Philostratus, Lives of the Sophists 1.480-481:
“It is necessary to consider the ancient sophistic art as a kind of rhetoric. For it presents discourses on the same things philosophers cover, but where the philosophers, in setting forth questions and in making small advances on their objects of investigations, assert that they still do not know anything, the ancient sophist claims that he does know the things he describes. At least, he recites as a beginning of his discourse phrases like “I know”, “I recognize”, and “I have noticed for some time,” or “Nothing is certain for man”. This species of introduction furnishes a sense of nobility and certainty to a speech along with implying a clear sense of what is real.”
Τὴν ἀρχαίαν σοφιστικὴν ῥητορικὴν ἡγεῖσθαι χρὴ φιλοσοφοῦσαν• διαλέγεται μὲν γὰρ ὑπὲρ ὧν οἱ φιλοσοφοῦντες, ἃ δὲ ἐκεῖνοι τὰς ἐρωτήσεις ὑποκαθήμενοι καὶ τὰ σμικρὰ τῶν ζητουμένων προβιβάζοντες οὔπω φασὶ γιγνώσκειν, ταῦτα ὁ παλαιὸς σοφιστὴς ὡς εἰδὼς λέγει. προοίμια γοῦν ποιεῖται τῶν λόγων τὸ „οἶδα” καὶ τὸ „γιγνώσκω” καὶ „πάλαι διέσκεμμαι” καὶ „βέβαιον ἀνθρώπῳ οὐδέν”. ἡ δὲ τοιαύτη ἰδέα τῶν προοιμίων εὐγένειάν τε προηχεῖ τῶν λόγων καὶ φρόνημα καὶ κατάληψιν
σαφῆ τοῦ ὄντος.