Passage
Karl Popper’s main contribution to the philosophy of science concerns the power of negative evidence. The fundamental point is simple: No number of white swans, for example, can ever prove that all swans are white, but a single black swan disproves the hypothesis. Popper gives this logical asymmetry between positive and negative a theory counts as scientific only if it makes predictions that are testable in this way.
However, Popper’s use of the logical asymmetry does not adequately capture the actual situation scientists face. If a theory deductively entails a false prediction, then the theory must be false as well. But a scientific theory rarely entails predictions on its own. When scientists actually derive a theory’s predictions, they almost always more than one possible explanation. Positive evidence is never conclusive. But negative evidence rarely is either.
Passage B When the planet Uranus was discovered, astronomers attempted to predict its orbit. They based their predictions on Newton’s laws and auxiliary assumptions about the mass of the sun and the masses, orbits, and velocities of other planets. One of the auxiliary assumptions was that no planets existed in the vicinity precise place it would have to be to bring their calculations into alignment with their observations.
Later astronomers, again using Newton’s laws, predicted the orbit of Mercury. Once again, the predictions were not borne out. They hypothesized the existence of another planet in the vicinity, which they called Vulcan. However, Vulcan was never found, and some scientists began to think that perhaps Newton’s laws were in error. Finally, to the rejection of Newton’s theory of gravity and to increased confidence in Einstein’s theory.
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