For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much--the wheel, New York, wars and so on--while all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man--for precisely the same reasons.

Summary

This quote from Douglas Adams' "The Hitchhiker's Guide to the Galaxy" highlights the irony of human arrogance. Man, with his progress and achievements, deems himself superior to dolphins, who seemingly idle away in water. However, dolphins view themselves as more intelligent based on their ability to enjoy life without causing destruction. It serves as a reminder that intelligence should not be judged solely based on achievements, but also on the ability to appreciate and coexist with the world in a harmonious way.