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The sequence can be understood as either of two sequences, each with four discrete sentences, by adding punctuation:
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.
This relates 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.