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Chapter XIV: On the discovery of Questions for DemonstrationEdit

To express questions for demonstration in a fitting manner a subject must be selected to which the quality to be demonstrated primarily belongs.

In order to find questions for solution we must make a selection from various partitions and divisions, taking care that a common generic notion lies at the base of all subordinate divisions, and assuming that all belong to a common genus. E.g. If Animals be the subject of investigation, we must first lay down what attributes are common to every animal. When this is done we should find what are the attributes of the first subdivision after the genus. Thus, if the subject be a bird, we should find the qualities possessed by every bird, and we must continue thus with the attributes of each lower term in the series. We shall then clearly be able to give the reason why the species included in the common genus possess such and such attributes; e.g. the reason why Man or Horse, as species of Animal, possess particular attributes. Let A be animal, B the attributes of every animal, C, D, E, particular species of animal. Now it is clear why B is an attribute of D, namely because of A, and it will similarly belong to the other species of animal. The same reasoning applies to other instances.

Hitherto we have spoken of cases where custom has sanctioned the application of the common class names to the particular species, but we should not limit ourselves to these. If anything else be seen to be an attribute of several things but to have no common generic name, we should take it for examination and look what are its attributes or of what it is an attribute. Thus, the possession of a ruminating stomach is a commonattribute of horned animals, as well as the possession of front teeth in one jaw only. We must then enquire what animals have the attribute of being horned. It will then be clear why the attribute mentioned belongs to these animals. It will belong to them because they have horns.[1] Another method consists in the observation of analogies. No single designation for instance exists for the spine of a fish, the pounce of a cuttle and ordinary bone, and yet all these parts have common qualities, as if their nature were the same in each case.


  1. Cf. Arist. de Part. Anim., III, 2.