FIG. 1.

Schematic illustration of Bayes' Theorem as applied to anthropic selection. p(x) is the probability distribution of property x for a general population of planets. p(I/x) models how the probability of the emergence of intelligence varies with x. In the case illustrated here, intelligence is most likely when x is small. p(x/I) is the resulting probability distribution of x for the subset of planets which happen to be inhabited. Note that the most likely value of x is smaller for inhabited worlds, that is, there is an anthropic selection effect.