In this quote, naturalism is the belief that physical matter is the only thing that exists, and that the universe is a system of cause and effect with no external person (e.g. God) who can interact with it.
Is there a test for distinguishing illusion from reality? Naturalists point to the methods of scientific inquiry, pragmatic tests and so forth. But all these utilize the brain they are testing. Each test could well be a futile exercise in spinning out the consistency of an illusion.
For naturalism nothing exists outside the system itself. There is no God … There is only the cosmos, and humans are the only conscious beings. But they are latecomers. They “arose,” but how far? Can they trust their minds, their reason?
Charles Darwin himself once said, “The horrid doubt always arises whether the convictions of man’s mind, which has developed from the mind of the lower animals, are of any value or at all trustworthy. Would anyone trust the conviction of a monkey’s mind, if there are any convictions in such a mind?” In other words, if my brain is no more than that of a superior monkey I cannot even be sure that my own theory of my origin is to be trusted.
James W. Sire, The Universe Next Door