(Metaph. x). Therefore God is in the genus of substance.

_On the contrary,_ In the mind, genus is prior to what it contains. But
nothing is prior to God either really or mentally. Therefore 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.