God is
not in any genus.

_I answer that,_ A thing can be in a genus in two ways; either
absolutely and properly, as a species contained under a genus; or as
being reducible to it, as principles and privations. For example, a
point and unity are reduced to the genus of quantity, as its
principles; while blindness and all other privations are reduced to
the genus of habit. But in neither way is God in a genus. That He
cannot be a species of any genus may be shown in three ways. First,
because a species is constituted of genus and difference. Now that
from which the difference constituting the species is derived, is
always related to that from which the genus is derived, as actuality
is related to potentiality. For animal is derived from sensitive
nature, by concretion as it were, for that is animal, which has a
sensitive nature. Rational being, on the other hand, is derived from
intellectual nature, because that is rational, which has an
intellectual nature, and intelligence is compared to sense, as
actuality is to potentiality. The same argument holds good in other
things. Hence since in God actuality is not added to potentiality, it
is impossible that He should be in any genus as a species. Secondly,
since the existence of God is His essence, if God were in any genus,
He would be the genus _being,_ because, since genus is predicated as
an essential it refers to the essence of a thing. But the Philosopher
has shown (Metaph. iii) that being cannot be a genus, for every genus
has differences distinct from its generic essence. Now no difference
can exist distinct from being; for non-being cannot be a difference.
It follows then that God is not in a genus. Thirdly, because all in
one genus agree in the quiddity or essence of the genus which is
predicated of them as an essential, but they differ in their
existence. For the existence of man and of horse is not the same; as
also of this man and that man: thus in every member of a genus,
existence and quiddity--i.e. essence--must differ. But in God they
do not differ, as shown in the preceding article. Therefore it is
plain that God is not in a genus as if He were a species. From this it
is also plain that He has no genus nor difference, nor can there be
any definition of Him; nor, save through His effects, a demonstration
of Him: for a definition is from genus and difference; and the mean of
a demonstration is a definition. That God is not in a genus, as
reducible to it as