A. 1), so from external properties
names are often imposed to signify essences. Hence such names are
sometimes taken strictly to denote the essence itself, the
signification of which is their principal object; but sometimes, and
less strictly, to denote the properties by reason of which they are
imposed. And so we see that the word "body" is used to denote a genus
of substances from the fact of their possessing three dimensions: and
is sometimes taken to denote the dimensions themselves; in which
sense body is said to be a species of quantity. The same must be said
of life. The name is given from a certain external appearance,
namely, self-movement, yet not precisely to signify this, but rather
a substance to which self-movement and the application of itself to
any kind of operation, belong naturally. To live, accordingly, is
nothing else than to exist in this or that nature; and life signifies
this, though in the abstract, just as the word "running" denotes "to
run" in the abstract.

Hence "living" is not an accidental but an essential predicate.
Sometimes, however, life is used less properly for the operations from
which its name is taken, and thus the Philosopher says (Ethic. ix, 9)
that to live is principally to sense or to understand.

Reply Obj. 1: The Philosopher here takes "to live" to mean an
operation of life. Or it would be better to say that sensation and
intelligence and the like, are sometimes taken for the operations,
sometimes for the existence itself of the operator. For he says
(Ethic. ix, 9) that to live is to sense or to understand--in other
words, to have a nature capable of sensation or understanding. Thus,
then, he distinguishes life by the four operations mentioned. For in
this lower world there are four kinds of living things. It is the
nature of some to be capable of nothing more than taking nourishment,
and, as a consequence, of growing and generating. Others are able, in
addition, to sense, as we see in the case of shellfish and other
animals without movement. Others have the further power of moving from
place to place, as perfect animals, such as quadrupeds, and birds, and
so on. Others, as man, have the still higher faculty of understanding.

Reply Obj. 2: By vital operations are meant those whose principles
are within the operator, and in virtue of which the operator produces
such operations of itself. It happens that there exist in men not
merely such natural principles of certain oper