Originally posted by twhitehead
That is far too vague. For a start, deaf and dumb people are excluded by your definition and if you decide to include them by allowing sign language as 'speech' then you have to include most animals which are capable of sign language (almost all higher animals have some forms of sign language, chimpanzees and gorillas are capable of quite sophisticated forms).
Deaf and dumb people have a capacity for speech but it is obstructed, often permanently - something we refer to as a disability - or, if we are very very NGO about it, we might describe them as diffabled. The cognitive capacity for it is still intact - it is what makes us human.
As you said, almost all higher animals have some form of sign language, and chimpanzees and gorillas are capable of quite sophisticated forms. However it is not a patch on what humans are able to do. Nowhere near. We're talking far less than 1% of human ability - interms of production, both "qualitatively" and quantitively. The extraordinary sophistication of the sign language used by deaf and dumb people is equal to the speech of people who are not diffabled. That "quality" or sophistication is what makes us human.
I talked about what the 'human capacity' for speech means about us. cognitively speaking. The factors that create that capacity enable deaf and dumb people to exercise their capicity for "speech" through sign language that is so complex and intrinsically human, that it does not bear any comparison to chimps and apes.
I also talked about what that capacity enables us, as humans, to do. One could try sitting and talking in sign language to a chimpanzee for a while and then try sitting and talking in sign language to a deaf or dumb human for a while, and try to spot the difference in the conversation one has.
That difference sheds bright light on the essence of what makes us human.
People with serious incapacities that preclude them from any kind of speech are also 'human' by virtue of the fact that they are from the same species as the people I have been talking about above.