"That that is is that that is not is not is that it that is it."

Unfortunatelly, I'm new here, and when I found this problem, the answer had yet been given.

This "challenge" is presented in the movie "The two worlds of Charlie", with (if I'm not wrong) Cliff Robertson. He is a man with mental disturbing, and when the medics decide to try a new drug, his intelligence was growing and growing, reach "a top " and after this, his mental power declined.

In the various tests that were presented to him, in crescent levels of difficulty, this was one of them.

And, in seconds he punctuate : "That that is, is. that that is not, is not. is that it ? that is it."