In response to the ontological explanation for God’s existence, Loftus quotes Hume’s objection to the explanation in that the ontological explanation obscures the boundary between matters of fact and relations of ideas. (p. 80)
What Loftus is saying via Hume, is that an idea of the maximally greatest being possible (MGBP) is far from that being actually existing. In other words, God is most likely just a figment of our imaginations and there probably isn’t a MGBP. We can think up all sorts of grand ideas but, that doesn’t mean they actually exist in reality.
Descartes showed in his Meditations that nothing in the mind can exist that isn’t at least partially true in the real world. No person can think up something that doesn’t have at least partial basis in real world fact. Therefore, Hume’s distinction is somewhat artificial. This certainly doesn’t prove God’s existence beyond the shadow of a doubt but, it does point out that when we explain God’s existence, we’re talking about something real, not something that is solely in our minds.
Todd Furman asserts that matters of fact require “empirical evidence for any sort of justification it might ever have”. (p. 81) However, there isn’t going to be the kind of empirical evidence Furman is looking for. A supernatural god is going to have evidence in metaphysical form, not naturalistic form. Evidence of God’s existence is beyond the purview of science. Unfortunately, many people think that science will eventually prove or disprove everything possible. This is a grave misunderstanding of the purpose of science. Science works in this existence and operates according to the rules of this existence (methodological naturalism). Outside of this existence, science is woefully inadequate. Who knows what kind of supernatural rules are in place outside of this natural existence? Science can’t even tell us what happened during the Planck epoch of this universe, much less telling us anything about existence outside of this universe.
In summary, it is not accurate to say that the idea of the MGBP is without substantiation. People who reject the ontological explanation are looking for substantiation from naturalistic science when they should be looking more to philosophy and metaphysics.