E.C. Marchant, Commentary on Thucydides: Book 3

ἐπὶ παντί—‘in every case’ of those just alluded to; cf. ἐπ᾽ οὐδενὶ τῶν τοιούτωι Demosth. XXI. 65.

ὁ μέν—first comes Desire, next Hope—and then the attempt. Note the personification here.

τὴν . . ἐπιβουλὴν ἐκφροντίζων—‘thinking out the plot.’ The schol. has ἐγχείρησιν on this word, so that the writer of it must have found ἐπιβολήν, ‘attempt,’ which most MSS. give, the only objection to which is that the subst. is not found elsewhere in this sense before Polybius.

τὴν εὐπορίαν τῆς τύχης—‘suggesting the ready help of Chanee.’ Hope deludes them into a fancy that at the critical moment Chance will favour them. Though a subjective gen. does not occur with εὐπορία elsewhere, τῆς τύχης is plainly subjective here; cf. VII. 61 τὸ τῆς τύχης κἂν μεθ᾽ ἡμῶν ἐλπίσαντες στῆναι, V. 113. (τύχη cannot=‘success’ here, cf. § 7, nor ‘their fortunes.’）

ὄντα ἀφανῆ—i.e. ἔρως and ἐλπίς, ‘because (not though) unseen.’ Were they φανεραί, men would see that they could not be counted upon. As it is, πλεῖστα βλάπτουσι by luring them on. Cf. v. 103 ἐπὶ τὰς ἀφανεῖς (ἐλπίδας) καθίστανται, and the next sentence.