sing 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 often 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 causes of
every affection, whether of sense or not, to be of the following nature,
remembering that we have already distinguished between the nature which
is easy and which is hard to move; fo