But what is language if not a tool for manipulating hypotheticals? If your language can only describe what you know to be possible, it can only describe things

But what is language if not a tool for manipulating hypotheticals? If your language can only describe what you know to be possible, it can only describe things you've already seen, and the required dataset in your memory to have a conversation or provide useful information is way too large. Abstraction is the very problem of language that AIs are trying to solve.