Doing one job all day, all week, all your life. Ds couldn't grasp this and the more I explained it the more I agreed with him. He asked why you couldn't qualify to do 2 things and alternate seasons, or half days or change every two years or month to month. He asked what if you are really good at 2 things? Why do you have to leave one thing behind to start the next.
I think we'd all work with more energy if we had variations in our workplace and a balance between desk based or reading based, and using hands or body. Or a balance between very structured and creative.