Duhem–Quine thesis

The Duhem–Quine thesis, also called the Duhem–Quine problem, after Pierre Duhem and Willard Van Orman Quine, is that it is impossible to test a scientific hypothesis in isolation, because an empirical test of the hypothesis requires one or more background assumptions (also called auxiliary assumptions or auxiliary hypotheses). In recent decades the set of associated assumptions supporting a thesis sometimes is called a bundle of hypotheses.

The Duhem–Quine thesis argues that no scientific hypothesis is by itself capable of making predictions. Instead, deriving predictions from the hypothesis typically requires background assumptions that several other hypotheses are correct; for example, that an experiment works as predicted or that previous scientific theory is sufficiently accurate. For instance, as evidence against the idea that the Earth 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 as classical and relativistic 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.

The pioneering work of Galileo Galilei in the application of the telescope 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 and satellites around Jupiter. In particular some prominent philosophers, most notoriously the highly respected Cesare Cremonini, refused to look through the telescope, arguing in effect that the instrument might have introduced artefacts producing illusions of mountains or satellites invisible to the unencumbered eye. To neglect such possibilities amounted to underdetermination in which argument for optical artefacts could be urged as being of merit equal to arguments for observation of new celestial effects, whether the latter were more parsimonious or not. On a similar principle in modern times a prevalent view is that extraordinary claims demand extraordinary proof.

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