A number of scientific and philosophical arguments have been proposed against the hypothesis that there exist alien beings. The success of the SETI programme would therefore refute these arguments, and call into question the premises on which they are based. In this chapter I shall summarize three such arguments: Carter's anthropic principle argument, Fermi's "Where are they?" argument and the neo-Darwinian argument from contingency.
In a memorable lecture at The Royal Society in London, delivered in 1983, the astrophysicist Brandon Carter further developed his ideas about the anthropic principle that has been the subject of numerous texts. In its most useful form, the anthropic principle can provide a plausible explanation for certain apparent coincidences or con-