Probability and Chance

The weather report says that the chance of a hurricane arriving later today is 90%. Forewarned is forearmed: expecting a hurricane, before leaving home you pack your hurricane lantern. Probability enters into this scenario twice, first in the form of a physical probability, sometimes called a chance, quantifying certain aspects of the local weather that make a hurricane very likely, and second in the form of an epistemic probability capturing a certain attitude to the proposition that a hurricane will strike, in this case one of considerable confidence. It is not immediately obvious that these two probabilities are two different kinds of thing, but a prima facie case can be made for their distinctness by observing that they can vary independently of one another: for example, if the meteorologists are mistaken, the chance of a hurricane may be very low though both they and I am confident that one is on its way. Most philosophers now believe that the apparent distinctness is real. They are therefore also inclined to say that my belief that the physical probability of a hurricane is very high is distinct from my high epistemic probability for a hurricane. There must be some principle of inference that takes me from one to the other, a principle that dictates the epistemic impact of the physical probabilities—or at least, of my beliefs about the physical probabilities—telling me, in the usual cases, to expect what is physically probable and not what is physically improbable. I call such a principle, me-