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PART FOUR
Part 1
We have now to consider the terms 'heavy'
and 'light'. We must ask what the bodies
so called are, how they are constituted,
and what is the reason of their possessing
these powers. The consideration of these
questions is a proper part of the theory
of movement, since we call things heavy and
light because they have the power of being
moved naturally in a certain way. The activities
corresponding to these powers have not been
given any name, unless it is thought that
'impetus' is such a name. But because the
inquiry into nature is concerned with movement,
and these things have in themselves some
spark (as it were) of movement, all inquirers
avail themselves of these powers, though
in all but a few cases without exact discrimination.
We must then first look at whatever others
have said, and formulate the questions which
require settlement in the interests of this
inquiry, before we go on to state our own
view of the matter.
Language recognizes (a) an absolute, (b)
a relative heavy and light. Of two heavy
things, such as wood and bronze, we say that
the one is relatively light, the other relatively
heavy. Our predecessors have not dealt at
all with the absolute use, of the terms,
but only with the relative. I mean, they
do not explain what the heavy is or what
the light is, but only the relative heaviness
and lightness of things possessing weight.
This can be made clearer as follows. There
are things whose constant nature it is to
move away from the centre, while others move
constantly towards the centre; and of these
movements that which is away from the centre
I call upward movement and that which is
towards it I call downward movement. (The
view, urged by some, that there is no up
and no down in the heaven, is absurd. There
can be, they say, no up and no down, since
the universe is similar every way, and from
any point on the earth's surface a man by
advancing far enough will come to stand foot
to foot with himself. But the extremity of
the whole, which we call 'above', is in position
above and in nature primary. And since the
universe has an extremity and a centre, it
must clearly have an up and down. Common
usage is thus correct, though inadequate.
And the reason of its inadequacy is that
men think that the universe is not similar
every way. They recognize only the hemisphere
which is over us. But if they went on to
think of the world as formed on this pattern
all round, with a centre identically related
to each point on the extremity, they would
have to admit that the extremity was above
and the centre below.) By absolutely light,
then, we mean that which moves upward or
to the extremity, and by absolutely heavy
that which moves downward or to the centre.
By lighter or relatively light we mean that
one, of two bodies endowed with weight and
equal in bulk, which is exceeded by the other
in the speed of its natural downward movement.
Part 2
Those of our predecessors who have entered
upon this inquiry have for the most part
spoken of light and heavy things only in
the sense in which one of two things both
endowed with weight is said to be the lighter.
And this treatment they consider a sufficient
analysis also of the notions of absolute
heaviness, to which their account does not
apply. This, however, will become clearer
as we advance. One use of the terms 'lighter'
and 'heavier' is that which is set forth
in writing in the Timaeus, that the body
which is composed of the greater number of
identical parts is relatively heavy, while
that which is composed of a smaller number
is relatively light. As a larger quantity
of lead or of bronze is heavier than a smaller-and
this holds good of all homogeneous masses,
the superior weight always depending upon
a numerical superiority of equal parts-in
precisely the same way, they assert, lead
is heavier than wood. For all bodies, in
spite of the general opinion to the contrary,
are composed of identical parts and of a
single material. But this analysis says nothing
of the absolutely heavy and light. The facts
are that fire is always light and moves upward,
while earth and all earthy things move downwards
or towards the centre. It cannot then be
the fewness of the triangles (of which, in
their view, all these bodies are composed)
which disposes fire to move upward. If it
were, the greater the quantity of fire the
slower it would move, owing to the increase
of weight due to the increased number of
triangles. But the palpable fact, on the
contrary, is that the greater the quantity,
the lighter the mass is and the quicker its
upward movement: and, similarly, in the reverse
movement from above downward, the small mass
will move quicker and the large slower. Further,
since to be lighter is to have fewer of these
homogeneous parts and to be heavier is to
have more, and air, water, and fire are composed
of the same triangles, the only difference
being in the number of such parts, which
must therefore explain any distinction of
relatively light and heavy between these
bodies, it follows that there must be a certain
quantum of air which is heavier than water.
But the facts are directly opposed to this.
The larger the quantity of air the more readily
it moves upward, and any portion of air without
exception will rise up out of the water.
So much for one view of the distinction between
light and heavy. To others the analysis seems
insufficient; and their views on the subject,
though they belong to an older generation
than ours, have an air of novelty. It is
apparent that there are bodies which, when
smaller in bulk than others, yet exceed them
in weight. It is therefore obviously insufficient
to say that bodies of equal weight are composed
of an equal number of primary parts: for
that would give equality of bulk. Those who
maintain that the primary or atomic parts,
of which bodies endowed with weight are composed,
are planes, cannot so speak without absurdity;
but those who regard them as solids are in
a better position to assert that of such
bodies the larger is the heavier. But since
in composite bodies the weight obviously
does not correspond in this way to the bulk,
the lesser bulk being often superior in weight
(as, for instance, if one be wool and the
other bronze), there are some who think and
say that the cause is to be found elsewhere.
The void, they say, which is imprisoned in
bodies, lightens them and sometimes makes
the larger body the lighter. The reason is
that there is more void. And this would also
account for the fact that a body composed
of a number of solid parts equal to, or even
smaller than, that of another is sometimes
larger in bulk than it. In short, generally
and in every case a body is relatively light
when it contains a relatively large amount
of void. This is the way they put it themselves,
but their account requires an addition. Relative
lightness must depend not only on an excess
of void, but also an a defect of solid: for
if the ratio of solid to void exceeds a certain
proportion, the relative lightness will disappear.
Thus fire, they say, is the lightest of things
just for this reason that it has the most
void. But it would follow that a large mass
of gold, as containing more void than a small
mass of fire, is lighter than it, unless
it also contains many times as much solid.
The addition is therefore necessary.
Of those who deny the existence of a void
some, like Anaxagoras and Empedocles, have
not tried to analyse the notions of light
and heavy at all; and those who, while still
denying the existence of a void, have attempted
this, have failed to explain why there are
bodies which are absolutely heavy and light,
or in other words why some move upward and
others downward. The fact, again, that the
body of greater bulk is sometimes lighter
than smaller bodies is one which they have
passed over in silence, and what they have
said gives no obvious suggestion for reconciling
their views with the observed facts.
But those who attribute the lightness of
fire to its containing so much void are necessarily
involved in practically the same difficulties.
For though fire be supposed to contain less
solid than any other body, as well as more
void, yet there will be a certain quantum
of fire in which the amount of solid or plenum
is in excess of the solids contained in some
small quantity of earth. They may reply that
there is an excess of void also. But the
question is, how will they discriminate the
absolutely heavy? Presumably, either by its
excess of solid or by its defect of void.
On the former view there could be an amount
of earth so small as to contain less solid
than a large mass of fire. And similarly,
if the distinction rests on the amount of
void, there will be a body, lighter than
the absolutely light, which nevertheless
moves downward as constantly as the other
moves upward. But that cannot be so, since
the absolutely light is always lighter than
bodies which have weight and move downward,
while, on the other hand, that which is lighter
need not be light, because in common speech
we distinguish a lighter and a heavier (viz.
water and earth) among bodies endowed with
weight. Again, the suggestion of a certain
ratio between the void and the solid in a
body is no more equal to solving the problem
before us. The manner of speaking will issue
in a similar impossibility. For any two portions
of fire, small or great, will exhibit the
same ratio of solid to void, but the upward
movement of the greater is quicker than that
of the less, just as the downward movement
of a mass of gold or lead, or of any other
body endowed with weight, is quicker in proportion
to its size. This, however, should not be
the case if the ratio is the ground of distinction
between heavy things and light. There is
also an absurdity in attributing the upward
movement of bodies to a void which does not
itself move. If, however, it is the nature
of a void to move upward and of a plenum
to move downward, and therefore each causes
a like movement in other things, there was
no need to raise the question why composite
bodies are some light and some heavy; they
had only to explain why these two things
are themselves light and heavy respectively,
and to give, further, the reason why the
plenum and the void are not eternally separated.
It is also unreasonable to imagine a place
for the void, as if the void were not itself
a kind of place. But if the void is to move,
it must have a place out of which and into
which the change carries it. Also what is
the cause of its movement? Not, surely, its
voidness: for it is not the void only which
is moved, but also the solid.
Similar difficulties are involved in all
other methods of distinction, whether they
account for the relative lightness and heaviness
of bodies by distinctions of size, or proceed
on any other principle, so long as they attribute
to each the same matter, or even if they
recognize more than one matter, so long as
that means only a pair of contraries. If
there is a single matter, as with those who
compose things of triangles, nothing can
be absolutely heavy or light: and if there
is one matter and its contrary-the void,
for instance, and the plenum-no reason can
be given for the relative lightness and heaviness
of the bodies intermediate between the absolutely
light and heavy when compared either with
one another or with these themselves. The
view which bases the distinction upon differences
of size is more like a mere fiction than
those previously mentioned, but, in that
it is able to make distinctions between the
four elements, it is in a stronger position
for meeting the foregoing difficulties. Since,
however, it imagines that these bodies which
differ in size are all made of one substance,
it implies, equally with the view that there
is but one matter, that there is nothing
absolutely light and nothing which moves
upward (except as being passed by other things
or forced up by them); and since a multitude
of small atoms are heavier than a few large
ones, it will follow that much air or fire
is heavier than a little water or earth,
which is impossible.
Part 3
These, then, are the views which have been
advanced by others and the terms in which
they state them. We may begin our own statement
by settling a question which to some has
been the main difficulty-the question why
some bodies move always and naturally upward
and others downward, while others again move
both upward and downward. After that we will
inquire into light and heavy and of the various
phenomena connected with them. The local
movement of each body into its own place
must be regarded as similar to what happens
in connexion with other forms of generation
and change. There are, in fact, three kinds
of movement, affecting respectively the size,
the form, and the place of a thing, and in
each it is observable that change proceeds
from a contrary to a contrary or to something
intermediate: it is never the change of any
chance subject in any chance direction, nor,
similarly, is the relation of the mover to
its object fortuitous: the thing altered
is different from the thing increased, and
precisely the same difference holds between
that which produces alteration and that which
produces increase. In the same manner it
must be thought that produces local motion
and that which is so moved are not fortuitously
related. Now, that which produces upward
and downward movement is that which produces
weight and lightness, and that which is moved
is that which is potentially heavy or light,
and the movement of each body to its own
place is motion towards its own form. (It
is best to interpret in this sense the common
statement of the older writers that 'like
moves to like'. For the words are not in
every sense true to fact. If one were to
remove the earth to where the moon now is,
the various fragments of earth would each
move not towards it but to the place in which
it now is. In general, when a number of similar
and undifferentiated bodies are moved with
the same motion this result is necessarily
produced, viz. that the place which is the
natural goal of the movement of each single
part is also that of the whole. But since
the place of a thing is the boundary of that
which contains it, and the continent of all
things that move upward or downward is the
extremity and the centre, and this boundary
comes to be, in a sense, the form of that
which is contained, it is to its like that
a body moves when it moves to its own place.
For the successive members of the scries
are like one another: water, I mean, is like
air and air like fire, and between intermediates
the relation may be converted, though not
between them and the extremes; thus air is
like water, but water is like earth: for
the relation of each outer body to that which
is next within it is that of form to matter.)
Thus to ask why fire moves upward and earth
downward is the same as to ask why the healable,
when moved and changed qua healable, attains
health and not whiteness; and similar questions
might be asked concerning any other subject
of aletion. Of course the subject of increase,
when changed qua increasable, attains not
health but a superior size. The same applies
in the other cases. One thing changes in
quality, another in quantity: and so in place,
a light thing goes upward, a heavy thing
downward. The only difference is that in
the last case, viz. that of the heavy and
the light, the bodies are thought to have
a spring of change within themselves, while
the subjects of healing and increase are
thought to be moved purely from without.
Sometimes, however, even they change of themselves,
ie. in response to a slight external movement
reach health or increase, as the case may
be. And since the same thing which is healable
is also receptive of disease, it depends
on whether it is moved qua healable or qua
liable to disease whether the motion is towards
health or towards disease. But the reason
why the heavy and the light appear more than
these things to contain within themselves
the source of their movements is that their
matter is nearest to being. This is indicated
by the fact that locomotion belongs to bodies
only when isolated from other bodies, and
is generated last of the several kinds of
movement; in order of being then it will
be first. Now whenever air comes into being
out of water, light out of heavy, it goes
to the upper place. It is forthwith light:
becoming is at an end, and in that place
it has being. Obviously, then, it is a potentiality,
which, in its passage to actuality, comes
into that place and quantity and quality
which belong to its actuality. And the same
fact explains why what is already actually
fire or earth moves, when nothing obstructs
it, towards its own place. For motion is
equally immediate in the case of nutriment,
when nothing hinders, and in the case of
the thing healed, when nothing stays the
healing. But the movement is also due to
the original creative force and to that which
removes the hindrance or off which the moving
thing rebounded, as was explained in our
opening discussions, where we tried to show
how none of these things moves itself. The
reason of the various motions of the various
bodies, and the meaning of the motion of
a body to its own place, have now been explained.
Part 4
We have now to speak of the distinctive properties
of these bodies and of the various phenomena
connected with them. In accordance with general
conviction we may distinguish the absolutely
heavy, as that which sinks to the bottom
of all things, from the absolutely light,
which is that which rises to the surface
of all things. I use the term 'absolutely',
in view of the generic character of 'light'
and 'heavy', in order to confine the application
to bodies which do not combine lightness
and heaviness. It is apparent, I mean, that
fire, in whatever quantity, so long as there
is no external obstacle moves upward, and
earth downward; and, if the quantity is increased,
the movement is the same, though swifter.
But the heaviness and lightness of bodies
which combine these qualities is different
from this, since while they rise to the surface
of some bodies they sink to the bottom of
others. Such are air and water. Neither of
them is absolutely either light or heavy.
Both are lighter than earth-for any portion
of either rises to the surface of it-but
heavier than fire, since a portion of either,
whatever its quantity, sinks to the bottom
of fire; compared together, however, the
one has absolute weight, the other absolute
lightness, since air in any quantity rises
to the surface of water, while water in any
quantity sinks to the bottom of air. Now
other bodies are severally light and heavy,
and evidently in them the attributes are
due to the difference of their uncompounded
parts: that is to say, according as the one
or the other happens to preponderate the
bodies will be heavy and light respectively.
Therefore we need only speak of these parts,
since they are primary and all else consequential:
and in so doing we shall be following the
advice which we gave to those whose attribute
heaviness to the presence of plenum and lightness
to that of void. It is due to the properties
of the elementary bodies that a body which
is regarded as light in one place is regarded
as heavy in another, and vice versa. In air,
for instance, a talent's weight of wood is
heavier than a mina of lead, but in water
the wood is the lighter. The reason is that
all the elements except fire have weight
and all but earth lightness. Earth, then,
and bodies in which earth preponderates,
must needs have weight everywhere, while
water is heavy anywhere but in earth, and
air is heavy when not in water or earth.
In its own place each of these bodies has
weight except fire, even air. Of this we
have evidence in the fact that a bladder
when inflated weighs more than when empty.
A body, then, in which air preponderates
over earth and water, may well be lighter
than something in water and yet heavier than
it in air, since such a body does not rise
in air but rises to the surface in water.
The following account will make it plain
that there is an absolutely light and an
absolutely heavy body. And by absolutely
light I mean one which of its own nature
always moves upward, by absolutely heavy
one which of its own nature always moves
downward, if no obstacle is in the way. There
are, I say, these two kinds of body, and
it is not the case, as some maintain, that
all bodies have weight. Different views are
in fact agreed that there is a heavy body,
which moves uniformly towards the centre.
But is also similarly a light body. For we
see with our eyes, as we said before, that
earthy things sink to the bottom of all things
and move towards the centre. But the centre
is a fixed point. If therefore there is some
body which rises to the surface of all things-and
we observe fire to move upward even in air
itself, while the air remains at rest-clearly
this body is moving towards the extremity.
It cannot then have any weight. If it had,
there would be another body in which it sank:
and if that had weight, there would be yet
another which moved to the extremity and
thus rose to the surface of all moving things.
In fact, however, we have no evidence of
such a body. Fire, then, has no weight. Neither
has earth any lightness, since it sinks to
the bottom of all things, and that which
sinks moves to the centre. That there is
a centre towards which the motion of heavy
things, and away from which that of light
things is directed, is manifest in many ways.
First, because no movement can continue to
infinity. For what cannot be can no more
come-to-be than be, and movement is a coming
to-be in one place from another. Secondly,
like the upward movement of fire, the downward
movement of earth and all heavy things makes
equal angles on every side with the earth's
surface: it must therefore be directed towards
the centre. Whether it is really the centre
of the earth and not rather that of the whole
to which it moves, may be left to another
inquiry, since these are coincident. But
since that which sinks to the bottom of all
things moves to the centre, necessarily that
which rises to the surface moves to the extremity
of the region in which the movement of these
bodies takes place. For the centre is opposed
as contrary to the extremity, as that which
sinks is opposed to that which rises to the
surface. This also gives a reasonable ground
for the duality of heavy and light in the
spatial duality centre and extremity. Now
there is also the intermediate region to
which each name is given in opposition to
the other extreme. For that which is intermediate
between the two is in a sense both extremity
and centre. For this reason there is another
heavy and light; namely, water and air. But
in our view the continent pertains to form
and the contained to matter: and this distinction
is present in every genus. Alike in the sphere
of quality and in that of quantity there
is that which corresponds rather to form
and that which corresponds to matter. In
the same way, among spatial distinctions,
the above belongs to the determinate, the
below to matter. The same holds, consequently,
also of the matter itself of that which is
heavy and light: as potentially possessing
the one character, it is matter for the heavy,
and as potentially possessing the other,
for the light. It is the same matter, but
its being is different, as that which is
receptive of disease is the same as that
which is receptive of health, though in being
different from it, and therefore diseasedness
is different from healthiness.
Part 5
A thing then which has the one kind of matter
is light and always moves upward, while a
thing which has the opposite matter is heavy
and always moves downward. Bodies composed
of kinds of matter different from these but
having relatively to each other the character
which these have absolutely, possess both
the upward and the downward motion. Hence
air and water each have both lightness and
weight, and water sinks to the bottom of
all things except earth, while air rises
to the surface of all things except fire.
But since there is one body only which rises
to the surface of all things and one only
which sinks to the bottom of all things,
there must needs be two other bodies which
sink in some bodies and rise to the surface
of others. The kinds of matter, then, must
be as numerous as these bodies, i. e. four,
but though they are four there must be a
common matter of all-particularly if they
pass into one another-which in each is in
being different. There is no reason why there
should not be one or more intermediates between
the contraries, as in the case of colour;
for 'intermediate' and 'mean' are capable
of more than one application.
Now in its own place every body endowed with
both weight and lightness has weightwhereas
earth has weight everywhere-but they only
have lightness among bodies to whose surface
they rise. Hence when a support is withdrawn
such a body moves downward until it reaches
the body next below it, air to the place
of water and water to that of earth. But
if the fire above air is removed, it will
not move upward to the place of fire, except
by constraint; and in that way water also
may be drawn up, when the upward movement
of air which has had a common surface with
it is swift enough to overpower the downward
impulse of the water. Nor does water move
upward to the place of air, except in the
manner just described. Earth is not so affected
at all, because a common surface is not possible
to it. Hence water is drawn up into the vessel
to which fire is applied, but not earth.
As earth fails to move upward, so fire fails
to move downward when air is withdrawn from
beneath it: for fire has no weight even in
its own place, as earth has no lightness.
The other two move downward when the body
beneath is withdrawn because, while the absolutely
heavy is that which sinks to the bottom of
all things, the relatively heavy sinks to
its own place or to the surface of the body
in which it rises, since it is similar in
matter to it.
It is plain that one must suppose as many
distinct species of matter as there are bodies.
For if, first, there is a single matter of
all things, as, for instance, the void or
the plenum or extension or the triangles,
either all things will move upward or all
things will move downward, and the second
motion will be abolished. And so, either
there will be no absolutely light body, if
superiority of weight is due to superior
size or number of the constituent bodies
or to the fullness of the body: but the contrary
is a matter of observation, and it has been
shown that the downward and upward movements
are equally constant and universal: or, if
the matter in question is the void or something
similar, which moves uniformly upward, there
will be nothing to move uniformly downward.
Further, it will follow that the intermediate
bodies move downward in some cases quicker
than earth: for air in sufficiently large
quantity will contain a larger number of
triangles or solids or particles. It is,
however, manifest that no portion of air
whatever moves downward. And the same reasoning
applies to lightness, if that is supposed
to depend on superiority of quantity of matter.
But if, secondly, the kinds of matter are
two, it will be difficult to make the intermediate
bodies behave as air and water behave. Suppose,
for example, that the two asserted are void
and plenum. Fire, then, as moving upward,
will be void, earth, as moving downward,
plenum; and in air, it will be said, fire
preponderates, in water, earth. There will
then be a quantity of water containing more
fire than a little air, and a large amount
of air will contain more earth than a little
water: consequently we shall have to say
that air in a certain quantity moves downward
more quickly than a little water. But such
a thing has never been observed anywhere.
Necessarily, then, as fire goes up because
it has something, e. g. void, which other
things do not have, and earth goes downward
because it has plenum, so air goes to its
own place above water because it has something
else, and water goes downward because of
some special kind of body. But if the two
bodies are one matter, or two matters both
present in each, there will be a certain
quantity of each at which water will excel
a little air in the upward movement and air
excel water in the downward movement, as
we have already often said.
Part 6
The shape of bodies will not account for
their moving upward or downward in general,
though it will account for their moving faster
or slower. The reasons for this are not difficult
to see. For the problem thus raised is why
a flat piece of iron or lead floats upon
water, while smaller and less heavy things,
so long as they are round or long-a needle,
for instance-sink down; and sometimes a thing
floats because it is small, as with gold
dust and the various earthy and dusty materials
which throng the air. With regard to these
questions, it is wrong to accept the explanation
offered by Democritus. He says that the warm
bodies moving up out of the water hold up
heavy bodies which are broad, while the narrow
ones fall through, because the bodies which
offer this resistance are not numerous. But
this would be even more likely to happen
in air-an objection which he himself raises.
His reply to the objection is feeble. In
the air, he says, the 'drive' (meaning by
drive the movement of the upward moving bodies)
is not uniform in direction. But since some
continua are easily divided and others less
easily, and things which produce division
differ similarly in the case with which they
produce it, the explanation must be found
in this fact. It is the easily bounded, in
proportion as it is easily bounded, which
is easily divided; and air is more so than
water, water than earth. Further, the smaller
the quantity in each kind, the more easily
it is divided and disrupted. Thus the reason
why broad things keep their place is because
they cover so wide a surface and the greater
quantity is less easily disrupted. Bodies
of the opposite shape sink down because they
occupy so little of the surface, which is
therefore easily parted. And these considerations
apply with far greater force to air, since
it is so much more easily divided than water.
But since there are two factors, the force
responsible for the downward motion of the
heavy body and the disruption-resisting force
of the continuous surface, there must be
some ratio between the two. For in proportion
as the force applied by the heavy thing towards
disruption and division exceeds that which
resides in the continuum, the quicker will
it force its way down; only if the force
of the heavy thing is the weaker, will it
ride upon the surface.
We have now finished our examination of the
heavy and the light and of the phenomena
connected with them.
End of Book Four
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Evans Experientialism 
