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Classification/Duhem: What is natural classification? Context of mental operations. It treats not concrete individuals, but abstractions: the types, e.g. vertebrates.
It explains, for example, that the swimming bubble of the fishes is the same as the lungs of the vertebrates. This is a purely ideal combination, which is not based on real organs but on simplified representations. More or less conspicuous similarities are seen by the zoologist as proof of a kinship.
If, on the other hand, paleontology were to explain to him that the kinship did not exist in this, the zoologist would nevertheless continue to believe that the system was a real relation. He would admit to having deceived himself about the nature of these relationships, but not about their existence.
Ziel und Struktur der physikalischen Theorien Hamburg 1998