Polybius 11.8.1-3 (tr. W.R. Paton):
There are three ways in which those who aim at acquiring the art of generalship may reasonably hope to do so, first by studying histories and availing themselves of the lessons contained in them, secondly by following the systematic instruction of experienced men, and thirdly by the habit and experience acquired in actual practice, and in all three the present Achaean strategi were absolutely unversed.
Ὅτι τριῶν ὄντων τρόπων, καθ᾿ οὓς ἐφίενται πάντες στρατηγίας οἱ κατὰ λόγον αὐτῇ προσιόντες, πρώτου μὲν διὰ τῶν ὑπομνημάτων καὶ τῆς ἐκ τούτων κατασκευῆς, ἑτέρου δὲ τοῦ μεθοδικοῦ καὶ τῆς παρὰ τῶν ἐμπείρων ἀνδρῶν παραδόσεως, τρίτου δὲ τοῦ διὰ τῆς ἐπ᾿ αὐτῶν τῶν πραγμάτων ἕξεως καὶ τριβῆς, πάντων ἦσαν τούτων ἀνεννόητοι οἱ τῶν Ἀχαιῶν στρατηγοὶ ἁπλῶς.
This passage is cited by W. Kendrick Pritchett, "Greek Military Training," The Greek State at War
, Part II (1974; rpt. Berkeley: University of California Press, 1992), pp. 208-231 (at 212, n. 17).