en they are in
the place that is proper and natural to them, then they are at rest.
Plants and other living things move with vital movement, in
accordance with the disposition of their nature, but not by
approaching thereto, or by receding from it, for in so far as they
recede from such movement, so far do they recede from their natural
disposition. Heavy and light bodies are moved by an extrinsic force,
either generating them and giving them form, or removing obstacles
from their way. They do not therefore move themselves, as do living
bodies.

Reply Obj. 3: Waters are called living that have a continuous
current: for standing waters, that are not connected with a
continually flowing source, are called dead, as in cisterns and
ponds. This is merely a similitude, inasmuch as the movement they are
seen to possess makes them look as if they were alive. Yet this is
not life in them in its real sense, since this movement of theirs is
not from themselves but from the cause that generates them. The same
is the case with the movement of other heavy and light bodies.
_______________________

SECOND ARTICLE [I, Q. 18, Art. 2]

Whether Life Is an Operation?

Objection 1: It seems that life is an operation. For nothing is
divided except into parts of the same genus. But life is divided by
certain operations, as is clear from the Philosopher (De Anima ii,
13), who distinguishes four kinds of life, namely, nourishment,
sensation, local movement and understanding. Therefore life is an
operation.

Obj. 2: Further, the active life is said to be different from the
contemplative. But the contemplative is only distinguished from the
active by certain operations. Therefore life is an operation.

Obj. 3: Further, to know God is an operation. But this is life,
as is clear from the words of John 18:3, "Now this is eternal life,
that they may know Thee, the only true God." Therefore life is an
operation.

_On the contrary,_ The Philosopher says (De Anima ii, 37), "In living
things, to live is to be."

_I answer that,_ As is clear from what has been said (Q. 17, A. 3),
our intellect, which takes cognizance of the essence of a thing as
its proper object, gains knowledge from sense, of which the proper
objects are external accidents. Hence from external appearances we
come to the knowledge of the essence of things. And because we name a
thing in accordance with our knowledge of it, as is clear from what
has already been said (Q. 13,