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In Categories, Aristotle claims that all things that exist are either complex or simple; and the simple things can be classified into ten categories.

The first and most basic category includes particular things such as this very chair in my room, or a water-pistol I once had; or indeed, a particular man such as Plato or particular woman such as Aspasia.

We can say what they are - their whatness - by giving their species ie a man or woman, or genus animal.

What distinguishes species from genus - other than by containment? And why two and not just one, or indeed more when one looks at the modern zoological classification.

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The concept to classify by division has already been used - and possibly invented - by Plato under the name dihairesis (= division). Plato starts his work Sophistes with several examples of this method.

One can describe dihairesis as creating a decision tree. In the most simple case it is a binary tree. The tree has several layers and the leaves of the tree are the terms to be classified. Of course the inner nodes of a tree are not distinguished by any structural property, only root and leaves are. Hence the principle of "definition = species plus genus proximum" seems a simplication. It takes into account only two neighbouring layers of the decision tree.

It has been discussed whether such decision trees are uniquely determined: To which degree are the properties used in the classification predetermined by the things to be classified and to which degree do they just mirror our point of view?

Apparently Aristotle uses the method for his biological classifications. In biology, the lowest category of species has an objective meaning - capable of interbreeding and producing fertile offspring. But the higher categories are much more arbitrary.

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