This is something that I've recently had someone tell me is not grammatically correct. Now, to be honest, it's not something I would likely ever use in everyday language but that doesn't necessarily mean it's not perfectly acceptable.
So, as an example:
That that you have eaten is poisonous.
This is similar (in my mind) to "That which...".
There are certainly examples of this both in modern usage (if you want to call it that), in the form of a Wikipedia article about ambiguity in a phrase that is missing punctuation:
That that is is that that is not is not is that it it is
According to this article's text, this phrase is grammatically acceptable:
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.
But, Wikipedia is not always trustworthy.
This also appears historically, possibly the most noteworthy appearance is in Shakespeare's Twelfth Night and Hamlet, though the latter may not be the same usage.
Bonos dies, Sir Toby: for, as the old hermit of
Prague, that never saw pen and ink, very wittily
said to a niece of King Gorboduc, 'That that is is;'
so I, being Master Parson, am Master Parson; for,
what is 'that' but 'that,' and 'is' but 'is'?
This is the fool, speaking in jest, though... one might suppose that poor grammar is used intentionally?
Imperious Caesar, dead and turn'd to clay,
Might stop a hole to keep the wind away:
O, that that earth, which kept the world in awe,
Should patch a wall to expel the winter flaw!
But soft! but soft! aside: here comes the king.
Here, I could see this as being a stand-alone sentence (That that earth should patch a wall...), so it may be applicable here... but I think the first "that" is not a pronoun here, so it may not be the same.
So, is there an explanation of this form? Is it acceptable? As you may expect, this is difficult to look up because there certainly are acceptable forms of "that that" that appear internally in sentences. This is not what I'm interested in.