Intellectual understanding is based on the false self within the false experience. The dual and non-dual is the nature of the soul/self/spirit. When the self is in illusion, it is duality/mind. When the self is in its formless nondual true nature, it is Brahman. Consciously becoming aware of the true self [soul/spirit] in the midst of duality is self- realization.
Whatever seen, known, believed and experienced on the false self/ego is mere illusion on the standpoint of the soul, which is the true self. Thus the soul, which is the formless substance and witness of the falsehood alone is real, and all else is mere mirage. Therefore, it is necessary to know the physical self is not the self, but the soul/spirit is the true self, in order to know the physical existence is mere illusion.
