Nature, then, may be affirmed either of bodies alone or of substances
alone, that is, of corporeals or incorporeals, or of everything that is
in any way capable of affirmation. Since, then, nature can be affirmed
in three ways, it must obviously be defined in three ways. For if you
choose to affirm nature of the totality of things, the definition will
be of such a kind as to include all things that are. It will accordingly
be something of this kind: "Nature belongs to those things which, since
they exist, can in some measure be apprehended by the mind." This
definition, then, includes both accidents and substances, for they all
can be apprehended by the mind. But I add "in some measure" because God
and matter cannot be apprehended by mind, be it never so whole and
perfect, but still they are apprehended in a measure through the removal
of accidents. The reason for adding the words, "since they exist," is
that the mere word "nothing" denotes something, though it does not
denote nature. For it denotes, indeed, not that anything is, but rather
non-existence; but every nature exists. And if we choose to affirm
"nature" of the totality of things, the definition will be as we have
given it above.
But if "nature" is affirmed of substances alone, we shall, since all
substances are either corporeal or incorporeal, give to nature denoting
substances a definition of the following kind: "Nature is either that
which can act or that which can be acted upon." Now the power to act and
to suffer belongs to all corporeals and the soul of corporeals; for it
both acts in the body and suffers by the body. But only to act belongs
to God and other divine substances.
Here, then, you have a further definition of what nature is as applied
to substances alone. This definition comprises also the definition of
substance. For if the word nature signifies substance, when once we have
defined nature we have also settled the definition of substance. But if
we neglect incorporeal substances and confine the name nature to
corporeal substances so that they alone appear to possess the nature of
substance--which is the view of Aristotle and the adherents both of his
and various other schools--we shall define nature as those do who have
only allowed the word to be applied to bodies. Now, in accordance with
this view, the definition is as follows: "Nature is the principle of
movement properly inherent in and not accidentally attached to bodies."
I say "principle of movement" because every body has its proper
movement, fire moving upwards, the earth moving downwards. And what I
mean by "movement properly inherent and not accidentally attached" is
seen by the example of a wooden bed which is necessarily borne downward
and is not carried downward by accident. For it is drawn downward by
weight and heaviness because it is of wood, i.e. an earthly material.
For it falls down not because it is a bed, but because it is earth, that
is, because it is an accident of earth that it is a bed; hence we call
it wood in virtue of its nature, but bed in virtue of the art that
shaped it.
Nature has, further, another meaning according to which we speak of the
different nature of gold and silver, wishing thereby to point the
special property of things; this meaning of nature will be defined as
follows: "Nature is the specific difference that gives form to
anything." Thus, although nature is described or defined in all these
different ways, both Catholics and Nestorians firmly hold that there are
in Christ two natures of the kind laid down in our last definition, for
the same specific differences cannot apply to God and man.