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In his letter last week, Robert Poole argues that "we need other grounds for judging the likelihood of life occurring elsewhere in the Universe" than mere probability ("Sample of one", 25 March). But this is incorrect. Probability theory alone will do for judging the "likelihood" of something occurring; it is certainty that is in dispute.

He then goes on to say: "I'll bet Smith ?100 that no (proof of alien life) will be discovered in our lifetimes. In fact, I'm so certain that he may as well pay up now."

I'm happy to accept the bet because, if I win, he can send me the cash at any time during the next 50 years or so (or anyway, while we are both still alive). But if he wins, what I want to know is where I should send his money once we are both dead?

Ken Smith, Bucks New University.