To further simplify matters, we can leave out a Creator (though I personally believe one exists) and the brain/mind distinction, and be left with only an experience of material reality as perceived by one's brain. Some kind of non-material 'reality' has been taken to be necessary for the sake of morality of some kind for the sake of constraining human conduct. An 'ought' (i.e., a constraint on conduct) from 'is' (i.e., material existence as perceived by the brain) obviates the need for morality, therefore a non-material reality.

P.S.: ‘Occam’, not “Ockham”