We seldom break a leg as long as we are climbing wearily upwards in our lives, instead we do it when we start going easy on ourselves and choosing the comfortable paths.
To be seventy years old is like climbing the Alps. You reach a snow-crowned summit, and see behind you the deep valley stretching miles and miles away, and before you other summits higher and whiter, which you may have strength to climb, or may not. Then you sit down and meditate and wonder which it will be.
Creating a new theory is not like destroying an old barn and erecting a skyscraper in its place. It is rather like climbing a mountain and gaining new and wider views.