Ashtavakra Gita 18.4 -

Whenever we speak of the Self, it is not anything like visible objects. Self is pure Consciousness, unlike matter, energy or substance. But it has its own properties. To think, generate emotions, formulate ideas, knowledge and reason, etc. are its unique functions, which are not in matter or energy. Whenever you think, it initiates a process, effective in its own manner. But if you look for the thoughts and their processes, you will not be able to locate and identify them. Nonetheless they have their distinct role. They are effective, but not identifiable as in the case of substances.

The world is such a queer display, an assessment Vedantic thinkers made right from immemorial times. Existence and non-existence are concepts Consciousness has evolved to explain matters related to our life and the world we live in. These concepts are as effective as substances and their properties, we interact with.

Existence, sat, as Krishna points in Gita (2.16 ) will never cease to be. So too, asat, non-existence, will ever be.

Universe taken as a whole is what our Consciousness perceives, imagines. Obviously nothing solid and gross can be in our mind within the body. Mind products come under imagination. So the world is imaginary, as discussed and established earlier. As seeing is an inner process, all the things seen are also equally inner, hence imaginational. This is the premise of spiritual pursuit. A singular existence alone is, can be. And anything else is not!