Whetstones are not themselves able to cut, but make iron sharp and capable of cutting. (tr. C. T. Ramage, 1869)

πρὸς τὸν ἐρόμενον διὰ τί οὐκ ὢν αὐτὸς ἱκανὸς ἄλλους ποιεῖ, εἶπεν ὅτι καὶ αἱ ἀκόναι αὐταὶ μὲν τέμνειν οὐ δύνανται τὸν δὲ σίδηρον τμητικὸν ποιοῦσιν. | When he was asked how he, not being a good speaker himself, could make others so, he replied that whetstones cannot themselves cut, but make iron fit to do so. (tr. H. N. Fowler, 1936)