[334] And how, bhikkhus, are a bhikkhu's conduct (cāra) and dwelling (vihāra) so followed that, as he conducts himself and dwells, evil unwholesome states of covetousness and grief (abhijjhā-domanassa) do not flow in upon him? Suppose, bhikkhus, a man were to enter a forest full of thorns: there would be a thorn in front of him, a thorn behind him, a thorn to the north, a thorn to the south, a thorn below, and a thorn above. Mindful he would go forward, mindful he would turn back, thinking: 'Let no thorn [pierce] me.' Even so, bhikkhus, whatever in the world has a pleasing nature (piyarūpa), an agreeable nature (sātarūpa) — this is called a thorn in the discipline of the Noble One. Having understood it thus, non-restraint and restraint should be understood.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.