A rule that strives to lay down the law about the significance of scientific evidence, then, must also lay down the law about the likelihood of all relevant auxiliary assumptions, in the same way that a procedure for determining chain strength must estimate the strength of every link. The rule’s judgments can be objectively valid only if its estimates of the auxiliary’s likelihoods are objectively valid. An objective rule for weighing any and every piece of evidence is therefore possible only if there is an objective fact of the matter about the likelihood of each relevant auxiliary
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The bayesians think they can do this.
