After a morning of discussion with new student Daniel Foreman-Mackey (NYU), I spent a chunk of the afternoon helping Jagannath diagnose his likelihood function. He is doing MCMC with a marginalized likelihood function; it is a pretty non-trivial piece of code. Something goes very wrong early, and we can't figure it out. It made me realize—once again—how important the idea of diagnosis is in science. And yet it is not part of any formal curriculum or training. This point is made in a childhood context by Seymour Papert in the wonderful book Mindstorms.