 However, the traditional locus classicus for underdetermination in science is the work of Pierre Duhem, a French physicist as well as historian and philosopher of science who lived at the turn of the 20th Century. In The Aim and Structure of Physical Theory, Duhem formulated various problems of scientific underdetermination in an especially perspicuous and compelling way, although he himself argued that these problems posed serious challenges only to our efforts to confirm theories in physics. In the middle of the 20th Century, W. V. O. Quine suggested that such challenges applied not only to the confirmation of all types of scientific theories, but to all knowledge claims whatsoever, and his incorporation and further development of these problems as part of a general account of human knowledge was one of the most significant developments of 20th Century epistemology. But neither Duhem nor Quine was careful to systematically distinguish a number of fundamentally distinct lines of thinking about underdetermination that may be discerned in their works. Perhaps the most important division is between what we might call holist and contrastive forms of underdetermination. Holist underdetermination (Section 2 below) arises whenever our inability to test hypotheses in isolation leaves us underdetermined in our response to a failed prediction or some other piece of disconfirming evidence. That is, because hypotheses have empirical implications or consequences only when conjoined with other hypotheses and/or background beliefs about the world, a failed prediction or falsified empirical consequence typically leaves open to us the possibility of blaming and abandoning one of these background beliefs and/or ‘auxiliary’ hypotheses rather than the hypothesis we set out to test in the first place. But contrastive underdetermination (Section 3 below) involves the quite different possibility that for any body of evidence confirming a theory, there might well be other theories that are also well confirmed by that very same body of evidence. Moreover, claims of underdetermination of either of these two fundamental varieties can vary in strength and character in any number of further ways: one might, for example, suggest that the choice between two theories or two ways of revising our beliefs is transiently underdetermined simply by the evidence we happen to have at present, or instead permanently underdetermined by all possible evidence. Indeed, the variety of forms of underdetermination that have been suggested to confront scientific inquiry, and the causes and consequences claimed for these different varieties, are sufficiently heterogeneous that attempts to address “the” problem of underdetermination for scientific theories have often engendered considerable confusion and argumentation at  cross-purposes.[2]