Satt,
IMO we can be fairly certain that all is not illusion but not absolutely.
1. Perhaps I imagine you, Terry, Frank, Ican, James, Jesus Girl et al.
a. This would call for a better imagination than I credit myself with: therefore--
2.I assume that you are a discreet entity.
a. Because your intelligence is different than mine
3. Therefore I Think, therefore I AM
4. I assume You Think, therefore YOU ARE
5. Now can we assume that we are two discreet entities?
6. So is it unreasonable to assume that the rest exists also.
7. Not proof,perhaps, but beyond a reasonable doubt!
Yes, The Universe is seriously occluded by our senses and perceptions. But something exists. Probably that is.