From Secular Dharma, a course given by Stephen and Martine Batchelor:
The Parable of the Raft
Imagine, friends, a man in the course of a journey who arrives at a great expanse of water, whose near bank is dangerous and whose far bank offers safety. But there is no ferryboat or bridge to take him across the water. So he thinks: ‘What if I collected grass, twigs, branches and leaves and bound them together as a raft? Supported by the raft and by paddling with my hands and feet, I should then be able to reach the far bank.’
“He does this and succeeds in getting across.
“On arriving at the far bank, it might occur to him: ‘This raft has been very helpful indeed. What if I were to hoist it on my head or shoulders, then proceed on my journey?’ Now, what do you think? By carrying it with him, would that man be doing what should be done with a raft?’
“’No, sir,’ replied his audience.
“’So what should he do with the raft? Having arrived at the far bank, he might think: ‘Yes, this raft has been very useful, but now I should just haul it onto dry land or leave it floating in the water, and then continue on my journey.’ In this way the man would be doing what should be done with that raft.
“The dharma too is like a raft. It serves the purpose of crossing over, not the purpose of grasping.
“When you understand that the dharma is like a raft, and that you should let go even of positive things (dhamma), then how much more so should you let go of negative things (adhamma).”