Plato,
Theaetetus 155 d (tr. Harold N. Fowler):
For this feeling of wonder shows that you are a philosopher, since wonder is the only beginning of philosophy.
μάλα γὰρ φιλοσόφου τοῦτο τὸ πάθος, τὸ θαυμάζειν: οὐ γὰρ ἄλλη ἀρχὴ φιλοσοφίας ἢ αὕτη.
Arrian,
Discourses of Epictetus 2.11.13 (tr. W.A. Oldfather):
Behold the beginning of philosophy!—a recognition of the conflict between the opinions of men, and a search for the origin of that conflict, and a condemnation of mere opinion, coupled with scepticism regarding it, and a kind of investigation to determine whether the opinion is rightly held, together with the invention of a kind of standard of judgement, as we have invented the balance for the determination of weights, or the carpenter's rule for the determination of things straight and crooked.
Ἴδ᾽ ἀρχὴ φιλοσοφίας· αἴσθησις μάχης τῆς πρὸς ἀλλήλους τῶν ἀνθρώπων καὶ ζήτησις τοῦ παρ᾽ ὃ γίνεται ἡ μάχη καὶ κατάγνωσις καὶ ἀπιστία πρὸς τὸ ψιλῶς δοκοῦν, ἔρευνα δέ τις περὶ τὸ δοκοῦν εἰ ὀρθῶς δοκεῖ καὶ εὕρεσις κανόνος τινός, οἷον ἐπὶ βαρῶν τὸν ζυγὸν εὕρομεν, οἷον ἐπὶ εὐθέων καὶ στρεβλῶν τὴν στάθμην.