1. In order to formulate the connexions we wish to prove we have to select our analyses and divisions. The method of selection consists in laying down the common genus of all our subjects of investigation - if e.g. they are animals, we lay down what the properties are which inhere in every animal. These established, we next lay down the properties essentially connected with the first of the remaining classes - e.g. if this first subgenus is bird, the essential properties of every bird - and so on, always characterizing the proximate subgenus. This will clearly at once enable us to say in virtue of what character the subgenera - man, e.g. or horse - possess their properties. Let A be animal, B the properties of every animal, C D E various species of animal. Then it is clear in virtue of what character B inheres in D - namely A - and that it inheres in C and E for the same reason: and throughout the remaining subgenera always the same rule applies.
2. We are now taking our examples from the traditional class-names, but we must not confine ourselves to considering these. We must collect any other common character which we observe, and then consider with what species it is connected and what.properties belong to it. For example, as the common properties of horned animals we collect the possession of a third stomach and only one row of teeth. Then since it is clear in virtue of what character they possess these attributes - namely their horned character - the next question is, to what species does the possession of horns attach?
3. Yet a further method of selection is by analogy: for we cannot find a single identical name to give to a squid's pounce, a fish's spine, and an animal's bone, although these too possess common properties as if there were a single osseous nature.