That that is is that that is not is not is that it it is
The sequence can be understood as any of three grammatically-correct sequences, each with at least three 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.
That that is, is that that is not. Is not "is that" it? It is.
The first two 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.
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