“The subject of a gestalt demonstration knows that his perception has shifted because he can make it shift back and forth repeatedly while he holds the same book or piece of paper in his hands. Aware that nothing in his environment has changed, he directs his attention increasingly not to the figure (duck or rabbit) but to the lines of the paper he is looking at. Ultimately he may even learn to see those lines without seeing either of the figures, and he may then say (what he could not legitimately have said earlier) that it is these lines that he really sees but that he sees them alternately as a duck and as a rabbit. …As in all similar psychological experiments, the effectiveness of the demonstration depends upon its being analyzable in this way. Unless there were an external standard with respect to which a switch of vision could be demonstrated, no conclusion about alternate perceptual possibilities could be drawn.”
Source: The Structure of Scientific Revolutions (1962), X. Revolutions as Changes of World View, p. 114 (3rd edn.)
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Thomas Kuhn 27
American historian, physicist and philosopher 1922–1996Related quotes

Letter to Oliver Evans (16 January 1814); published in The Writings of Thomas Jefferson (1905) Vol. 13, p. 66
1810s
Context: A man has a right to use a saw, an axe, a plane, separately; may he not combine their uses on the same piece of wood? He has a right to use his knife to cut his meat, a fork to hold it; may a patentee take from him the right to combine their use on the same subject? Such a law, instead of enlarging our conveniences, as was intended, would most fearfully abridge them, and crowd us by monopolies out of the use of the things we have.
Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 277.

In a letter to his son w:Lucien Pissarro, 14th May 1887, as quoted in Renoir – his life and work, Francois Fosca, Book Club Associates /Thames and Hudson Ltd, London 1975, p. 188
Pissarro's critical quote on Renoir's painting art
1880's

Source: The Dangerous Summer (1985), Ch. 13

On his friend, George Orwell, in The Sunday Times (1968-09-29); reprinted in The Evening Colonnade (New York, 1973)
The Heat in the Room (Jarkko Laine: Amerikan Cowboy. Helsinki: Otava, 1981. ISBN 951-1-06597-1

As quoted in Walt Disney, Magician of the Movies (1966) by Bob Thomas p. 116