Ugly duckling theorem

The ugly duckling theorem is an argument showing that classification is not really possible without some sort of bias.

The theorem is named after Hans Christian Andersen's 1843 story "The Ugly Duckling", because it shows that a duckling is just as similar to a swan as two swans are to each other. It was derived by Satosi Watanabe, in 1969.^[1]^{: 376–377}