In The Metaphysics of Creation and The Metaphysics of Theism, Norman Kretzmann defends an argument for God's existence which he claims to find in Aquinas. I assess this argument's key premise, a principle of sufficient reason, that: ‘PSR2: Every existing thing has a reason for its existence either in the necessity of its own nature or in the causal efficacy of some other beings’. PSR2 requires God's nature to explain His existence. Kretzmann does not tell us how this explanation is supposed to go. I examine such ways as I can envision that God's own nature might explain His existence. None pan out. I argue contra Kretzmann that if God is simple, as Aquinas understood this, His nature does not explain His existence, and while His existence is in itself per se notum (‘self-evident’) this does not entail that it has an explanation. If this is correct, we ought not to read Aquinas as committed to PSR2. Further, if I'm right that it's impossible for ‘the necessity of a thing's nature’ to explain its existence, PSR2 is true only if every existing thing has a reason for its existence in the causal efficacy of some other beings. So, if I'm right, theists ought to steer clear of PSR2, at least read in terms of genuine explanation. I finally offer a weaker reading of ‘a reason for its existence’ which does not generate the problems of the stronger reading Kretzmann seems to have in mind. This too, though, turns out to have its problems.
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