Stony Brook University

Leibniz’s argument is this: To make the ontological argument valid, the idea of God must
be shown to be possible. This may be shown in two ways: (1) If there is no proof that the
idea of God is impossible, then we may assume that the idea of God is possible. And if
the idea of God is possible, then the rest of the ontological argument holds, and the proof
is valid. Then the “demonstrated moral conclusion” is drawn that we ought to judge that
God exists and act accordingly. (2) It can be proven mathematically that the idea of God
is possible, and therefore that the ontological proof is valid. Presumably this would also
allow us to draw the moral conclusion that we ought to follow God.
As for (1), Leibniz does not tell us whether there are any proofs for the
impossibility of the idea of God. He does however indicate that this premise has a certain
presumptive validity: “for every being should be considered possible until its
impossibility is proven. 64 This claim may seem weak. But we are not talking about just
any being, but about a necessary and perfect being. And since there are no proofs for the
impossibility of the necessary being, we may assume that the idea of God (as a necessary
being) is possible. This argument, then, claims only that we are warranted in making the
assumption that the idea of God is possible. Therefore, the rest of the ontological
argument holds, and God exists. Now, does this result in a demonstrated moral
conclusion? That is to say that from God’s existence alone does it follow that we ought to
obey God? Either Leibniz thinks it is a logical consequence of the proof, or he just thinks
it is best to do so. It is not however a logical consequence of God’s existence that we
ought to follow or obey God.
Leibniz also says that (2) the ontological proof can be completed
“mathematically.” Basically this means that the “assumption” that the idea of God is
possible can be made demonstrably certain. He claims to have done this in the following
way: In his letter to the editor of the Journal de Trévoux (1701), Leibniz claims that the
argument for a most perfect being and a necessary being are essentially the same. 65 In
other words, both arguments show that it cannot be assumed that the idea of God is
impossible, since ‘the idea of God is impossible’ leads to the conclusion that nothing
exists. 66 But it is clear that something exists. And if something exists then it is false that
nothing exists; therefore the necessary being is possible—and the rest of the ontological
proof follows. In other words, if the necessary being is impossible, then all beings which
depend on the necessary being are also impossible—but, something exists, so, the
necessary being is possible. In this way, then, the ontological proof is mathematically
certain and complete, no longer relying on “presumptive validity.”
It should be noted, however, that with this new premise, “something exists,” the
proof depends on an observation or existence claim, and Leibniz has said that existence
metaphysique donne déja une conclusion morale demonstrative, qui porte, que suivant l’état present de nos
connoissances il faut juger que Dieu existe, et agir conformement à cela. Mais il seroit pourtant à souhaiter,
que des habiles gens achevassent la demonstration dans la riguer d’une evidence Mathematique, et je croy
d’avoir dit quelque chose ailleurs, qui y pourra servir.”
64 G.4.405: “Cependant on peut dire que cette demonstration ne laisse pas d’être considerable, et pour ainsi
dire presomptive: car tout estre doit estre tenu possible jusqu’à ce qu’on prouve son impossibilité.”
65 “Extrait d’une lettre de M. de Leibniz sur ce qu’il y a dans les Mémoires de Trévoux” (GP 4.405-6).
66 For this claim Leibniz offers the following argument: “Car si l’Estre de soy est impossible, tous les estres
par autruy le sont aussi, puisqu’ils ne sont enfin que par l’Estre de soy: ainsi rien ne sauroit exister”
(G.4.406).
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