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In his review of Dirk Schulze-Makuch and David Darling's book (We Are Not Alone, 4 March), Ian Crawford criticises the authors for claiming that we have already discovered extraterrestrial life elsewhere in the Universe and insists that this is not the case. However, in making his argument, Crawford badly understates the important contribution of probability theory to this question.

Because he confines his criticism of their book to our own solar system, what Crawford says makes good sense: as yet there is no solid empirical evidence to support their thesis. But when we extend this argument to consider the question of the likelihood of life elsewhere in our Universe - as Schulze-Makuch and Darling do and Crawford should have done - then it simply is not statistically possible, with all of the billions of stars in our own galaxy alone, and all of the other billions of galaxies that we are told exist in our own Universe, that our planet is the only one to support life.

We already know that life exists in our own solar system - we ourselves, life on Earth, are the proof of this - but since this is the case, this must mean that life exists elsewhere in the Universe. It is not merely improbable, but it is impossible, that this should not be the case. All that "actual, definitive evidence for extraterrestrial life" elsewhere in our solar system (the empirical standard that Crawford wishes to apply) will ever do is prove what we already know to be the case.

Ken Smith, Bucks New University.