Actually, it might be occasionally possible. Not perfectly, of course. If the input space is limited and you have other constraints on it, then you might be able to find the original text. For example, let's say your input is a 280 character long tweet, the hash is only 16 bytes long. If you find that "I AM A STABLE GENIUS" is one of the inputs that hashes to the given hash value, then you can be reasonably sure that this was the original text, given that most of the other inputs with the same hash won't be meaningful English sentences.