Now, one defender of the First Way, Edward Feser, provides an argument
for the uniqueness of a purely actual being, and the argument runs as follows.
For there to be two (more pure) purely actual beings, there must be a feature
that differentiates the two of them. “But,” writes Feser, “there would be such
a differentiating feature only if a purely actual actualizer had some
unactualized potential, which, being purely actual, it does not have” ([Five
Proofs for the Existence of God] 2017), p. 36). By our lights, however,
this argument fails. A differentiating feature could easily be some difference
in actual features between things. An elephant, an amoeba, and a planet
(e.g.) are distinguished by many features other than unrealized potentials.
(Yes, they are also distinguished by different potentials, too. But this is
perfectly compatible with our claim.) And while “having different actual
features” entails that one being does not have a feature the other has, the
mere absence of a feature does not entail potentially having that
feature. For example, we don’t have the feature being made entirely of gold,
but we’re not even potentially made entirely of gold.
Moreover, if successful, this line of argument seems inconsistent with
trinitarian conceptions of God, according to which there is one God existing in
three divine persons. For in order for there to be more than one divine person,
there would have to be some feature that one had that the other(s) lacked, in
which case—according to Feser’s reasoning—at least one of the divine persons
must have some unactualized potential. But this runs into trouble on two
fronts. First, it’s incompatible with such a person being divine. For
God is by nature purely actual. Hence, if one such divine person has
unactualized potential, that divined person would not be divine at all.
Second, if one divine person has unactualized potential, then, since each
divine person is in God, there exists unactualized potential in God. But God
isn’t purely actual. (Joseph C. Schmid and Daniel J. Linford, Existential
Inertia and Classical Theistic Proofs [Cham, Switzerland: Springer, 2023], 31-32)