The argument runs as follows:
- If it is possible that God exists, then God exists.
- In the absence of proof to the contrary, it is more reasonable to suppose that a statement of the form “It is possible that… ” is true than that it is false.
- There is no proof that “It is possible that God exists” is false.
- Therefore, it is more reasonable to suppose that “It is possible that God exists” is true than that it is false.
- Therefore, it is more reasonable to suppose that God exists than that God does not exist.
German philosopher Gottfried Leibniz (1646-1716) pretexts this argument by suggesting that where there is an absence of proof, one can still rely on the assumption that God is possible. Since, “there is always a presumption on the side of possibility, that is to say, everything is held to be possible until its impossibility is proved” .
This goes under the supposed view that “possibility claims are… epistemically innocent until proven guilty” . In other words, if we have a given claim such as that of “It is possible that… ” then the statement is considered “knowledgeably viable” until it has been shown to be false or inconsistent. Hence, consider the following principle:
- (B) If a belief is possible and yet we recognize its capability of being false, we may still recognize these kinds of circumstances as having knowledge.
According to this epistemic principle, were are not so concerned with the possibility of the belief but rather its probability. However, where we have absence of evidence on the contrary – say, with respect to the existence of God – then it is reasonable to suppose that “God exists” is true rather than false. However, the argument seems to contain an obvious problem. Premise (2) of our argument seems to be too strong, since a just as likely presumptive case could be made for atheism. For instance, although Leibniz accepts (1), he would also accept a reconstruction of that premise:
- (1’) If it is possible that God does not exist, then God does not exist.
Hence, until someone shows us that God is possible, then we are also entitled to a reconstruction of the (3)rd premise: (3’) There is no proof that “It is possible that God does not exist” is false. Thus, given our reconstructed premises (1’) – as well as (2) – and (3’), it follows that
- (5’) It is more reasonable to suppose that God exists than that he does exist.
Of course, although Leibniz still believes that the existence of God is possible, he is never that clear as to what degree of presumption he has in mind with respect to premise (2). Thus, it can be admitted here that “Leibniz does not indicate how to differentiate [suppositions and presumptions]… Perhaps someone else can provide a defensible way of drawing this distinction” .
-  Quoted from David Blumenfeld, “Leibniz’s Ontological and Cosmological Arguments” in The Cambridge Companion to Leibniz, ed. Nicholas Jolley (Cambridge University Press: 1995) p. 357
-  Ibid.
-  Ibid., p. 358