[451] Ibid. a. 21.
5. Instead of asking the two contraries in plain and direct terms, you may ask the two contraries in different propositions, yet necessarily implicated with the first two.[452]
[452] Ibid. a. 22.
There is this difference between Petitio Principii, and Petitio Contrariorum: the first has reference to the conclusion which you have to prove, and the wrong procedure involved in it is relative to that conclusion; but in the second the wrong procedure affects only the two propositions themselves and the relation subsisting between them.[453]
[453] Ibid. a. 24: διαφέρει δὲ τὸ τἀναντία λαμβάνειν τοῦ ἐν ἀρχῇ, ὅτι τοῦ μέν ἐστιν ἡ ἁμαρτία πρὸς τὸ συμπέρασμα (πρὸς γὰρ ἐκεῖνο βλέποντες τὸ ἐν ἀρχῇ λέγομεν αἰτεῖσθαι), τὰ δ’ ἐναντία ἐστὶν ἐν ταῖς προτάσεσι τῷ ἔχειν πως ταύτας πρὸς ἀλλήλας.
Aristotle now, finally, proceeds to give some general advice for exercise and practice in Dialectic. You ought to accustom yourself to treat arguments by converting the syllogisms of which they consist; that is, by applying to them the treatment of which the Reductio ad Absurdum is one case.[454] You ought to test every thesis by first assuming it to be true, then assuming it to be false, and following out the consequences on both sides.[455] When you have hunted out each train of arguments, look out at once for the counter-arguments available against it. This will strengthen your power both as questioner and as respondent. It is indeed an exercise so valuable, that you will do well to go through it by yourself, if you have no companion.[456] Put the different trains of argument, bearing on the same thesis, into comparison with each other. A wide command of arguments affirmative as well as negative will serve you well both for attack and for defence.[457]
[454] Ibid. xiv. p. 163, a. 29: πρὸς δὲ γυμνασίαν καὶ μελέτην τῶν τοιούτων λόγων πρῶτον μὲν ἀντιστρέφειν ἐθίζεσθαι χρὴ τοὺς λόγους. For Conversion of Syllogism, see [p. 174].
[455] Topic. VIII. xiv. p. 163, a. 36: πρὸς ἅπασάν τε θέσιν καὶ ὅτι οὕτως καὶ ὅτι οὐχ οὕτως τὸ ἐπιχείρημα σκεπτέον.
[456] Ibid. b. 3: κἂν πρὸς μηδένα ἄλλον ἔχωμεν, πρὸς αὑτούς.
[457] Ibid. b. 5: τοῦτο γὰρ πρός τε τὸ βιάζεσθαι πολλὴν εὐπορίαν ποιεῖ καὶ πρὸς τὸ ἐλέγχειν μεγάλην ἔχει βοήθειαν, ὅταν εὐπορῇ τις καὶ ὅτι οὕτως καὶ ὅτι οὐχ οὕτως· πρὸς τὰ ἐναντία γὰρ συμβαίνει ποιεῖσθαι τὴν φυλακήν.