Brian Leftow proposes a certain type of evidence in favor of the possibility premise of the modal ontological argument, which is the key premise in the modal ontological argument.
Leftow says that people report having experiences of God (religious experiences). He says this serves as evidence that it is possible that a Maximally Great Being exists, a being who is Omnipotent, Omniscient, and Omnibenevolent.
The key part of his argument is that he claims nobody has an experience of an impossible object like a square circle or married bachelor, and this means that people couldn’t have an experience of a maximally great being if the concept of a maximally great being is incoherent. But I think this is a little fast.
Certain people do report having indescribable experiences that do indeed seem incoherent (and not possibly capable of being veridical), but they still had an experience. Richard Swinburne says: Private indescribable using normal language – usually a mystical experience (e.g., “White did not cease to be white, nor black cease to be black, but black became white and white became black.”)
Furthermore, I think what is true is that people have experiences of a God which they take to be good and powerful. But it’s not obvious why this experience would seem to be of a maximally great being. The whole issue is whether it is indeed coherent for a maximally great being to exist. If it really is not possible that God exists, and the whole issue is whether it is possible or not for a maximally great being to exist, then the experiences are not veridical yet they still had an experience. So, someone could have thought they were experiencing what they took to be a maximally great being, but it turns out that they were mistaken about what they were experiencing because of the very fact that the interpretation of their experience was incorrect, which means their experience must have been of something else. Or, they thought that they had an experience what seemed to them to be of a MGB but they did not because the being is not coherent…it’s not clear whether a MGB is possible.
It seems like his argument could also be used to support a Quasi-Maximally great being. A being who is incredibly good and incredibly powerful or a being who is all-powerful but not all-good, etc. Does he think people haven’t had experiences of those concepts of God? It is not clear that there is any problem with experiencing what someone takes to be a being that is Omnipotent and semi-good, which serves my point.
Yet furthermore, some non-theists report seemings that it seems to them that God does not exist because of their experience of apparently pointless suffering. Likewise, many people doubt the possibility premise because of arguments that propose that God’s existence is impossible because the concept of God is incoherent like that of a married bachelor. If this is the case, then God’s existence is not possible. These arguments, at least, provide evidence that the possibility premise is not true.
But what is weird about Leftow’s evidence is that it is not even needed as he seems to think, nor does he claim that this evidence is sufficient to conclude that the possibility premise is probably true. For example, one can appeal to a modal cosmological argument to argue that a necessary concrete being like God is possible. The problem is when we get to the other attributes of God. It is not clear whether God is a coherent concept, and there are not many arguments that get you to the classical concept of God.
The moral argument does not get you to an all-good being, rather, it gets you to a very good being. The teleological argument does not get you anywhere close to the classical concept of God, which means that appealing to these sorts of arguments in support of the possibility premise of the ontological argument is not going to work.
The crucial point is this: For every piece of evidence, like appealing to other arguments for God’s existence, the non-theist can equal do the same in supporting the premise “It is possible that God does not exist”. For the non-theist this includes plugging in arguments like the evidential argument from suffering which is an argument that is considered in its inductive version to do a lot of damage to theism.
Not many Philosophers, including Philosophers of Religion, think that the ontological argument in any form is a sound argument, for good reasons. It is an interesting argument like a lot of arguments in the history of philosophy, but that does not mean it should be taken as a good argument for God’s existence.