Willard Van Orman Quine

Underdetermination of Physical Theory 95
According to Quine, an observation sentence for a given speech community
is an occasion sentence that is directly and firmly associated
with sensory stimulations for every member of the community and
on which all members give the same verdict when witnessing the
same situation (PTb 3). (Clearly, this is an idealization. People are
sometimes mistaken about the truth-value of observation sentences.
Possibly, no sentence satisfies Quine’s definition, strictly speaking.) 2
Examples of observation sentences are ‘It’s cold’, ‘That is a dog’, ‘This
is a flower’. Most observation sentences report physical things and
events, but some – for example, ‘Tom perceives a dog’ – are mentalistic
(PTb 62).
Observation sentences are occasion sentences; that is, they are
true on some occasions and false on others. Therefore, they cannot
be implied by scientific theories, which are either true or false once
and for all. However, two observation sentences can be combined
into a general sentence of the form ‘Whenever this, that’. An example
would be ‘Whenever there is a raven, it is black’, or simply ‘All
ravens are black’. Such sentences are called observation categoricals;
they are true or false once and for all, and they can be implied
by scientific theories. An observation categorical is synthetic for a
given speaker if the stimulations associated with the antecedent are
not completely included among the stimulations associated with
the consequent. Synthetic observation categoricals can be tested in
experiments. Two observation categoricals are synonymous for a
speaker if their respective components are associated with the same
stimulations. The empirical content of a theory for a given speaker
consists of the set of synthetic observation categoricals implied by it,
plus all synonymous ones. Moreover, Quine says that two theories
are empirically equivalent for a given community if they have the
same empirical content for each member (PTb 16–17). Presumably,
the empirical content that is common to the two theories can be different
for different speakers. Therefore, it is unclear what would be
the empirical content of a theory for a given community. Maybe we
could say that it is the union – or the intersection? – of the empirical
contents for all members.
Notice that, on this account, the empirical content of a theory
is relative to speakers of communities. It seems that it is also relative
to times, since the stimulations associated with an observation
sentence for a speaker can be expected to vary from time to time.
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