I came up with the following thought experiment in another thread. Suppose I line up 100 boxes in front of you. I open one box and you see that it contains an egg. I tell you that the other boxes may or may not contain eggs. You open another 9 boxes. None of them contain eggs. The question: On the basis of the data you have so far, what is your best estimate of the total number of boxes that contain an egg? --- Obviously, this is a crude analogy for estimating the probability of life on other planets. What similarities and differences do you see?