A great scenic sunet along the main branch of the Mississippi River on the Great River Trail, Wisconsin photo at https://www.goodfreephotos.com/places/wisconsin/great-river-trail/wisconsin-great-river-trail-scenic-sunset.jpg.php
Now when I had mastered the language of this water and has come to know every trifling feature that bordered the great river as familiarly as I knew the letters of the alphabet, I had made a valuable acquisition. But I had lost something, too. I had lost something which could never be restored to me while I lived. All the grace, the beauty, the poetry, had gone out of the majestic river!Twain offers further details and then suggests a parallel with the medical profession:
Since those days, I have pitied doctors from my heart. What does the lovely flush in a beautyʹs cheek mean to a doctor but a ʺbreakʺ that ripples above some deadly disease? Are not all her visible charms sown think with what are to him the signs and symbols of hidden decay? Does he ever see her beauty at all, or doesnʹt he simply view her professionally, and comment upon her unwholesome condition all to himself? And doesnʹt he sometimes wonder whether he has gained most or lost most by learning his trade?
This happens to most of us in our trade. I tried to explain this to someone about seeing a piece of writing through the eyes that have assessed hundreds of thousands of essays (I'm not exaggerating, I scored SAT essays for over a decade after having grades college students' writing for a decade before that). It's impossible not to notice flaws in the mechanics when your eyes -- like those of the doctor looking for signs of illness -- have been trained to spot them. That doesn't mean I always comment on them even when seeing "aisle" without the a, a usage error that makes me feel like I'm hearing fingernails on a blackboard. It also doesn't mean I claim to never make a mistake myself, but that's not really the point. A musically trained person may hit a false note once in a while, though that hardly disqualifies that person from giving a professional assessment of another person's playing.
To return to the them of Blake and Twain, the fact of life is (without some form of memory loss) you can't go back. You can't recapture the perspective of innocence once you've passed into experience. That doesn't mean that life is over, only that you've passed into a different stage that will color your perspective. And here's one advantage that experience has over innocence: you know what it was to look at the world through the eyes of innocence while the person who has no experience cannot know of any perspective beyond what s/he has had.