[442] Ibid. xiii. p. 162, b. 31: τὸ δ’ ἐν ἀρχῇ καὶ τὰ ἐναντία πῶς αἰτεῖται ὁ ἐρωτῶν, κατ’ ἀλήθειαν μὲν ἐν τοῖς Ἀναλυτικοῖς (Priora, II. xvi.) εἴρηται, κατὰ δόξαν δὲ νῦν λεκτέον.
Five distinct modes may be enumerated of committing the fault called Petitio Principii:—
1. You may put as a question the very conclusion which it is incumbent on you to prove, in refutation of the thesis of the respondent. If this is done in explicit terms, your opponent can hardly fail to perceive it; but he possibly may fail, if you substitute an equivalent term or the definition in place of the term.[443]
[443] Ibid. b. 34. πρῶτον εἴ τις αὐτὸ τὸ δείκνυσθαι δέον αἰτήσει· τοῦτο δ’ ἐπ’ αὐτοῦ μὲν οὐ ῥᾴδιον λανθάνειν, ἐν δὲ τοῖς συνωνύμοις, καὶ ἐν ὅσοις τὸ ὄνομα καὶ ὁ λόγος τὸ αὐτὸ σημαίνει, μᾶλλον.
2. If the conclusion which you are seeking to prove is a particular one, you may put as a question the universal in which it is comprised. Thus, if you are to prove that the knowledge of Contraries is one and the same, you may put as a question, Is not the knowledge of Opposites one and the same? You are asking the very point which it was your business to show; but you are asking along with it much more besides.[444]
[444] Ibid. p. 163, a. 1.
3. If you are seeking to prove an universal conclusion, you may put as a question one of the particulars comprised therein. Thus, if you are to prove that the knowledge of Contraries is one and the same, you may put as a question, Is not the knowledge of white and black, good and evil, or any other pair of particular contraries, one and the same? It was your business to prove this particular, along with many others besides; but you are now asking it as a question separately.[445]
[445] Ibid. a. 5.
4. If the conclusion which you are seeking to prove has two terms conjointly, you may put as a question one or the other of these separately. Thus, when you are trying to show that the healing art is knowledge of what is wholesome and unwholesome, you may ask, Is it a knowledge of the wholesome?[446]
[446] Ibid. a. 8.