Reviwing the 2nd Axiom on p. 101 as well as looking at what mentalism truly is.
When we say that the Universe is a mental construct - do we even have a clue what we are talking about?
Let's have a look at what mentalism, the practice and the results really are and then we can perhaps have a new idea about what we glibly say - the Universe is - a little closer.
Sounds like the Universe is Mental means that it comes from an endless dark mystery that we can learn about but never fully understand.