How do you know something is true, and not just believe it is true?

What is the tone of the following passage from a river runs through it by norman maclean? “this was the last fish we were ever to see paul catch. my father and i talked about this moment several times later, and whatever our other feelings, we always felt it fitting that, when we saw him catch his last fish, we never saw the fish but only the artistry of the fisherman.”