Theism and Explanation - Appeared-to-Blogly

120 Theism and Explanation
to the deduction of hitherto unobserved facts. When Albert Michelson and
Edward Morley set up their famous experiment in 1887, they did not yet
know what its outcome would be. All they knew is that if there were a
luminiferous aether, you would expect a difference in the speed of light
when measured parallel to and at right angles to the movement of the earth
through the aether. Their failure to observe this difference constituted
strong evidence against the theory. But the term “prediction” is sometimes
used in a looser sense. We have seen that to test his theory of the electron,
J. J. Thomson “predicted” that if cathode rays were negatively charged
particles, they would carry their charge into an enclosure. But he already
knew the outcome of the experiment that tested the prediction, which had
been performed in 1895 by Jean Perrin. 9
The Popperian view. The question that arises here is: Do we need successful
“predictions” in the strict sense of that word? That is to say, is it necessary
that the confi rming observation should occur only after the formulation of
our hypothesis? Could a theory be corroborated by facts that are already
known? A strictly Popperian view of testability denies this; it insists that
the facts that corroborate a theory must be previously unknown. For Popper,
what counts is a severe test, and a severe test of a theory is one whose
outcome is unlikely to be observed, given background knowledge alone. 10
A theory is corroborated insofar as it has passed severe tests. But if the
outcome of the test is already part of our background knowledge, then a
“prediction” of that outcome can hardly constitute a severe test.
The problem with the Popperian view is one at which I have already
hinted. It is simply too demanding, given actual scientifi c practice. As John
Worrall points out, the facts concerning the perihelion of Mercury—the
rate at which its elliptical orbit around the sun rotates—were known well
in advance of Einstein’s theory of general relativity. (The apparent anomaly
here, on a Newtonian view, had been noted as early as 1859.) But the fact
that Einstein’s physics could explain this movement while Newton’s could
not has generally been taken as evidence in support of general relativity. 11
On a Popperian view, this conclusion would be illegitimate. 12
In addition to this, there appear to be good theoretical reasons to allow a
theory to be corroborated by known facts. After all, it could be argued, what
matters when we are testing a theory is that its consequences are true.
If the evidence shows that some consequence of a theory is true, then
this cannot depend on whether the evidence came to be known before
the theory was proposed or afterwards. Such historical considerations,
interesting as they may be, ought not to affect questions of confi rmation
or evidential support. 13
So on logical grounds alone, it would seem reasonable to allow a theory to
be corroborated by facts that are already known.