It reminded me of one of my formative childhood books:

What is the probability there is some form of life on Titan? We apply the principle of indifference and answer 1/2. What is the probability of no simple plant life on Titan? Again, we answer 1/2. Of no one-celled animal life? Again, 1/2.

--Martin Gardner, Aha! Gotcha

He goes on to demonstrate the obvious contradiction, and points out some related fallacies. The whole book is great, as is its companion Aha! Insight. (They're bundled into a book called Aha! now.)