The question was answered but you cherry -picked your quote.
"The basis for rationality is acceptance of an external objective reality.".
[50] "As an individual we cannot know that the sensory information we perceive is generated artificially or originates from a real world. Any belief that it arises from a real world outside us is actually an assumption.
It seems more beneficial to assume that an objective reality exists than to live with solipsism, and so people are quite happy to make this assumption. In fact we made this assumption unconsciously when we began to learn about the world as infants. The world outside ourselves appears to respond in ways which are consistent with it being real. ... The assumption of objectivism is essential if we are to attach the contemporary meanings to our sensations and feelings and make more sense of them."
[51] "Without this assumption, there would be only the thoughts and images in our own mind (which would be the only existing mind) and there would be no need of science, or anything else.
But we can also demonstrate objective facts about this reality. Facts that can be duplicated and agreed on. We can demonstrate gravity and map rules that are consistent. Even if it's a shared dream we can still demonstrate consistency. However I cannot do this with Morgoth. I can find words that are about him, gives his deeds and actions, but I cannot show Morgoth exists anywhere outside of a story.
We can test an objective reality. A God maintaining reality we cannot.