Generics, Frequency Adverbs, and Probability

proportion of black moths in these histories is roughly the same as it was in
the late 19th century.
It is the restriction of histories to admissible ones which accounts for the
fact that generics and frequency statements are not parametric on possible
worlds. There are any number of worlds where birds do not fly and where
John does not jog in the park, and correspondingly there are any number of
histories where birds lose the faculty of flight and John quits jogging. But
such histories will not be admissible. In order for them to be admissible they
would have to continue the actual history; but in the actual history birds do
fly and John does jog, so these histories would fail to be homogeneous. Only
histories in which things happen pretty much the way they occur in the actual
world will be admissible, hence generics and frequency statements are not
parametric on possible worlds. Since these histories are similar to the actual
world, with its contingent properties, generics and frequency statements are
contingent and not necessary.
The notion of homogeneity also accounts for the observation, discussed in
section 1.6 above, that generics and frequency statements require a regular
distribution of events in time. In a state of affairs where the distribution
of events is not regular, any history with respect to which the sentence is
evaluated will not be homogeneous. Hence there will not be any admissible
histories. Since, as we have noted, the proposed interpretation of probability
presupposes the existence of admissible histories, the sentence would be ruled
out as a case of presupposition failure.
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