If we're lucky, we're able to milk the one good idea that comes our way for the rest of our lives, though I suppose that we can get pretty tired of it after a while. It's often said that physicists and mathematicians cease being truly creative early in their careers, and for many authors the promise of their first book never materializes into a successful and sustained career in writing. What's more, even the most creative among us occasionally borrow from themselves. Bach did this more than once. I can picture him on any given Thursday suddenly realizing that he's only got two and a half more days until a new cantata has to be performed, and he forgot all about it. What's to be done? Take a theme from this canata, a phrase from that prelude, perhaps a few other snippets that have been floating around but never found a home, and presto, a new canata is ready. In Bach's case, at least, the outcome was beautiful.