timaeus   

parted into two regions, separate from and opposite to each other, the
one a lower to which all things tend which have any bulk, and an upper
to which things only ascend against their will. For as the universe is
in the form of a sphere, all the extremities, being equidistant from
the centre, are equally extremities, and the centre, which is
equidistant from them, is equally to be regarded as the opposite of
them all. Such being the nature of the world, when a person says
that any of these points is above or below, may he not be justly
charged with using an improper expression? For the centre of the world
cannot be rightly called either above or below, but is the centre
and nothing else; and the circumference is not the centre, and has
in no one part of itself a different relation to the centre from
what it has in any of the opposite parts. Indeed, when it is in
every direction similar, how can one rightly give to it names which
imply opposition? For if there were any solid body in equipoise at the
centre of the universe, there would be nothing to draw it to this
extreme rather than to that, for they are all perfectly similar; and
if a person were to go round the world in a circle, he would often,
when standing at the antipodes of his former position, speak of the
same point as above and below; for, as I was saying just now, to speak
of the whole which is in the form of a globe as having one part
above and another below is not like a sensible man.
The reason why these names are used, and the circumstances under
which they are ordinarily applied by us to the division of the
heavens, may be elucidated by the following supposition:-if a person
were to stand in that part of the universe which is the appointed
place of fire, and where there is the great mass of fire to which
fiery bodies gather-if, I say, he were to ascend thither, and,
having the power to do this, were to abstract particles of fire and
put them in scales and weigh them, and then, raising the balance, were
to draw the fire by force towards the uncongenial element of the
air, it would be very evident that he could compel the smaller mass
more readily than the larger; for when two things are simultaneously
raised by one and the same power, the smaller body must necessarily
yield to the superior power with less reluctance than the larger;
and the larger body is called heavy and said to tend downwards, and
the smaller body is called light and said to tend upwards. And we
may detect ourselves who are upon the earth doing precisely the same
thing. For we of separate earthy natures, and sometimes earth
itself, and draw them into the uncongenial element of air by force and
contrary to nature, both clinging to their kindred elements. But
that which is smaller yields to the impulse given by us towards the
dissimilar element more easily than the larger; and so we call the
former light, and the place towards which it is impelled we call
above, and the contrary state and place we call heavy and below
respectively. Now the relations of these must necessarily vary,
because the principal masses of the different elements hold opposite
positions; for that which is light, heavy, below or above in one place
will be found to be and become contrary and transverse and every way
diverse in relation to that which is light, heavy, below or above in
an opposite place. And about all of them this has to be
considered:-that the tendency of each towards its kindred element
makes the body which is moved heavy, and the place towards which the
motion tends below, but things which have an opposite tendency we call
by an opposite name. Such are the causes which we assign to these
phenomena. As to the smooth and the rough, any one who sees them can
explain the reason of them to another. For roughness is hardness
mingled with irregularity, and smoothness is produced by the joint
effect of uniformity and density.
The most important of the affections which concern the whole body
remains to be considered-that is, the cause of pleasure and pain in
the perceptions of which I have been speaking, and in all other things
which are perceived by sense through the parts of the body, and have
both pains and pleasures attendant on them. Let us imagine the