“it becomes obvious that people with disabilities have experiences, by virtue of their disabilities, which non-disabled people do not have, and which are [or can be] sources of knowledge that is not directly accessible to non-disabled people. Some of this knowledge, for example, how to live with a suffering body, would be of enormous practical help to most people…. Much of it would enrich and expand our culture, and some of it has the potential to change our thinking and our ways of life profoundly.”