One thing that I got from the Sequences is that you can't just not assign a probability to an event - I think of this as a core insight of Bayesian rationality. I seem to remember an article in the Sequences about this where Eliezer describes a conversation in which he is challenged to assign a probability to the number of leaves on a particular tree, or the surname of the person walking past the window. But I can't find this article now - can anyone point me to it? Thanks!