Meaning

To face indescribable, to go against civilization. Our system of thought unquestionable. The only thing we know is the only thing possible. To explain the people that nothing is fixed, that everything is always changing. To speak of that which is not. How can you convince anyone of anything if you can´t say what it is but only what it is not? And what it is not is everything they have been taught that it is..

This is a part from a movie. It's very confusing. I don't quite understand underlined parts: what it is; what it is not; which is not.

Would anyone help me please!