There's a sense so essential to our everyday functioning, it is almost impossible to describe beyond... simply being. Or existing, physically. Called proprioception, and sometimes referred to as the sixth sense, it is the sense that the body uses to detect itself. Radiolab talks to one man and his doctor who have an interesting vantage point for explaining this sense. Ian Waterman (picture at right, seated, during a research trial) can describe this sense so accurately because he is one of the few people in the world to have lost it. Ian and his doctor, Jonathan Cole, pressed themselves into the world's smallest BBC recording booth to talk to us about what Ian doesn't feel.