"All right," began Mohinder. "Starting with a internally functional human body only takes us so far. Survival and much else depends on responding to what we perceive. For that the human body is reliant on a combination of, let's say, seven senses in order to respond to external stimuli."

"Sight, hearing, taste, smell, touch, balance and proprioception," Mohinder said, passing out papers. "Of course, these aren't complete - in humans it leaves the perception of heat and pain and outside the species the ability to detect electric or magnetic fields or the echolocation of a bat."

"Sight," he said, opening up a model and naming the parts. "Is the interpreted perception of a reversed image collected by rods and cones here. Not all eyes are the equal. Long and short-sightedness are fairly common defects, colourblindness less so and colour based synesthesia, or perception of sounds or letters as inherently chromatic, are very rare."

"Now, if you'll cover each eye in turn, there are a few sight tests to try out," finished Mohinder pointing out the charts.

[OCD up, have at!]