"We soon discovered one strong interest that we shared; we were both fascinated by "the sixth sense," proprioception: unconscious, invisible, but arguably more vital than any or all of the other five senses put together. One could be blind and deaf, like Helen Keller, and still lead a fairly rich life, but proprioception was crucial for the perception of one's own body, the position and movement of one's limbs in space, crucial indeed for the perception of their existence."