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Inphilosophy of science, theDuhem–Quine thesis, also called theDuhem–Quine problem, says that unambiguousfalsifications of ascientific hypothesis are impossible, because anempirical test of the hypothesis requires one or more background assumptions. Rather than disproving the main hypothesis, the blame can be placed on one of the background beliefs or "auxiliary" hypotheses.[1][2] It is named after French theoretical physicistPierre Duhem and American logicianWillard Van Orman Quine, who wrote about similar concepts.
In recent decades, the set of associated assumptions supporting a thesis sometimes is called abundle of hypotheses, i.e. a hypothesis and its background assumptions. Although a bundle of hypotheses as a whole can be tested against the empirical world and be falsified if it fails the test, the Duhem–Quine thesis says it is impossible to isolate a single hypothesis in the bundle, a viewpoint calledconfirmation holism.
The Duhem–Quine thesis argues that no scientific hypothesis is by itself capable of making predictions.[3] Instead, deriving predictions from the hypothesis typically requires background assumptions that several other hypotheses are correct — that an experiment works as predicted, or that previous scientific theory is accurate. For instance, as evidence against the idea that theEarth is in motion, some people objected that birds did not get thrown off into the sky whenever they let go of a tree branch. Later theories of physics and astronomy, such asclassical andrelativistic mechanics could account for such observations without positing a fixed Earth, and in due course they replaced the static-Earth auxiliary hypotheses and initial conditions.
Although a bundle of hypotheses (i.e. a hypothesis and its background assumptions)as a whole can be tested against the empirical world and be falsified if it fails the test, the Duhem–Quine thesis says it is impossible to isolate a single hypothesis in the bundle. One solution to the dilemma thus facing scientists is that when we have rational reasons to accept the background assumptions as true (e.g. explanatory scientific theories together with their respective supporting evidence) we will have rational—albeit nonconclusive—reasons for thinking that the theory under test probably is wrong in at least one respect if the empirical test fails.
AsAllan Franklin understands,
Consider themodus ponens. If a hypothesish entails evidencee then note entails noth. As Duhem and Quine, in slightly different ways, pointed out it is noth alone that entailse, but ratherh andb, that entailse, whereb is background knowledge. Thus, note entails noth or notb, and one doesn’t know where to put the not. [sic][4]
The work ofGalileo Galilei in the application of thetelescope to astronomical observation met with rejection from influential sceptics. They denied the truth of his most startling reports, such as that there were mountains on the Moon andsatellites around Jupiter.[5] In particular, some prominent philosophers, most notablyCesare Cremonini, refused to look through the telescope, arguing that the instrument itself might have introducedartefacts that produced the illusion of mountains or satellites invisible to the naked eye.[6] To neglect such possibilities amounted tounderdetermination in which argument for optical artefacts could be urged as being of equal merit to arguments for observation of new celestial effects. On a similar principle in modern times a prevalent view is that "extraordinary claims demand extraordinary proof."
In the early 17th century the modern version of the Duhem–Quine thesis had not been formulated, but common sense objections to such elaborate and ad hoc implicit auxiliary assumptions were urged. To begin with, the mechanism of the (Galilean) telescopes had been explained in terms ofgeometrical optics and the nature of the objects that they imaged was consistent; for example a distant lake would not resemble a tree when seen through a telescope. The behaviour of telescopes on Earth denied any basis for arguing that they could create systematic artefacts in the sky, such as apparent satellites that behaved in the predictable manner ofJovian moons. Evidence also offered no basis to suggest that they could present yet other, more elaborate artefacts, fundamentally different from the satellites, such as lunar mountains that cast shadows varying consistently with the direction of solar illumination.
In practice thepolitics and theology of the day determined the result of the dispute, but the nature of the controversy was a clear example of how different bundles of (usually implicit) auxiliary assumptions could support mutually inconsistent hypotheses concerning a single theory. In terms of either version of the Duhem–Quine thesis it therefore is necessary to study the defensibility of the auxiliary assumptions, together with the primary hypothesis, to arrive at the most viable working hypothesis.
As popular as the Duhem–Quine thesis may be in philosophy of science, in realityPierre Duhem andWillard Van Orman Quine stated very different theses. Duhem believed thatonly in the field of physics can a single individual hypothesis not be isolated for testing. He says in no uncertain terms that experimental theory in physics is not the same as in fields like physiology and certain branches of chemistry.[citation needed] Also, Duhem's conception of "theoretical group" has its limits, since he states that not all concepts are connected to each other logically.[citation needed] He did not include at alla priori disciplines such as logic and mathematics within the theoretical groups in physics, since they cannot be tested.[citation needed]
In "Two Dogmas of Empiricism", Quine presents a stronger form ofunderdetermination, extending it to allknowledge, even logic and mathematics. For Quine, knowledge constitutes a single web ofempirical significance, making even the most abstract componentsepistemologically akin tomythological posits devised to explain experience. Even logic and mathematics might be revised in light of experience, he once held, citingquantum logic.
He later recanted, writing inPhilosophy of Logic that revising logic would amount to "changing the subject". Inclassical logic, connectives are defined bytruth values. Inmulti-valued logic or quantum logics, however, they diverge in meaning, with quantum logic abandoning truth-functionalsemantics altogether. Quine argued thatdeviant logics tend to lack the simplicity and fruitfulness of classic logic.
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