Rational Corpora

Abstract

Probability, in the sense that it is useful as a guide in life or as a guide to reasonable decisions in the face of uncertainty, must be relativized in some way to a body of knowledge. If we know that Mr. Jones is dead, we do not assign to him the canonical probability of survival for another year, as determined by mortality tables for people of his age and class. If Mr. Smith has a serious heart ailment, but lied about his health on his application for insurance, we can do nothing else than offer him insurance at the standard rates, unless we find out about his prevarication. If you know that an ordinary die was thrown and landed with an even number of spots up, the appropriate probability that the two landed uppermost to take as a basis for bets and indeed for other sorts of actions is not $\frac{1}{6}$ but $\frac{1}{3}$ It would be nice if we could always act on the basis of what is the case. As I hold a newly manufactured die in my hand, it is the case that it will be tossed a hundred times and land with the three up on the 100th toss, or that is not the case. Alas, the future is (partly) inaccessible to us. It is, on almost everyone’s view, this inaccessibility of the future that renders the concept of probability important and useful.