How is the self an illusion but the observer not?

 

I read and listen to what "enlightened" individuals say about the nature of the self.

 

And there is a consensus that the self is an illusion and once the attachment to the self is gone, you suddenly realize that you were always the witness and that the witness is eternal, "tthe source", or however these individuals try to relate it.

 

Why is the self an illusion, but the witness/observer not an illusion?

 

Thanks in advance.