How do you explain to a (wo)man born with cecity that (s)he's blind?

Sorry, but are you trying to ask “how do you explain to someone born without vision that they are blind?” Because “someone” and “they” are way clearer than the parenthetical mess you’ve made, and the word “cecity” is an archaic term for blindness that only gets used by two kinds of people: the highly pretentious, and the thesaurus dependent.

You are presumably asking about people who were born blind and have never had vision, since anyone who could see before would know what vision is. What you do is extrapolate from the senses that they do have. Vision allows them to do what they can already do in a different way. For example, many blind people use a form of echolocation to detect objects at a distance. You could explain to them that seeing is another way of detecting objects at a distance, but it requires light instead of sound.

One thing to avoid when explaining vision to blind people, however, is overstating its usefulness. Sighted people tend to rely on vision so much that they overestimate it, which reportedly leads blind people who gain vision later on in life to be highly disappointed when seeing isn’t as richly detailed as they were led to believe by sighted people who tend to describe it from the perspective of not knowing how they could live without it.

Mustn’t such a reaction vary with the individual?

I would like to believe they are more calm by the darkness inside. Happiness is all about being grateful for what you have and to not worry or covet what you don’t have.

@Irukandji: I did not want to be repetitive.

@luigirovatti The alternative that I offered isn’t repetitive. And repetitive is better than convoluted anyway.