Should we be more sur­prised to find some ancient evi­dence of liv­ing organ­isms on Mars or not to find such evi­dence? There has been a lot of debate about the proa­bil­i­ty of life ocur­ring on any giv­en plan­et in the uni­verse. Although this prob­a­bil­i­tiy (call it P0) has not been sat­is­fac­to­ri­ly deter­mined, “Amir Aczel”:–0052059-3270336 esti­mates the prob­a­bil­i­ty of life exist­ing on a plan­et orbit­ing at least one oth­er star in the uni­verse as 1, to a rea­son­able approx­i­ma­tion. His hypoth­e­sis has been “criticized”: but the crit­i­cisms, which relate prin­ci­pal­ly to his esti­mate of the P0 vari­able —do not seem to make much dif­fer­ence to his result. The uni­verse is so vast that the immense size of the “sam­ple space” swamps all oth­er vari­ables in the prob­a­bil­i­ty expres­sion. What, how­ev­er, is the prob­a­bil­i­ty of life occur­ring more than once in a solar-sys­tem (e.g. ours ). The prob­a­blil­i­ty must be large in com­par­i­son to P0 because many of fun­da­men­tal pre-con­di­tions for life have already been met (e.g. age, size and type of the star). And what, if any­thing, does the dis­cov­ery of this life, occur­ring in a cos­mic neigh­bor­hood where life has already occurred once, tell us about the val­ue of P0? Noth­ing, I sug­gest. The Mars probe bud­get needs some oth­er justification.

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