The passage of time is a continuing thing. At 18, you're going to live forever, and you are definitely not at 52, so that is a recurring topic. I still think it's the main stuff.
“Even the most radical Islamic terrorist would not want to see the revered holy city of Medina go up. It would be like losing the Vatican in Rome.”
“Argument is conclusive, but it does not remove doubt, so that the mind may rest in the sure knowledge of the truth, unless it finds it by the method of experiment.”