I am reading "The Quantum Theory of Measurement," by Busch, Lahti, and Mittelstaedt, and I came across this statement (p. 44, 1996 ed.): "The difficulties encountered in giving a precise formulation of this idea are due to the facts that relative frequencies are not probabilities, and probabilities need not be relative frequencies." Again, on p. 47, they mention that "... the concept of probability cannot be reduced to that of relative frequency." Now, I was taught at my mother's knee (well, my physics professor's) that probability was defined in terms of relative frequency. Can anyone help me understand what probability means, if not relative frequency, and under what conditions "probabilities need not be relative frequencies"?