OF SPIRITS.

We have said, on the other side of this page, that the definition of
a spirit is a power conjoined to a body; because it cannot move of
its own accord, nor can it have any kind of motion in space; and if
you were to say that it moves itself, this cannot be within the
elements. For, if the spirit is an incorporeal quantity, this
quantity is called a vacuum, and a vacuum does not exist in nature;
and granting that one were formed, it would be immediately filled up
by the rushing in of the element in which the vacuum had been
generated. Therefore, from the definition of weight, which is
this--Gravity is an accidental power, created by one element being
drawn to or suspended in another--it follows that an element, not
weighing anything compared with itself, has weight in the element
above it and lighter than it; as we see that the parts of water have
no gravity or levity compared with other water, but if you draw it
up into the air, then it would acquire weight, and if you were to
draw the air beneath the water then the water which remains above
this air would acquire weight, which weight could not sustain itself
by itself, whence collapse is inevitable. And this happens in water;
wherever the vacuum may be in this water it will fall in; and this
would happen with a spirit amid the elements, where it would
continuously generate a vacuum in whatever element it might find
itself, whence it would be inevitable that it should be constantly
flying towards the sky until it had quitted these elements.

AS TO WHETHER A SPIRIT HAS A BODY AMID THE ELEMENTS.

We have proved that a spirit cannot exist of itself amid the
elements without a body, nor can it move of itself by voluntary
motion unless it be to rise upwards. But now we will say how such a
spirit taking an aerial body would be inevitably melt into air;
because if it remained united, it would be separated and fall to
form a vacuum, as is said above; therefore it is inevitable, if it
is to be able to remain suspended in the air, that it should absorb
a certain quantity of air; and if it were mingled with the air, two
difficulties arise; that is to say: It must rarefy that portion of
the air with which it mingles; and for this cause the rarefied air
must fly up of itself and will not remain among the air that is
heavier than itself; and besides this the subtle spiritual essence
disunites itself, and its nature is modified, by which that nature
loses some of its first virtue. Added to these there is a third
difficulty, and this is that such a body formed of air assumed by
the spirits is exposed to the penetrating winds, which are
incessantly sundering and dispersing the united portions of the air,
revolving and whirling amidst the rest of the atmosphere; therefore
the spirit which is infused in this