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For as the boundary-stone separates what is proper to each from what is another's, so also the definition separates the nature of each from the nature of another. The soundness of a definition is to be neither deficient nor excessive in its words, but its fault is both to be deficient and to be excessive in its words. A perfect definition is one that is convertible with the thing defined, but an imperfect one is that which is not convertible. Therefore, neither the one deficient in words is convertible, nor the one that is excessive. For when it is excessive in words, it is deficient in things; but when it is deficient in words, it is excessive in things. So that one might say: nature has invented a wonderful contrivance, a deficiency that is rich and a richness that feigns deficiency. For instance, the perfect definition of man is "rational mortal animal." Behold, it is convertible; for every rational mortal animal is a man, and every man is a rational mortal animal. If, then, it is deficient by one word, it is excessive in things, for instance, "rational animal." Behold, it was deficient in a word, for I did not say "mortal," and it was excessive in things, for not only man is a rational animal but also an angel, and it is not convertible. But if I say "rational, mortal, literate animal," again it is not convertible; for it was excessive in a word by saying "literate," but deficient in things. For it did not define every man, but only the literate ones. For every rational, mortal, literate animal is a man, but not every man is a rational, mortal, literate animal; for not every man is literate. Therefore, perfect definitions are those that are convertible with the thing defined. But since the property is also convertible with that of which it is a property—for if something is a man, it is capable of laughter, and if something is capable of laughter, it is a man—we must further specify and say that perfect definitions are those derived from genus and constitutive differentiae, and are neither deficient nor excessive in words, and are convertible with the thing defined. Likewise also those derived from the combination of both the subject and the end are perfect, and those from the combination of both matter and form, and sometimes also those from the subject alone, when that subject is not subject to another art, as glass is to the art of glass-making, and from the end alone, when that end cannot be the end of another art, as in the case of shipbuilding. So from all these things one must know that the perfection of a term is to be convertible. Definition differs from term in respect to what is more universal and more particular; for 'term' is more universal than 'definition'. For 'term' signifies the boundary-stone, and it also signifies a decision, as we say "the king decided," it also signifies that into which a proposition is resolved, as with God's help we will learn in what follows, and it also signifies the definition. But 'definition' only signifies the concise account that declares the nature of the underlying thing. And one must know that a definition is applied only to substance and its species. And we cannot make a definition of either an individual or of accidents, but rather a description, because a definition is composed of a genus and constitutive differentiae, but a description is from non-essential attributes. 9 Concerning genus. It must be known that concerning homonyms, one must seek three things. If it is one of the homonyms, and how many things it signifies, and about which of them the discussion is. Therefore, one must first say what a homonym is. Homonyms, then, are when two or more things have one name, but each of them signifies something different and is of a different substance, that is, it receives a different definition, as is now the case with genus; for 'genus' is one of the homonyms. For 'genus' is said, first, of that which is from one's fatherland and from one's parent, and each of these in two ways: either proximately or remotely. From the proximate fatherland, as someone is called a Hierosolymite, but from the remote, a Palestinian. Similarly from the proximate parent, as when Achilles, being the son of Peleus, is called Peleides, but from the remote, as when the same Achilles is called Aeacides from his grandfather Aeacus; for he was the father of Peleus. Again, 'genus' is said of the relation of one to the many who come from him, as when all who have their descent from Israel are called Israelites. Concerning, then, the aforementioned genera

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γὰρ τὸ ὁροθέσιον χωρίζει τὸ ἴδιον ἑκάστου ἀπὸ τοῦ ἀλλοτρίου, οὕτω καὶ ὁ ὁρισμὸς τὴν φύσιν ἑκάστου χωρίζει ἀπὸ τῆς τοῦ ἑτέρου φύσεως. Ὑγίεια δὲ ὁρισμοῦ ἐστι τὸ μὴ ἐλλείπειν μήτε πλεονάζειν ταῖς λέξεσι, κακία δὲ τό τε ἐλλείπειν καὶ τὸ πλεονάζειν ταῖς λέξεσι. 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. Ὥστε λέγειν· θαυμαστὸν ἐφεῦρεν ἡ φύσις μηχάνημα, ἔνδειαν πλουτοῦσαν καὶ πλοῦτον ὑποκρινόμενον ἔνδειαν. 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 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. Τέλειοι οὖν ὁρισμοί εἰσιν οἱ ἀντιστρέφοντες πρὸς τὸ ὁριστόν. Ἀλλ' ἐπεὶ καὶ τὸ ἴδιον ἀντιστρέφει πρὸς τὸ οὗ ἐστιν ἴδιον-εἴ τι γὰρ ἄνθρωπος, τοῦτο γελαστικόν, καὶ εἴ τι γελαστικόν, τοῦτο ἄνθρωπος-, δεῖ ἡμᾶς προσδιορισαμένους εἰπεῖν, ὅτι τέλειοι ὁρισμοί εἰσιν οἱ ἐκ γένους καὶ συστατικῶν διαφορῶν παραλαμβανόμενοι καὶ μήτε ἐλλείποντες τῇ λέξει μήτε πλεονάζοντες καὶ ἀντιστρέφοντες πρὸς τὸ ὁριστόν. Ὡσαύτως δὲ καὶ οἱ λαμβανόμενοι ἐκ τοῦ συναμφοτέρου, τοῦ τε ὑποκειμένου καὶ τοῦ τέλους, τέλειοί εἰσι, καὶ οἱ ἐκ τοῦ συναμφοτέρου, τῆς τε ὕλης καὶ τοῦ εἴδους, ἔστι δὲ ὅτε καὶ οἱ ἐκ μόνου τοῦ ὑποκειμένου, ὅταν τὸ ὑποκείμενον ἐκεῖνο οὐχ ὑπόκειται ἑτέρᾳ τέχνῃ ὡς ἡ ὕελος τῇ ὑελοψικῇ, καὶ ἐκ τοῦ τέλους μόνου, ὅταν τὸ τέλος ἐκεῖνο οὐ δύναται τέλος εἶναι ἑτέρας τέχνης ὡς ἐπὶ τῆς ναυπηγικῆς. Ὥστε ἐκ πάντων τούτων δεῖ γνῶναι, ὅτι τελειότης ἐστὶν ὅρου τὸ ἀντιστρέφειν. ∆ιαφέρει δὲ ὁρισμὸς ὅρου τῷ καθολικωτέρῳ καὶ μερικωτέρῳ· ὁ γὰρ ὅρος καθολικώτερός ἐστι τοῦ ὁρισμοῦ. Ὁ μὲν γὰρ ὅρος δηλοῖ τὸ ὁροθέσιον, δηλοῖ δὲ καὶ τὴν ἀπόφασιν, ὥς φαμεν «ὥρισεν ὁ βασιλεύς», δηλοῖ καί, εἰς ὃν ἀναλύεται ἡ πρότασις, ὡς ἐν τοῖς ἑξῆς σὺν θεῷ μαθησόμεθα, δηλοῖ δὲ καὶ τὸν ὁρισμόν. Ὁ δὲ ὁρισμὸς μόνον δηλοῖ τὸν λόγον τὸν σύντομον, τὸν δηλωτικὸν τῆς φύσεως τοῦ ὑποκειμένου πράγματος. ∆εῖ δὲ γινώσκειν, ὅτι ἐπὶ τῆς οὐσίας μόνης λαμβάνεται ὁρισμὸς καὶ τῶν εἰδῶν αὐτῆς. Οὔτε δὲ ἀτόμου οὔτε συμβεβηκότων δυνάμεθα ποιήσασθαι ὁρισμὸν ἀλλ' ὑπογραφήν, διὰ τὸ τὸν ὁρισμὸν ἐκ γένους καὶ συστατικῶν διαφορῶν συγκεῖσθαι, τὴν δὲ ὑπογραφὴν ἐξ ἐπουσιωδῶν. 9 Περὶ γένους. Ἰστέον, ὡς ἐπὶ τῶν ὁμωνύμων τρία δεῖ ζητεῖν. Ἐάν ἐστι τῶν ὁμωνύμων, καὶ πόσα σημαίνει καὶ περὶ ποίου αὐτῶν ἐστιν ὁ λόγος. ∆εῖ οὖν πρότερον εἰπεῖν, τί ἐστιν ὁμώνυμον. Ὁμώνυμα οὖν εἰσιν, ὅταν δύο ἢ καὶ πλείονα ἓν ἔχουσιν ὄνομα, ἕκαστον δὲ αὐτῶν ἄλλο τι σημαίνει καὶ ἄλλης οὐσίας ἐστὶν ἤγουν ἕτερον ὁρισμὸν δέχεται, ὡς καὶ νῦν ἐπὶ τοῦ γένους· τὸ γὰρ γένος τῶν ὁμωνύμων ἐστί. Λέγεται γὰρ γένος πρῶτον μὲν τὸ ἀπὸ τῆς πατρίδος καὶ τοῦ τεκόντος, καὶ ἕκαστον αὐτῶν διχῶς· ἢ προσεχῶς ἢ πόῤῥω. Ἀπὸ μὲν τῆς προσεχοῦς πατρίδος, ὡς λέγεταί τις Ἱεροσολυμίτης, ἀπὸ δὲ τῆς πόῤῥω Παλαιστινός. Ὁμοίως ἀπὸ τοῦ προσεχοῦς τεκόντος, ὡς ὅταν ὁ Ἀχιλλεὺς υἱὸς ὢν τοῦ Πηλέως λέγεται Πηλεΐδης, ἀπὸ δὲ τοῦ πόῤῥω, ὡς ὅταν ὁ αὐτὸς Ἀχιλλεὺς ἀπὸ τοῦ πάππου αὐτοῦ Αἰακοῦ Αἰακίδης· οὗτος γὰρ ἦν τοῦ Πηλέως πατήρ. Λέγεται πάλιν γένος ἡ ἑνὸς πρὸς πολλοὺς τοὺς ἐξ αὐτοῦ σχέσις, ὡς ὅταν πάντες οἱ ἔχοντες τὸ καταγώγιον ἐξ Ἰσραὴλ λέγονται Ἰσραηλῖται. Περὶ μὲν οὖν τῶν προειρημένων γενῶν