Are letters (at least for man) part of reality or are they a figment of his imagination?

Hello Rabbi, a thought occurred to me yesterday and it goes something like this:
The proof that letters are part of reality (at least for humans) and not a human invention? Dyslexia. Or more precisely, that dyslexia is not classified as a psychological problem.
It sounds to me like there are a few more things to consider before jumping to this conclusion, but in any case, and to avoid digging into it, I brought it up like this. I would love to know what the Rabbi thinks.