Bhagavad-gita 7.15 Purport
The typical example of the beast of burden is the ass. This humble beast is made to work very hard by his master. The ass does not really know for whom he works so hard day and night. He remains satisfied by filling his stomach with a bundle of grass, sleeping for a while under fear of being beaten by his master, and satisfying his sex appetite at the risk of being repeatedly kicked by the opposite party. The ass sings poetry and philosophy sometimes, but this braying only disturbs others. This is the position of the foolish fruitive worker who does not know for whom he should work. He does not know that karma (action) is meant for yajna (sacrifice).