Holism

The advocates of the empiricist consensus assumed either the H-D account of justification, or possibly the Popperian view of falisificationism.  Both of these accounts of why we are justified in believing general statements ("laws" or "theories") hold that each  proposed general hypothesis has to meet the "test" of observational evidence, or in other words what we "test" by the scientific method are hypotheses one by one. The philosophical thesis known as "holism" presents a serious challenge to this apparently innocent assumption.
 "Holism," is the view that what is in fact "tested" by observational evidence (if anything is) are not individual laws or theories, but rather large constellations of belief which include not only the one "theory" that is allegedly undergoing empirical "testing" but also a whole array of "auxiliary hypotheses" which are more or less tacitly taken for granted.
This point was first made by the non-consensus French philosopher/historian of science, Pierre Duhem. Later the American pragmatist Willard van Orman Quine made effectively the same point, but extended the set of "auxiliary hypotheses" to include not only those statements which the positivists had considered "synthetic" but also even those once regarded as "analytic," i.e. true by definition. Quine's point was that faced with "recalcitrant evidence" (observations which do not square with our expectations), there is always the option of altering meanings, or even logical laws, to save a cherished hypothesis from potential refutation. Thus in famous phrases Quine argues that our whole system of scientific claims, our "web of belief," faces the "tribunal of experience" only as a "collective body" not as individual hypotheses.
 
Thus the "Quine-Duhem thesis" has come to be the name for the (presumably true) view that what is actually required for a test is not merely one hypothesis, but the conjunction of that hypothesis with a wide variety of often tacit auxiliary hypotheses
The occurrence of an observation which refutes a prediction derived from the conjunction of the theory under test with all the necessary auxiliaries only shows that at least some one member of that conjunction is false, not that the false assumption is necessarily the theory under test. Whether the false prediction resulted from the falsity of the tested hypothesis or the falsity of some other one or more of the auxiliary assumptions is not known. Thus, according to the Quine-Duhem thesis, by suitable modification or buttressing of the proper auxiliary hypotheses, any theory can always be "saved" from potential refutation. Furthermore, it is often claimed that historical research shows that scientists do frequently do precisely this. Hence it would seem that, contra Popper, theories cannot be definitively refuted any more than they can be confirmed.