Generics, Frequency Adverbs, and Probability

as judgments of generics. I propose that the reason for this difference between
generics and frequency adverbs stems from the different criteria for
saliency. Since for frequency adverbs, only temporal partitions are relevant
for satisfying the homogeneity constraint, judgments on the truth or falsity
of Birds usually fly do not depend on whether or not partitioning the domain
according to biological families is considered salient.
Judgments on the saliency of a partition may be influenced by the linguistic
form of the relevant sentence. Consider the following pair of examples:
(29) a. Mammals are placental mammals.
b. Mammals have a placenta.
The majority of mammals are, indeed, placental mammals; yet (29.a) is bad.
In this case, partition into biological families, orders, or infraclasses would
cause the homogeneity constraint to be violated, since some of the subsets
of mammals induced by the partition would consist of marsupials. This, I
propose, is the explanation for the unacceptability of (29.a). Note that (29.b)
is considerably better, and many people would judge it to be true. Taxonomic
partitions are less salient when (29.b) is evaluated, because, unlike (29.a),
here there is no direct reference to the taxonomy of mammals.
Note that, as mentioned above, if all members of a domain satisfy a given
property, the domain is trivially homogeneous with respect to any partition.
Thus, since all dogs, without exception, are placental mammals, (30) is unproblematic,
in fact true:
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