High Quality Content by WIKIPEDIA articles! The Ugly Duckling theorem is an argument asserting that classification is impossible without some sort of bias. It is named for Hans Christian Andersen's famous story of "The Ugly Duckling." It gets its name because it shows that, all things being equal, an ugly duckling is just as similar to a swan as two swans are to each other, although it is only a theorem in a very informal sense. It was proposed by Satoshi Watanabe in 1969. Suppose there are n things in the universe, and we want to put them into classes or categories. We have no preconceived ideas or biases about what sorts of categories are "natural" or "normal" and what are not. So we have to consider all the possible classes that could be, all the possible ways of making sets out of the n objects.