If the predicate of the thesis be a true genus, you may convert the thesis simply, having substituted for the predicate the denial of its Contradictory; if not, not. Vice versâ, if the new proposition so formed be true, the predicate of the thesis will be a true genus; if not, not. Thus, if good be the true genus of pleasurable, nothing that is not good will be pleasurable. This locus also will serve both for making out an affirmative and for making out a negative.[178]
[178] Topic. IV. iv. p. 124, b. 7-14: πάλιν ἐπὶ τῶν ἀποφάσεων σκοπεῖν ἀνάπαλιν, &c.
If the subject (species) of the thesis be a Relative, you will examine whether the predicate (genus) be relative also; if not, it will not be the true genus of the subject. The converse of this rule, however, will not hold; and indeed the rule itself is not absolutely universal.[179] You may also argue that, if the correlate of the genus be not the same as the correlate of the species, the genus cannot be truly predicated of that species: thus, half is the correlate of double, but half is not the proper correlate of multiple; therefore, multiple is not the true genus of double. But your argument may here be met by contradictory instances; thus, cognition has reference to the cognitum, but habitus and dispositio (the genera to which cognitio belongs) do not refer to cognitum but to anima.[180] You may also examine whether the correlate, when applied to the genus, is put in the same case (e.g., genitive, dative, &c.) as when it is applied to the species: if it be put into a different case, this affords presumption that the genus is not a true genus; though here again instances may be produced showing that your presumption will not hold universally. Farther, you will observe whether the correlates thus similarly inflected reciprocate like the species and genus; if not, this will furnish you with the same adverse presumption.[181]
[179] Ibid. b. 15-22.
[180] Ibid. b. 23-34.
[181] Ibid. b. 35, seq.
Again, examine whether the correlate of the genus is genus to the correlate of the species; if it be not so, you may argue that the genus is not truly predicated. Thus, if the thesis affirms that perceptio is the genus of cognitio, it will follow that percipibile is the genus of cognoscibile. Now this cannot be maintained; for there are some cognoscibilia which are not perceivable, e.g., some cogitabilia (intelligibilia, νοητά). Since therefore percipibile is not the true genus of cognoscibile, neither can perceptio be the true genus of cognitio.[182]
[182] Ibid. p. 125, a. 25-32: ὁρᾶν δὲ καὶ εἰ τοῦ ἀντικειμένου τὸ ἀντικείμενον γένος, οἷον εἰ τοῦ διπλασίου τὸ πολλαπλάσιον καὶ τοῦ ἡμίσεος τὸ πολλοστημόριον· δεῖ γὰρ τὸ ἀντικείμενον τοῦ ἀντικειμένου γένος εἶναι.
We must take note here of the large sense in which Aristotle uses Ἀντικείμενα — Opposita, including as one of the four varieties Relata and Correlata = Relativé-Opposita (to use a technical word familiar in logical manuals). I have before (supra, [p. 105]) remarked the inconvenience of calling the Relative opposite to its Correlate; and have observed that it is logically incorrect to treat Relata as a species or mode of the genus Opposita. The reverse would be more correct: we ought to rank Opposita or a species or mode under the genus Relata. Since Aristotle numbers Relata among the ten Categories, he ought to have seen that it cannot be included as a subordinate under any superior genus.
Suppose the thesis predicates of memory that it is — a continuance of cognition. This will be open to attack, if the predicate be affirmed as the genus (or even as the accident) of the subject. For every continuance must be in that which continues. But memory is of necessity in the soul; it cannot therefore be in cognition.[183] There is another ground on which the thesis will be assailable, if it defines memory to be — a habit or acquirement retentive of belief. This will not hold, because it confounds habit or disposition with act; which last is the true description of memory. The opposite error will be committed if the respondent defines perceptivity to be a — movement through or by means of the body. Here perceptivity, which is a habit or disposition, is ranked under movement, which is the act exercising the same, i.e., perceptivity in actual exercise.[184] Or, the mistake may be made of ranking some habit or disposition under the power consequent on the possession thereof, as if this power were the superior genus: thus the respondent may define gentleness to be a continence of anger; courage, a continence of fears; justice, a continence of appetite of lucre. But the genus here assigned is not a good one: for a man who feels no anger is called gentle; a man who feels no fear is called courageous; whereas the continent man is he who feels anger or fear, but controls them. Such controlling power is a natural consequence of gentleness and courage, insomuch that, if the gentle man happened to feel anger, or the courageous man to feel fear, each would control these impulses; but it is no part of the essence thereof, and therefore cannot be the genus under which they fall.[185] A like mistake is made if pain be predicated as the genus of anger, or supposition as the genus of belief. The angry man doubtless feels pain, but his pain precedes his anger in time, and is the antecedent cause thereof; now the genus can never precede its species in time. So also a man may have the same supposition sometimes with belief, sometimes without it; accordingly, supposition cannot be the genus of belief any more than the same animal can be sometimes a man, sometimes a brute.[186] And indeed the same negative conclusion would follow, even if we granted that every supposition was always attended with belief. For, in that case, supposition and belief would be co-extensive terms; but the generic term must always be more extensive than its specific.[187]