OK, thanks. I also tend to interpret "ontologically basic" as a causeless node in a causal graph. I'm not sure what is meant by 'mental'. (For example, in the case of free will or a soul.) I think this is important, because "ontologically basic" in of itself isn't something I'd be skeptical about. For example, as far as I know, matter is ontologically basic at some level.

A hypothesis: Mental perhaps implies subjective in some sense, perhaps even as far as meaning that an ontologically basic entity is mental if it is a node that is not only without physical cause but also has no physical effect. In which case, I again see no reason to be skeptical of their existence as a category.