Enchiridion · §52

ὁ πρῶτος καὶ ἀναγκαιότατος τόπος ἐστὶν ἐν φιλοσοφίᾳ ὁ τῆς χρήσεως τῶν θεωρημάτων, οἷον τὸ μὴ ψεύδεσθαι· ὁ δεύτερος ὁ τῶν ἀποδείξεων, οἷον πόθεν ὅτι οὐ δεῖ ψεύδεσθαι· ὁ τρίτος ὁ τῶν τούτων αὐτῶν βεβαιωτικὸς καὶ διαρθρωτικός, οἷον πόθεν τοῦτο ἀπόδειξίς ἐστιν· τί γάρ ἐστιν ἀπόδειξις, τί ἀκολουθία, τί μάχη, τί ἀληθές, τί ψεῦδος;ὁ τρίτος τοίνυν τόπος ἀναγκαῖος μέν ἐστι διὰ τὸν δεύτερον, ὁ δεύτερος διὰ τὸν πρῶτον· ὁ δὲ ἀναγκαιότατος καὶ ἐν ᾧ δεῖ ἀναπαύεσθαι, ὁ πρῶτος.

ho prōtos kai anankaiotatos topos estin en philosophia ho tēs chrēseōs tōn theōrēmatōn, hoion to mē pseudesthai; ho deuteros ho tōn apodeixeōn, hoion pothen hoti ou dei pseudesthai; ho tritos ho tōn toutōn autōn bebaiōtikos kai diarthrōtikos.

In philosophy, the first and most necessary topic is the practical application of principles — for example, not lying. The second is demonstrations — for example, why one ought not to lie. The third is the one that confirms and analyzes these demonstrations — for example, what makes something a proof, what follows from what, what contradicts what, what is true, what is false. The third topic is necessary because of the second, and the second because of the first. But the most necessary, and the one where we ought to rest, is the first.

— Enchiridion 52

Source: Greek (Schenkl/Oldfather critical edition tradition) + Long 1877 English translation