"Given that a repeated series of trials is required, frequentists are unable to assign probabilities to single events. Thus, with regard to whether it was raining in Manchester yesterday, there is no way of creating a large number of `yesterdays' in order to determine the probability. Frequentists would say that, even though they might not know, in actual fact it either was raining or it wasn't, and so this is not a matter for assigning a probability. And the same remains true even if we replace `Manchester' by `the Sahara Desert'.

Another example would be the unwillingness of a frequentist to assign a probability to the statement that `the first astronaut to set foot on Mars will return to Earth alive.' This does not mean it is an uninteresting question, especially if you have been chosen to be on the first manned-mission to Mars, but then, don't ask a frequentist to assess the probability."

Louis Lyons, "Bayes and Frequentism: a Particle Physicist's Perspective"