Passage 2.1.5
καθήκει δὲ ὁ τῶν Κορινθίων ἰσθμὸς τῇ μὲν ἐς τὴν ἐπὶ Κεγχρέαις, τῇ δὲ ἐς τὴν ἐπὶ Λεχαίῳ θάλασσαν· τοῦτο γὰρ ἤπειρον ποιεῖ τὴν ἐντὸς χώραν. ὃς δὲ ἐπεχείρησε Πελοπόννησον ἐργάσασθαι νῆσον, προαπέλιπε διορύσσων ἰσθμόν· καὶ ὅθεν μὲν διορύσσειν ἤρξαντο δῆλόν ἐστιν, ἐς δὲ τὸ πετρῶδες οὐ προεχώρησαν ἀρχήν· μένει δὲ ὡς πεφύκει καὶ νῦν ἤπειρος ὤν. Ἀλεξάνδρῳ τε τῷ Φιλίππου διασκάψαι Μίμαντα ἐθελήσαντι μόνον τοῦτο οὐ προεχώρησε τὸ ἔργον· Κνιδίους δὲ ἡ Πυθία τὸν ἰσθμὸν ὀρύσσοντας ἔπαυσεν. οὕτω χαλεπὸν ἀνθρώπῳ τὰ θεῖα βιάσασθαι.
The Isthmus of the Corinthians stretches on one side towards the sea near Cenchreae, and on the other towards the sea at Lechaeum; it is this that makes the inland region a part of the mainland. He who first attempted to turn the Peloponnese into an island abandoned his work of digging through the Isthmus before completion. The place where they began digging is clearly visible, but they failed entirely to proceed into the solid rock, so the region remains as nature formed it, continuing even now as part of the mainland. Even Alexander, the son of Philip, who wished to cut through Mount Mimas, found only in this endeavor that his efforts were unsuccessful. Likewise, the Pythian oracle stopped the Cnidians as they were digging through their isthmus. Thus difficult it is for man to force what the gods ordain.