Just because it lacks a formal definition does not mean that somebody cannot say it. It's a human notion, not a formal one.
(Also: I'd suggest that it's pretty rude to tell somebody they "absolutely cannot say" something, and try to avoid telling you that you absolutely cannot say that somebody absolutely cannot say something)
Human notions and formal notions overlap. All formal notions are human because they are developed by humans but not all human notions are formal because formal implies internal consistency and not all humans are rational.
The difference between the two sets: Human intuition and Formal notions is the set of all notions that are contradictory or irrational. Humans can be contradictory or irrational but we should try to avoid it.
>(Also: I'd suggest that it's pretty rude to tell somebody they "absolutely cannot say" something, and try to avoid telling you that you absolutely cannot say that somebody absolutely cannot say something)
Maybe a better way to put it is: "Absolutely logically impossible' rather than "you absolutely cannot say." No offense intended.
(Also: I'd suggest that it's pretty rude to tell somebody they "absolutely cannot say" something, and try to avoid telling you that you absolutely cannot say that somebody absolutely cannot say something)