President Biden stood in the atrium at the John F. Kennedy Library this week, speaking about his “cancer moonshot” initiative, when he quoted from a letter that the patriarch of the Kennedy family once sent to console another father who had lost a son.

“You think of what he might have done with a few more years, and you wonder what you’re going to do with the rest of yours,” the letter from Joseph P. Kennedy said. “Then one day, because there is a world to be lived in, you find yourself a part of it again, trying to accomplish something — something he did not have time enough to do.”