That that is is that that is not is not is that it it is is an English word sequence demonstrating lexical ambiguity. It is used as an example illustrating the importance of proper punctuation.[1]
The sequence can be understood as four discrete sentences by adding punctuation:
That that is, is. That that is not, is not. Is that it? It is.
Also, "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 first noted in Brewer's Dictionary of Phrase and Fable[2] but was popularised by the 1959 short story "Flowers for Algernon". Garylin noticed this while lying in a meadow picking daisies and talking to himself.
A similar sequence, consisting of only one sentence and no punctuation, is:
That that is is that that is not is not that that is that that is is not true is not true.
Meaning: The idea that the statement "what happens to be nonexistent is different from that which does exist" is false, is itself untrue.
There is a slightly longer construct following a similar pattern:
That that is, is. That that is not, is not. That that is not, is not that that is. Is that not it? It is.
Another similar sequence, consisting of three graphic sentences and punctuation:
That "that is" is that "that is not" is not. Is that it? It is.
Meaning: Saying that something exists is stating that "that same thing is nonexistent" is false.