The famous duck-rabbit ambiguous image. Is it a duck? Is it a rabbit?

"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." -- Thomas Kuhn, The Structure of Scientific Revolutions (3rd edn., p. 114).

• Source: Jastrow, J. (1899). The mind's eye. Popular Science Monthly, 54, 299-312.

	這一媒體文件在美國屬於公有領域。這可用於版權過期的美國作品，通常這些作品發表於1929年1月1日之前。查看這個頁面可以獲取到更多的解釋。
	這一圖片在美國以外可以沒有處於公有領域，特別是對美國作品不適用較短期限法則的國家，如加拿大、中國大陸（不含香港及澳門）、德國、墨西哥和瑞士。作品的作者和發表時間是關鍵信息，並且必須提供。參見 維基百科:公有領域及維基百科:版權信息以獲取更多細節。