Every definition must be an universal proposition, applicable, not exclusively to one particular object, but to a class of greater or less extent. The lowest species is easier to define than the higher genus; this is one reason why we must begin with particulars, and ascend to universals. It is in the higher genera that equivocal terms most frequently escape detection.[49] When you are demonstrating, what you have first to attend to is, the completeness of the form of syllogizing: when you are defining, the main requisite is to be perspicuous and intelligible; i.e. to avoid equivocal or metaphorical terms.[50] You will best succeed in avoiding them, if you begin with the individuals, or with examples of the lowest species, and then proceed to consider not their resemblances generally, but their resemblances in certain definite ways, as in colour or figure. These more definite resemblances you will note first; upon each you will found a formula of separate definition; after which you will ascend to the more general formula of less definite resemblance common to both. Thus, in regard to the acute or sharp, you will consider the acute in sound, and in other matters (tastes, pains, weapons, angles, &c.), and you will investigate what is the common point of identity characterizing all. Perhaps there may be no such identity; the transfer of the term from one to the other may be only a metaphor: you will thus learn that no common definition is attainable. This is an important lesson; for as we are forbidden to carry on a dialectical debate in metaphorical terms, much more are we forbidden to introduce metaphorical terms in a definition.[51]
[49] Analyt. Post. II. xiii. p. 97, b. 29: καὶ γὰρ αἱ ὁμωνυμίαι λανθάνουσι μᾶλλον ἐν τοῖς καθόλου ἢ ἐν τοῖς ἀδιαφόροις.
[50] Analyt. Post. II. xiii. p. 97, b. 31: ὥσπερ δε ἐν ταῖς ἀποδείξεσι δεῖ τό γε συλλελογίσθαι ὑπάρχειν, οὕτω καὶ ἐν τοῖς ὅροις τὸ σαφές.
By τὸ σαφές, he evidently means the avoidance of equivocal or metaphorical terms, and the adherence to true genera and species. Compare Biese, Die Philosophie des Aristot. pp. 308-310.
[51] Analyt. Post. II. xiii. p. 97, b. 35-39. — (διαλέγεσθαί φησι, τὸ διαλεκτικῶς ὁμιλεῖν. — Schol. p. 248, b. 23, Brand.). Aristotle considers it metaphorical when the term acute is applied both to a sound and to an angle.
The treatment of this portion of the Aristotelian doctrine by Prantl (Geschichte der Logik, vol. I. ch. iv. pp. 246, 247, 338), is instructive. He brings out, in peculiar but forcible terms, the idea of “notional causality� which underlies Aristotle’s Logic. “So also ist die Definition das Aussprechen des schöpferischen Wesensbegriffes.… Soweit der schöpferische Wesensbegriff erreicht werden kann, ist durch denselben die begriffliche Causalität erkannt; und die Einsicht in diese primitive Ursächlichkeit wird in dem Syllogismus vermittelst des Mittelbegriffes erreicht. Ueber den schöpferischen Wesensbegriff hinauszugehen, ist nicht möglich.… Sobald die Definition mehr als eine blosse Namenserklärung ist — und sie muss mehr seyn — erkennt sie den Mittelbegriff als schöpferische Causalität.… Die ontologische Bedeutung des Mittelbegriffes ist, dass er schöpferischer Wesensbegriff ist.� Rassow (pp. 51, 63, &c.) adopts a like metaphorical phrase:— “Definitionem est, explicare notionem; quæ quidem est creatrix rerum causa.�
To obtain and enunciate correctly the problems suitable for discussion in each branch of science, you must have before you tables of dissection and logical division, and take them as guides;[52] beginning with the highest genus and proceeding downward through the successively descending scale of sub-genera and species. If you are studying animals, you first collect the predicates belonging to all animals; you then take the highest subdivision of the genus animal, such as bird, and you collect the predicates belonging to all birds; and so on to the next in the descending scale. You will be able to show cause why any of these predicates must belong to the man Sokrates, or to the horse Bukephalus; because it belongs to the genus animal, which includes man and horse. Animal will be the middle term in the demonstration.[53] This example is taken from the class-terms current in vulgar speech. But you must not confine yourself to these; you must look out for new classes, bound together by the possession of some common attribute, yet not usually talked of as classes, and you must see whether other attributes can be found constantly conjoined therewith. Thus you find that all animals having horns, have also a structure of stomach fit for rumination, and teeth upon one jaw only. You know, therefore, what is the cause that oxen and sheep have a structure of stomach fit for rumination. It is because they have horns. Having-horns is the middle term of the demonstration.[54] Cases may also be found in which several objects possess no common nature or attribute to bind them into a class, but are yet linked together, by analogy, in different ways, to one and the same common term.[55] Some predicates will be found to accompany constantly this analogy, or to belong to all the objects quâ analogous, just as if they had one and the same class-nature. Demonstration may be applied to these, as to the former cases.
[52] Analyt. Post. II. xiv. p. 98, a. 1. πρὸς δὲ τὸ ἔχειν τὰ προβλήματα, λέγειν δεῖ τάς τε ἀνατομὰς καὶ τὰς διαιρέσεις, οὕτω δὲ διαλέγειν, ὑποθέμενον τὸ γένος τὸ κοινὸν ἁπάντων. This is Waitz’s text, which differs from Julius Pacius and from Firmin Didot.
Themistius (pp. 94-95) explains τὰς ἀνατομὰς to be anatomical drawings or exercises prepared by Aristotle for teaching: καὶ τὰς ἀνατομὰς ἔχειν δεῖ προχείρως, ὅσαι πεποίηνται Ἀριστοτέλει.
The collection of Problems or questions for investigation was much prosecuted, not merely by Aristotle but by Theophrastus (Schol. p. 249, a. 12, Br.).