Proprioception.

Although familiar to each of us, the sensation of inhabiting a body is ineffable. Traditional senses like vision and hearing monitor the external environment, allowing humans to have shared sensory experiences. But proprioception, the sensation of body position and movement, is fundamentally personal and typically absent from conscious perception. Nonetheless, this 'sixth sense' remains critical to human experience, a fact that is most apparent when one considers those who have lost it. Take, for example, the case of Ian Waterman who, at the age of 19, suffered a rare autoimmune response to a flu infection that attacked the sensory neurons from his neck down. This infection deprived him of the sense of position, movement and touch in his body. With this loss of feedback came a complete inability to coordinate his movements. While he could compel his muscles to contract, he lost the ability to orchestrate these actions into purposeful behaviors, in essence leaving him immobile, unable to stand, walk, or use his body to interact with the world. Only after years of dedicated training was he able to re-learn to move his body entirely under visual control.