Abstract

Consider Dretske's measles example (from page 74 in his Knowldege and the Flow of Information (MIT/Bradford: 1981) ): since the question of whether Alice's being one of Herman's children carries the information that she has the measles is a question about conditional probabilities, we must be careful about our specification of the condition, the antecedent. Although we are to suppose that it is a true generalization that all of Herman's children have the measles, since that is a coincidence, we can just as well suppose that Alice is an only child with the measles. It is of course true that the conditional probability of Alice's having measles given that she has the measles is 1; but that is not relevant to the question Dretske raises. In Dretske's example, the question is whether Alice's being Herman's child carries the information that she has measles. And so the relevant condition in this example is simply Alice's being Herman's child. While it is in fact true that Alice has the measles, that isn't part of the condition: for the question is, "how probable is the one state of affairs given some other state of affairs,"