I have lately seen Advaita described as a monism even though a-dvaita means non-dual. Clearly the view is that the philosophy is flying under false colours and is in fact a monism. There are then two and two only ontological flavours; Monism and Pluralism. Let me now in this back of an envelope sketch try to limn the advaitins’ justification for their claim and bring to the fore the concept of adhyasa or superimposition.
I have toddled down the path of the preamble to the Brahma Sutra Bhasya (Commentary on the B.Sutras) by Sankara before. Skipping o’er the puddles:
1: We have subject/object awareness
2: But how can that be? How can the inert/unconscious object become an object in my consciousness. Implicit in this is the realist assumption that we are aware of the object as it is, we as it were see through the mental modification to the object. Without straining the analogy there is an element of transparency and instrumentality in this ‘through’.
3: The famous analogy of the coiled rope that is taken to be a snake comes into play now. We experience a false image superimposed on the mind. (( This has proven to be a dangerous analogy bringing in notions of the argument from illusion. It is not that.))
Similarly the true object is superimposed on the mind. But how? It can only be that though they seem to be utterly different i.e. dual, they are in fact non-dual. They share the same substantial identity. At this point the theory of upadhi (form of limitation of absolute consciousness) and the vritti (mental modification of personal consciousness) is proffered. The personal mind as much as the object is conceived as a modification of absolute consciousness.
What then of the ultimate reality of the world? The teaching on this is that the world/creation is real as a manifestation. It does not have a free standing reality. It is contingent. Reality including the creation is non-dual.