Chapter 8

A definition is a concise statement setting forth the nature of the thing in question, that is to say, such statement as expresses in brief the nature of the thing in question. For example, man is a rational mortal animal capable of intelligence and knowledge. Now, many men have discoursed at length on the nature of man, that is, they have written long and extensive treatises on the subject. But these are not concise and, therefore, arc not definitions. There are also consise statements, such as apophthegms, but, since they do not .set forth the nature of a thing, they are not definitions. A name, too, oftentimes indicates the nature of the thing in question, but it is not a definition. For the name is one word, while the definition is a statement, and a statement is made up of at least two words. (Therefore, the definition is a name explained, whereas a name is a term of a proposition, when it is in conjunction.)

The definition is made up of genus and constituent, that is to say, essential differences. Thus it is with the definition of animal, for animal is an animate sentient substance. Here the genus is substance, while the constituent differences are the being animate and sentient. The definition may also be taken from matter and form, as, for example: A statue is that which is made of bronze and represents the form of a man. In this case the bronze is the matter, while the representation of the shape of the man is the form of the statue. The matter corresponds to the genus and the form to the specific difference. The definition may also be taken from subject and purpose. Medicine, for example, is concerned with human bodies and is productive of health. Here the subject of medicine is the human body, whereas its purpose is health.

Now, the description is made up of non-essential elements, that is to say, of properties and accidents. For example, man is an animal which is able to laugh, walks erect, and has broad nails. These elements are non-essential. For this reason it is called description, since it outlines, bringing out not the essential substance but only the things consequential to it. The descriptive definition is a combination of essentials and non-essentials, as, for example: Man is rational animal walking erect and having broad nails.

Definition is the term for the setting of land boundaries taken in a metaphorical sense. For, just as the boundary separates that which belongs to one from that which belongs to another, so does the definition set off the nature of one thing from that of any other.

Now, the soundness of a definition lies in its having neither too few nor too many terms, while its vice lies in its having either too few or too many terms. A perfect definition is one which is convertible with the thing defined, while an imperfect one is one which is not. Neither is that which has too few terms convertible (nor that which has too many), for, when it has too many terms, it covers too few things, whereas, when it has too few terms, it covers too many things. (And so one may say that nature has discovered a wonderful device—poverty that is wealthy and wealth feigning poverty.) For example, the perfect definition of man is: Man is rational mortal animal. Notice how this is convertible, for every rational mortal animal is a man and every man is a rational mortal animal. Now, if one term were to be left out, the definition would cover too many things. Take it, for example, as ‘rational animal.’ Here there are too few terms, because I did not say ‘mortal.’ And it covers too many things, because man is not the only rational animal; the angel is one, too. Therefore, it is not convertible. If, on the other hand, I should say ‘a rational, mortal, literate animal, again it is not convertible. For by my saying ‘literate it has received too many terms, while it covers too few things. This is because it has not defined every man, but only those men who are literate. Thus, every rational, mortal, and literate animal is a man, but not every man is a rational, mortal, and literate animal, because not every man is literate.

Therefore, those definitions are perfect which are convertible with the thing defined. Since, however, a property is also convertible with the thing of which it is a property— for, if anything is a man, it will be capable of laughter; and if anything is capable of laughter, it will be a man— then we must make an additional specification and say that perfect definitions are those which are taken from genus and constituent differences, which are neither deficient nor excessive in terms, and which are convertible with the thing defined. In the same way, those are perfect which are taken from the pairs of subject and purpose and of matter and form. Sometimes this is also true of those taken from the subject alone, as when the subject is not subject to any other art—as glass is not subject to any other art than that of the glass-maker. The same is also true of those taken from the purpose alone, in the case that that purpose is not the purpose of- any other art—as with the art of shipbuilding, As a result of all this one must know that the perfection of a definition is in its convertibility.

Definition differs from term by the one being more particular and the other more general. For term is more general than definition, because it means the setting of limits. It also means a decree, as when we say that the king ‘decreed/ It still further means that into which a proposition is resolved, as with God’s help, we shall learn in that which is to follow. It also means definition. Definition, however, means only the concise statement setting forth the nature of the thing in question.

One must know, furthermore, that a definition is given only in the case of the substance and its species, and that we cannot give a definition of an individual or of accidents, but only a description, because of the fact that the definition is made up of genus and constituent differences, while the description is made up of non-essentials.

[16] {Περὶ ὁρισμοῦ.} Ὁρισμός ἐστι λόγος σύντομος δηλωτικὸς τῆς φύσεως τοῦ ὑποκειμένου πράγματος ἤγουν ὁ λόγος ὁ δι' ὀλίγου σημαίνων τὴν φύσιν τοῦ ὑποκειμένου πράγματος, οἷον ἄνθρωπός ἐστι ζῷον λογικόν, θνητόν, νοῦ καὶ ἐπιστήμης δεκτικόν. Πολλοὶ μὲν γὰρ ἐποίησαν περὶ φύσεως ἀνθρώπου λόγους κατεπεκτάδην ἤγουν ἐκτεταμένους καὶ μεγάλους, ἀλλ' οὔκ εἰσι σύντομοι διὸ οὐδὲ ὁρισμοί εἰσιν. Εἰσὶ δὲ καὶ σύντομοι λόγοι ὡς τὰ ἀποφθέγματα, ἀλλ' ἐπειδὴ οὐ δηλοῦσι φύσιν πράγματος, οὔκ εἰσιν ὁρισμοί. Καὶ τὸ ὄνομα δὲ πολλάκις δηλοῖ τὴν φύσιν τοῦ ὑποκειμένου πράγματος, ἀλλ' οὐκ ἔστιν ὁρισμός: τὸ γὰρ ὄνομα μία λέξις ἐστίν, ὁ δὲ ὁρισμὸς λόγος ἐστί. Λόγος δὲ ἐκ δύο τὸ ἐλάχιστον σύγκειται λέξεων. Ὥστε ὁ ὁρισμὸς θέλει ἐκ διαφόρων λέξεων συγκεῖσθαι καὶ σύντομος εἶναι καὶ δηλοῦν τὴν φύσιν τοῦ ὑπ' αὐτοῦ ὁριζομένου πράγματος. Συνίσταται δὲ ὁ ὁρισμὸς ἐκ γένους καὶ συστατικῶν διαφορῶν ἤγουν οὐσιωδῶν, ὡς ἐπὶ τοῦ ὅρου τοῦ ζῴου: ζῷον γάρ ἐστιν οὐσία ἔμψυχος αἰσθητική. Ἰδοὺ ἡ μὲν οὐσία γένος ἐστί, τὸ δὲ ἔμψυχον καὶ τὸ αἰσθητικὸν συστατικαὶ διαφοραί. Λαμβάνεται δὲ καὶ ἐξ ὕλης καὶ εἴδους, οἷον ἀνδριάς ἐστιν ἐκ χαλκοῦ πεποιημένος, ἀνδρὸς εἶδος σημαίνων: ὁ μὲν οὖν χαλκὸς ὕλη ἐστί, τὸ δὲ τοῦ ἀνδρὸς εἶδος εἶδος. Ἀναλογεῖ δὲ ἡ μὲν ὕλη τῷ γένει, τὸ δὲ εἶδος διαφορᾷ. Λαμβάνεται δὲ καὶ ἐξ ὑποκειμένου καὶ τέλους, οἷον ἰατρική ἐστι τέχνη περὶ τὰ ἀνθρώπινα σώματα καταγινομένη ὑγιείας περιποιητική. Ἰδοὺ ὑποκείμενά εἰσι τῇ ἰατρικῇ τὰ ἀνθρώπινα σώματα, τέλος δὲ αὐτῆς ἡ ὑγίεια. Ἡ δὲ ὑπογραφὴ ἐκ τῶν ἐπουσιωδῶν σύγκειται ἤγουν ἰδίων καὶ συμβεβηκότων, οἷον ἄνθρωπός ἐστι γελαστικόν, ὀρθοπεριπατητικόν, πλατυώνυχον: ταῦτα γὰρ ὅλα ἐπουσιώδη εἰσί. Διὸ καὶ ὑπογραφὴ λέγεται, ὡς σκιάζουσα καὶ οὐ τὴν οὐσιώδη ὕπαρξιν τοῦ ὑποκειμένου δηλοῦσα ἀλλὰ τὰ παρεπόμενα. Ὁ δὲ ὑπογραφικὸς ὁρισμὸς μικτός ἐστιν ἐξ οὐσιωδῶν καὶ ἐπουσιωδῶν, οἷον ἄνθρωπός ἐστι ζῷον λογικόν, ὀρθοπεριπατητικόν, πλατυώνυχον. Εἴρηται δὲ ὁρισμὸς ἀπὸ μεταφορᾶς τῶν τῆς γῆς ὁροθεσίων: ὥσπερ γὰρ τὸ ὁροθέσιον χωρίζει τὸ ἴδιον ἑκάστου, οὕτω καὶ ὁ ὁρισμὸς τὴν φύσιν ἑκάστου χωρίζει ἐκ τῆς τοῦ ἑτέρου φύσεως. Ὑγίεια δὲ ὁρισμοῦ ἐστι τὸ μήτε ἐλλείπειν μήτε πλεονάζειν ταῖς λέξεσι, κακία δὲ τὸ ἐλλείπειν καὶ πλεονάζειν. Τέλειος δὲ ὁρισμός ἐστιν ὁ ἀντιστρέφων πρὸς τὸ ὁριστόν, ἀτελὴς δὲ ὁ μὴ ἀντιστρέφων. Οὔτε οὖν ὁ ἐλλείπων ταῖς λέξεσιν ἀντιστρέφει οὔτε ὁ πλεονάζων. Ὅτε μὲν γὰρ πλεονάσει ταῖς λέξεσι, λείπει τοῖς πράγμασιν: ὅτε δὲ λείψει ταῖς λέξεσι, πλεονάζει τοῖς πράγμασιν. Οἷον ὁ τέλειος ὁρισμὸς τοῦ ἀνθρώπου ‘ζῷον λογικὸν θνητόν’. Ἰδοὺ ἀντιστρέφει: πᾶν γὰρ ζῷον λογικὸν θνητὸν ἄνθρωπός ἐστι, καὶ πᾶς ἄνθρωπος ζῷόν ἐστι λογικὸν θνητόν. Ἐὰν οὖν λείψῃ μιᾷ λέξει, πλεονάζει τοῖς πράγμασιν, οἷον ζῷον λογικόν. Ἰδοὺ ἔλειψε τῇ λέξει, οὐ γὰρ εἶπον ‘θνητόν’, καὶ ἐπλεόνασε τοῖς πράγμασιν, οὐ γὰρ μόνος ἐστὶν ἄνθρωπος ζῷον λογικὸν ἀλλὰ καὶ ἄγγελος, καὶ οὐκ ἀντιστρέφει. Ἐὰν δὲ εἴπω ‘ζῷον λογικόν, θνητόν, γραμματικόν’, πάλιν οὐκ ἀντιστρέφει: ἐπλεόνασε γὰρ τῇ λέξει διὰ τοῦ εἰπεῖν ‘γραμματικόν’, ἔλειψε δὲ τοῖς πράγμασιν. Οὐ πάντα γὰρ ἄνθρωπον ὡρίσατο, ἀλλ' ἦ μόνον τοὺς γραμματικούς. Πᾶν μὲν γὰρ ζῷον λογικόν, θνητόν, γραμματικὸν ἄνθρωπός ἐστιν, οὐ πᾶς δὲ ἄνθρωπος ζῷον λογικόν, θνητόν, γραμματικόν ἐστιν: οὐ γὰρ πᾶς ἄνθρωπος γραμματικός ἐστι.