[279] Topic. VI. iv. p. 142, a. 10.
[280] Ibid. a. 12; also, a. 32.
The canon being, That what is posterius must be defined by its prius, — the definer may sin against this in defining the prius by its posterius; e.g., if he defines the stationary and the determinate by means of the moveable and the variable.[281] Also, when his definition is neither prius, nor posterius, but of equal position with the definiend, he is at fault. This may happen (1) when he defines by an Opposite (for, according to some, the science of Opposites is one and the same, and it is impossible that either one of a pair can be absolutely more knowable than the other; though it is true that no relative can be understood or explained without the knowledge of its correlative, e.g., double and half); or (2) when he includes the definiend itself in his definition, either under its proper name or any other name;[282] or (3) when he defines by means of a contra-specific to the definiend — by something of equal specific rank or position, which is therefore simul naturâ therewith (e.g., Odd is that which is greater than even by unity); or (4) when he defines by something specifically subordinate (e.g., An even number is that which may be bisected, where bisected means divisible by two, itself one among the even numbers[283]).
[281] Ibid. a. 20: πρότερον γὰρ τὸ μένον καὶ τὸ ὡρισμένον τοῦ ἀορίστου καὶ ἐν κινήσει ὄντος.
[282] Topic. VI. iv. p. 142, a. 22-b. 6.
[283] Ibid. b. 7-19: πάλιν, εἰ τῷ ἀντιδιῃρημένῳ τὸ ἀντιδιῃρημένον ὥρισται — ὁμοίως δὲ καὶ εἰ διὰ τῶν ὑποκάτω τὸ ἐπάνω ὥρισται.
2. The second locus (after that bearing on the Prius et Notius) of argument for impugning a definition is, where it does not enunciate the genus in which the definiend is really included. The mention of the genus, as enunciating the fundamental essence of the definiend, ought to stand first in the definition. If your opponent defines body — that which has three dimensions, or man — that which knows how to count, you attack him by asking, What is it that has three dimensions? What is it that knows how to count? No genus has been assigned.[284]
[284] Ibid. v. p. 142, b. 22-29.
3. A third locus is, where the definiend is a complex whole having reference to several distinct facts or phenomena, while the definition indicates only one of them. Thus, if grammar be defined — the knowing how to write from dictation, you will object that it is just as much — the knowing how to read. The definition is incomplete unless it includes both.[285]
[285] Ibid. b. 30.