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Aristotle says in the second [book] of Topica: converting[1] an appropriate name derived from an accidental[2] [is] very difficult. One would say very difficult because it is not simple nor readily done, as in the case of the others.

Χαλεπώτατον: φησὶν Ἀριστοτέλης ἐν δευτέρῳ Τοπικῶν: χαλεπώτατον τὸ ἀντιστρέφειν τὴν ἀπὸ τοῦ συμβεβηκότος οἰκείαν ὀνομασίαν. χαλεπώτατον λέγοι ἄν, ὅτι μὴ ἁπλοῦν μηδὲ πρόχειρον, ὡς ἐπὶ τῶν ἄλλων.

Alexander of Aphrodisias, Commentaries on Aristotle's Topica 131.20-23 Wallies. The Suda's headword is extracted from the body of the entry.
[1] Aristotle shortly illustrates an example of correct conversion in 109a14-16: “For example, if ‘to be a biped pedestrian animal is an attribute of A,’ it will be true to say by conversion that ‘A is a biped pedestrian animal.’” For a formal definition of "conversion" see the first link below.
[2] In 109a22-24: “ . . . it is not enough to show that whiteness or justice is an attribute of a man in order to show that he is white or just; for it is possible to argue that he is only partly white or just.” In other words, whiteness and justice are not necessary attributes of a man but being a biped is. From Aristotle on, “accidentals,” or simply “accidents” play a great role in philosophical definitions. For a formal definition of “accident” see the second link below.

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