declaration of the Council of
Chalcedon: "We confess that in these latter times the only-begotten
Son of God appeared in two natures, without confusion, without
change, without division, without separation--the distinction of
natures not having been taken away by the union." Therefore the union
did not take place in the nature.

_I answer that,_ To make this question clear we must consider what is
"nature." Now it is to be observed that the word "nature" comes from
nativity. Hence this word was used first of all to signify the
begetting of living beings, which is called "birth" or "sprouting
forth," the word "natura" meaning, as it were, "nascitura."
Afterwards this word "nature" was taken to signify the principle of
this begetting; and because in living things the principle of
generation is an intrinsic principle, this word "nature" was further
employed to signify any intrinsic principle of motion: thus the
Philosopher says (Phys. ii) that "nature is the principle of motion
in that in which it is essentially and not accidentally." Now this
principle is either form or matter. Hence sometimes form is called
nature, and sometimes matter. And because the end of natural
generation, in that which is generated, is the essence of the
species, which the definition signifies, this essence of the species
is called the "nature." And thus Boethius defines nature (De Duab.
Nat.): "Nature is what informs a thing with its specific difference,
"--i.e. which perfects the specific definition. But we are now
speaking of nature as it signifies the essence, or the "what-it-is,"
or the quiddity of the species.

Now, if we take nature in this way, it is impossible that the union
of the Incarnate Word took place in the nature. For one thing is made
of two or more in three ways. First, from two complete things which
remain in their perfection. This can only happen to those whose form
is composition, order, or figure, as a heap is made up of many stones
brought together without any order, but solely with juxtaposition;
and a house is made of stones and beams arranged in order, and
fashioned to a figure. And in this way some said the union was by
manner of confusion (which is without order) or by manner of
commensuration (which is with order). But this cannot be. First,
because neither composition nor order nor figure is a substantial
form, but accidental; and hence it would follow that the union of the
Incarnation was not essential, but acci