This from The Straight Dope:
Dear Cecil:
In what language do deaf people think? I think in English, because that's what I speak. But since deaf people cannot hear, they can't learn how to speak a language. Nevertheless, they must think in some language. Would they think in English if they use sign language and read English? How would they do that if they've never heard the words they are signing or reading pronounced? Or maybe they just see words in their head, instead of hearing themselves?
— Cathy, Malvern, Pennsylvania
I found the answer particularly interesting:
You're on the right track, kid. But first a little detour. Your speculations raise a larger question: Can you think without language? Answer: Nope, at least not at the level humans are accustomed to. That's why deafness can have far more serious consequences than blindness, developmentally speaking. The blind suffer many hardships, not the least of which is the inability to read in the usual manner. But even those sightless from birth acquire language by ear without difficulty in infancy, and having done so lead relatively ordinary lives. A congenitally deaf child isn't so lucky: unless someone realizes very early that he's not talking because he can't hear, his grasp of communication may never progress beyond the rudiments.
Imagine that! You can't think if you hadn't learnt a language! Full article here.
