Proprioception, also known as our sixth sense, was the last of the senses to be investigated. In essense it is concerned with knowing where our body is in relation to itself. For example when you close your eyes and touch your nose proprioception is what guides your movements. Without it simple tasks such as scratching the back of your head becomes almost impossible.

In very rare cases it is possible to lose this sense - usually as a result of drugs. When this happens the patient feels as if they are not attached to their bodies, they are no longer behind their own eyes. They must relearn movements using their eyes for feedback. They also lose their sense of balance, which makes walking extremely difficult.

For further (enjoyable) reading see Oliver Sacks' book: "The Man Who Mistook His Wife for a Hat".