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Probabilistic thinking was a mid-17th century artifact originating in a famous correspondence between Fermat and Pascal -- a correspondence on which Huygens based a widely read textbook: On Calculating in Games of Luck (1657). The probabilistic framework didn't exist until those people cobbled it together. It remains in use today, much as in Huygens's book.

Within the framework, we make up our minds by adopting probability functions -- or, anyway, features of such functions. This is not a matter of eliciting what is already in mind; rather, it is a matter of artifice, a family of arts of probabilistic judgment. And that includes making up our minds about how to change our minds. With the founders (so I think), and certainly with Ramsey and de Finetti, who revived the view of probability as a mode of judgment, I see the probability calculus as a logic. Mere logic does not tell us how to make up our minds. It does help us spot inconsistencies within projected mental makeups, but fails to underwrite the further gradations we make in classifying survivors of the first cut as reasonable or not, or in grading them as more or less reasonable. These finer distinctions separate cases on the basis of standards rooted in our actual appreciation of ongoing methodological experience.

 The basic ideas, floated in 1 and 3, are applied in 2 and 4 to troubling questions about scientific method and practical decision-making. The question of normativity is addressed in 5.

 I'd be glad to have corrigenda and other suggestions.

Richard Jeffrey

7 Dec 99

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