Objection #1: The Problem of Old Evidence
We already know that life exists, and, consequently, we know that whatever is physically
required for life to exist must be actual. Any hypothesis that purports to "explain" the
coincidences is explaining something we already know to be true. It is not thereby making a
risky prediction that may or may not be borne out by subsequent observation. Hypotheses can be
confirmed or made more probable only when they make such risky predictions, as when Halley
predicted the return of Halley's comet, or Einstein predicted the bending of light by the sun's
gravity.
This objection can be illustrated by using Bayes's theorem, a basic theorem of probability theory.
Bayesians stipulate that the posterior probability of a hypothesis, after observing result E, is
equal to P(H/E). According to Bayes's theorem, P(H/E) is equal to the product of P(H), the prior
probability of H, and P(E/H), the degree to which H made E probable, divided by P(E), the prior
probability of E. The probability of H is increased if two conditions are met: (i) P(H) is not zero,
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 Fall '09
 JorgeRigol
 Probability, anthropic coincidences, old evidence

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