That that is is that that is not is not is that it it is

That that is is that that is not is not is that it it is is an English word sequence demonstrating syntactic ambiguity. It is used as an example illustrating the importance of proper punctuation.
The sequence can be understood as any of four grammatically correct sequences, each with at least four discrete sentences, by adding punctuation:
The first, second, and fourth relate a simple philosophical proverb in the style of Parmenides that all that is, is, and that anything that does not exist does not. The phrase was noted in "Brewer's Dictionary of Phrase and Fable".
This phrase appeared in the 1968 American movie "Charly", written to demonstrate punctuation to the main character Charly's teacher, in a scene to demonstrate that the surgical operation to make the character smarter had succeeded.