The Blackwell Guide to the Philosophy of Science - The Department ...

Experiment and Observation
example of the epistemically significant interplay between theoretical commitments
and data production (Galison, 1997, pp. xvii–63). 17
Hidden possibilities
If an empirical result agrees with a theory, does that entitle us to conclude anything
more than that the result agreed with the theory? Bacon is supposed to have
thought so for cases of crucial experiments in which the result also rules out the
theory’s known competitors. 18 But Duhem argues that, to prove the correctness
of a theory, a crucial experiment would have to test every possible alternative, and
scientists are never in a position to know what alternatives remain to be discovered
(Duhem, 1991, pp. 180–90). Thus, Duhem would deny that empirical evidence
which favored GTR over all of its known competitors establishes its
correctness. The most it establishes is that GTR did better than the alternatives
with which it was compared. That wouldn’t be so bad if the evidence told the
investigators what probability to assign to GTR. But as long as the possibility space
remains uncharted, it would seem that probability could be assigned only relative
to the known possibilities, or by assigning a prior probability to the disjunction
of unknown alternative possibilities. The latter alternative is unattractive because
it is as hard to see how the priors could be assigned non-arbitrarily, as to see what
should be made of probabilities which depend on arbitrarily stipulated priors. The
former conjures up memories of defective theories which were accepted on evidence
which would have been unimpressive if alternative theories had been developed
(Earman, 1992, ch. 7). Thus a pressing question for empirical epistemology
is the issue of how empirical results can contribute to the exploration and limitation
of possibility spaces.
Replacing empiricism
Old-fashioned empiricists thought the test of a scientific claim is how well it stacks
up against evidence delivered by the senses – unaided, or aided by magnifying or
amplifying devices. But what about evidence produced by equipment like Geiger
counters and galvanometers which are attuned to signals which the senses cannot
pick up? It doesn’t take too much of a stretch for empiricists to extend their notion
of empirical evidence to apply to what registers only on observational equipment.
But some of the evidence scientists rely on presents a more severe challenge to
empiricism. For example, PET and fMRI imaging are so far the best, noninvasive
techniques for comparing levels of cognitively significant neuronal
activity in anatomically different regions of the brain during the performance of a
mental task. The quantities exhibited by PET images are usually calculated from
signals emitted by an oxygen isotope which has been introduced into the circula-
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