Confirmation holism

Confirmation holism, also called epistemological holism is the claim that a single scientific theory cannot be tested in isolation; a test of one theory always depends on other theories and hypotheses.
For example, in the first half of the 19th century, astronomers were observing the path of the planet Uranus to see if it conformed to the path predicted by Newton's law of gravitation; it didn't. There were an indeterminate number of possible explanations, such as that the telescopic observations were wrong because of some unknown factor; or that Newton's laws were in error, or that God was causing the perturbation in order to show the hubris of modern science. However, it was eventually accepted that an unknown planet was affecting the path of Uranus, and that the hypothesis that there are seven planets in our solar system was false. Le Verrier calculated the approximate position of the interfering planet and its existence was confirmed in 1846. We now call the planet Neptune.
There are two aspects of confirmation holism. The first is that observations are dependent on theory (sometimes called theory-laden). Before accepting the telescopic observations one must look into the optics of the telescope, the way the mount is constructed in order to ensure that the telescope is pointing in the right direction, and that light travels through space in a straight line (which itself is sometimes not so, as Einstein demonstrated). The second is that evidence alone is insufficient to determine which theory is correct. Each of the alternatives above might have been correct, but only one was in the end accepted.
That theories can only be tested as they relate to other theories implies that one can always claim that test results that seem to refute a favoured scientific theory have not refuted that theory at all. Rather, one can claim that the test results conflict with predictions because some other theory is false or unrecognised. Maybe the test equipment was out of alignment because the cleaning lady bumped into it the previous night. Or, maybe, there is dark matter in the universe that accounts for the strange motions of some galaxies.
That one cannot unambiguously determine which theory is refuted by unexpected data means that scientists must use judgments about which theories to accept and which to reject. Logic alone does not guide such decisions.