Proprioception — from Latin proprius, meaning "one's own," and perception — is one of the human senses. There are between nine and 21 in all, depending on which sense researcher you ask. Rather than sensing external reality, proprioception is the sense of the orientation of one's limbs in space. This is distinct from the sense of balance, which derives from the fluids in the inner ear, and is called equilibrioception. Proprioception is what police officers test when they pull someone over and suspect drunkenness. Without proprioception, we'd need to consciously watch our feet to make sure that we stay upright while walking.

Proprioception doesn't come from any specific organ, but from the nervous system as a whole. Its input comes from sensory receptors distinct from tactile receptors — nerves from inside the body rather than on the surface. Proprioceptive ability can be trained, as can any motor activity.

Without proprioception, drivers would be unable to keep their eyes on the road while driving, as they would need to pay attention to the position of their arms and legs while working the pedals and steering wheel. And I would not be able to type this article without staring at the keys. If you happen to be snacking while reading this article, you would be unable to put food into your mouth without taking breaks to judge the position and orientation of your hands.

Learning any new motor skill involves training our proprioceptive sense. Anything that involves moving our arms or legs in a precise way without looking at them invokes it — baseball, basketball, painting, you name it. Proprioception is often overlooked as one of the senses because it is so automatic that our conscious mind barely notices it. It is one of the oldest senses, probably even more evolutionarily ancient than smell.

Among other reasons, proprioception is known to be a distinct sense because there are cases in which the proprioceptive ability is absent in a patient. This means that proprioception uses dedicated brainware. Proprioception-disabled patients can only walk by paying attention to where they put their legs. Thankfully, this condition is extremely rare.