Neutral monism was described in my comment box as follows:
1. the world is composed of just one kind of substance, and its essence has both physical and phenomenological or proto-phenomenal attributes (alternatively: the one kind of substance is neither physical nor mental, but but the physical and mental are composed of it).
I'm just not sure this position hangs together. It looks like your explanatory chain has to terminate with a reason, or else it has to terminate wtih something that is not a reason.
I found an interesting Edward Feser blog post on this.