PTS vp En 348 PTS cs 20 ‘Venerable Nāgasena, those three qualities of the serpent you say he ought to take, which are they?’
‘Just, O king, as the serpent progresses by means of its belly; just so, O king, should the strenuous Bhikshu, earnest in effort, progress by means of his knowledge. For the heart of the recluse, O king, who progresses by knowledge, continues in perception (of the four Truths), that which is inconsistent with the characteristics of a recluse does he put away, that which is consistent with them does he develop in himself. This, PTS vp Pali 406 O king, is the first quality of the serpent he ought to have.
PTS cs 21 ‘And again, O king, just as the serpent as it moves avoids drugs ; just so, O king, should the strenuous Bhikshu, earnest in effort, go on his way avoiding unrighteousness. This, O king, is the second quality of the serpent he ought to have.
PTS cs 22 ‘And again, O king, as the serpent on catching sight of men is anxious, and pained, and seeks a way of escape ; just so, O king, should the strenuous Bhikshu, earnest in effort, when he finds himself thinking wrong thoughts, or discontent arising within him, be anxious and pained, and seek a way of escape, saying to himself: “This day must I have spent in carelessness, and never shall I be able to recover it.” This, O king, is the third quality of the PTS vp En 349 serpent he ought to have. For it is a saying, O king, of the two fairy birds in the Bhallāṭiya Jātaka:
“’Tis one night only, hunter, that we’ve spent
Away from home, and that against our will,
And thinking all night through of one another,
Yet that one night is it that we bemoan,
And grieve; for nevermore can it return!”’