ever the produce of
chance (s. 3).
So much for the hypothesis, that both stars and circles are in
motion. Let us now assume that one is at rest, and the other in
motion; and first, let the circles be at rest, and the stars in
motion. This again will lead to absurdities; for we shall still be
unable to explain how it happens that the outermost stars are moved
most quickly, and that their velocities are proportioned to the
magnitudes of the circles.
Since then we cannot assume either that both are moved, or that the
star alone is moved, we must adopt the third supposition, that the
circles are moved, and that the stars, being themselves at rest, are
fastened in the circles and carried round along with them. This is
the only hypothesis which entails no unreasonable consequences. For
it is reasonable that, of circles fastened round the same centre, the
greater velocity should belong to the greatest. For, as in all the
varieties of body the heavier fragment is carried with greater
velocity than the lighter in its appropriate motion, so it happens
with the encyclical body. When two straight lines are drawn from the
centre, the segment of the greater circle intercepted between them
will be greater than the segment of the smaller; and it is consistent
with reason that the greater circle should be carried round in equal
time. This is one reason why the Kosmos is not split into separate
parts; another reason is, because the universe has been shown to be
continuous (s. 4, 5).
Now we all agree that the stars are of spherical figure: and
spherical bodies have two motions of their own--rolling and rotatory
([Greek: ku/lisis kai\ di/nêsis]). If they were moved of themselves,
they would be moved in one or other of these two ways; but we see
that they are so in neither. They do not rotate; for, if they did,
they would remain always in the same place, which contradicts
universal observation and belief. Besides, it is reasonable to
suppose that all the stars move in the same manner, but the Sun is
the only one that is seen so to move, when he rises or sets; and he
too, not by any movement of his own, but through the distance of our
vision, which when stretched to a great distance, rotates from
weakness (s. 6). This is perhaps the reason why the stars fastened
(in the outer sphere) twinkle, while the planets do not twinkle; for
the planets are near to us, so that our vision reaches them while yet
strong; whereas in regard to the unmoved stars it is made to quiver
in consequence of the great distance from being stretched out too
far, and its quivering causes the appearance of motion in the star.
For there is no difference between moving the vision and moving the
object seen ([Greek: ou)the\n ga\r diaphe/rei kinei=n tê\n o)/psin
ê)\ to\ o(rô/menon]--s. 6).
Again, neither do the stars roll nor revolve forward. For that which
rolls forward must necessarily turn round; but the same side of the
moon--what is called the face of the moon--is always clearly visible
to us (s. 7).
Since it is reasonable to believe, therefore, that, if the stars were
moved in themselves, they would be moved in their own special variety
of motion (_i.e._, rolling or rotatory), and since it has been shown
that they are not moved in either of these two ways, we see plainly
that they cannot be moved in themselves (but are carried round in the
revolution of the Aplanês).
Besides, if they were moved in **themselves, it is unreasonable that
Nature should have assigned to them no organ suitable for motion,
since Nature does nothing by haphazard; and that she should have been
considerate in providing for animals, while she overlooked objects so
honourable as the stars. The truth rather is, that she has withheld
from them, as it were by express purpose, all aids, through which it
was possible for them to advance forward in themselves, and has
placed them at the greatest possible distance from objects furnished
with organs for motion (s. 8).
Hence it would seem to be the reasonable doctrine--That the entire
Heaven is spherical, and that each of the stars (fastened in it) is
also spherical. For the sphere is the most convenient of all figures
for motion in the same place, so that the Heaven being spherical
would be moved most rapidly and would best maintain its own place.
But for forward motion the sphere is of all figures the most
inconvenient; for it least resembles self-moving bodies; it has no
outlying appendage or projecting end, as rectilinear figures have,
and stands farthest removed from the figures of marching bodies.
Since therefore it is the function of ([Greek: dei=]) the Heaven to
be moved by a motion in the same place ([Greek: kinei=sthai tê\n e)n
au(tô=| ki/nêsin]), and that of the stars not to make any advance
by themselves ([Greek: ta\ a)/lla d' a)/stra mê\ proi+e/nai di'
au(tô=n]), it is with good reason that both of them are spherical.
For thus will the Heaven best be moved, and the stars will best be at
rest.
CH. 9.--From what I have said, it is plain that those who affirm that
the revolving celestial bodies emit in their revolutions sounds
harmonious to each other, speak cleverly and ingeniously, but not
consistently with the truth. There must necessarily be sound (they
say) from the revolution of such vast bodies. Since bodies near to us
make sound in motion, the sun, moon, and stars, being so much larger
and moving with so much greater velocity, must make an immense sound;
and, since their distances and velocities are assumed to be in
harmonic proportion, the sounds emitted in their revolution must also
be in harmony. To the question put to them--Why do we not hear this
immense sound? they reply, that we have been hearing it constantly
from the moment of our birth; that we have no experience of an
opposite state, or state of silence, with which to contrast it, and
that sound and silence are discriminated only by relation to each
other ([Greek: ô(/ste mê\ dia/dêlon ei)=nai pro\s tê\n e)nanti/an
sigê/n; pro\s a)/llêla ga\r phônê=s kai\ sigê=s ei)=nai tê\n
dia/gnôsin]); that men thus cease to be affected by it, just as
blacksmiths from constant habit cease to be affected by the noise of
their own work (s. 1).
The reasoning of these philosophers (the Pythagoreans), as I have
just said, is graceful and poetical, yet nevertheless inadmissible.
For they ought to explain, upon their hypothesis, not merely why we
hear nothing, but why we experience no uncomfortable impressions
apart from hearing. For prodigious sounds pierce through and destroy
the continuity even of inanimate bodies; thus thunder splits up
stones and other bodies of the greatest strength. The impression