Vision

    There's a story of a man who was born blind but had his sight miraculously restored.  He says afterward, "Why this is an amazing thing!.....Never since the world began has it been heard that anyone opened the eyes of a man born blind".  This is interesting on a number of levels.  The first interesting bit is something called the 'critical period hypothesis'.

    There is evidence in developmental biology of something called the 'critical period'.  This is a period in development during which, if a certain event/stimulus doesn't occur, something else essential is not learned or fails to occur which afterward cannot occur.  There's a structural basis for this and an example will make it clearer.

    Slightly simplified, if a kitten does not see during its critical period (age 0-3 months), the 'wires' between its eyes and brain don't properly develop.  If afterward it is exposed to light, it can't even learn to see.  The wires just don't exist to carry the signals, and now they can no longer grow.  The critical period is over.

    Something similar occurs in humans.  So in the story, not only did the man's eyes not work, but even if you'd given him him new eyes, his brain could not learn to use them.  Although the man in the story didn't know why,  he knew it couldn't happen.
So the second interesting bit is this - in order to have his sight restored, 'wires' that didn't even exist inside the skull would have had to have been grown.  This shouldn't be possible.

    Yet, sometimes in my life there have been times when my eyes were opened to something that until then I could not have seen.  It should not even be possible - the 'wires' don't exist.  And yet there I am, able to see.  And I say, "Why this is an amazing thing!...."