and
sometimes, that it is neither heavy nor light. For in his Second Book of
Motion he says, that the fire, being without gravity, ascends upwards,
and the air like to that; the water approaching more to the earth, and
the air to the fire. But in his Physical Arts he inclines to the other
opinion, that the air of itself has neither gravity nor levity.

He says that the air is by nature dark, and uses this as an argument of
its being also the first cold; for that its darkness is opposite to the
brightness, and its coldness to the heat of fire. Moving this in his
First Book of Natural Questions, he again in his treatise of Habits
says, that habits are nothing else but airs; for bodies are contained by
these, and the cause that every one of the bodies contained in any
habit is such as it is, is the containing air, which they call in iron
hardness, in stone solidness, in silver whiteness. These words have in
them much absurdity and contradiction. For if the air remains such as it
is of its own nature, how comes black, in that which is not white, to
be made whiteness; and soft, in that which is not hard, to be made
hardness; and rare, in that which is not thick, to be made thickness?
But if, being mixed with these, it is altered and made like to them, how
is it a habit or power or cause of these things by which it is subdued?
For such a change, by which it loses its own qualities, is the property
of a patient, not of an agent, and not of a thing containing, but of a
thing languishing. Yet they everywhere affirm, that matter, being of its
own nature idle and motionless, is subjected to qualities, and that the
qualities are spirits, which, being also aerial tensions, give a form
and figure to every part of matter to which they adhere. These things
they cannot rationally say, supposing the air to be such as they affirm
it. For if it is a habit and tension, it will assimilate every body to
itself, so that it shall be black and soft. But if by the mixture with
these things it receives forms contrary to those it has, it will be in
some sort the matter, and not the cause or power of matter.

It is often said by Chrysippus, that there is without the world an
infinite vacuum, and that this infinity has neither beginning, middle,
nor end. And by this the Stoics chiefly refute that spontaneous motion
of the atoms downward, which is taught by Epicurus; there not being in
infinity any difference according to which one thing is thought t