What is certainly one of the Aeneid’s best known passages (6.129ish),
Trojan, son of Anchises, easy is the descent to Avernus:
night and day do the doors of black Dis lay open;
but to retrace your step and escape to the upper air,
this is the task, this the labor
Tros Anchisiade, facilis descensus Averno:
noctes atque dies patet atri ianua Ditis;
sed revocare gradum superasque evadere ad auras,
hoc opus, hic labor est.
Which, with consistent temperamental flippancy, I cannot read without recalling Ovid’s corrupting echo in Ars Amatoria 1.453:
But what you haven’t given, seem always on the cusp of giving:
In this way a barren field has often deceived its owner:
In this way the gambler – so that he won’t lose – does not leave off losing
and often the dice call back his greedy hands.
This is the the task, this the labor – to get her to bed without a preceding gift;
And so that what she’s given won’t have been given for nothing she’ll keep on giving.
At quod non dederis, semper videare daturus:
Sic dominum sterilis saepe fefellit ager:
Sic, ne perdiderit, non cessat perdere lusor,
Et revocat cupidas alea saepe manus.
Hoc opus, hic labor est, primo sine munere iungi;
Ne dederit gratis quae dedit, usque dabit.