Zhuangzi, known as Zhuang Zhou, was a philosopher from the Warring States period, born in Meng, Song State (present-day northeast of Shangqiu County, Henan).
One day, the philosopher Zhuangzi and his friend Huizi, another thinker from the Song State, were strolling along the Hao River (in present-day Fengyang, Anhui) and watching the fish swim. Zhuangzi remarked, "Look how the fish dart about freely—this is the joy of fish." Huizi challenged him, "You are not a fish—how can you possibly know the joy of fish?" Zhuangzi retorted, "You are not me—how can you know that I don't know the joy of fish?" Huizi pressed, "I am not you, so I cannot know what you know; but you are not a fish, so you cannot know the joy of fish—that's the end of it." Zhuangzi smiled and replied, "Let's go back to the beginning. You asked me, 'How can you know the joy of fish?' That question already shows you knew I knew it—otherwise, why would you ask? I know it from right here, standing on the bridge."
Zhuangzi remarked, "Look at how freely and contentedly this fish swims in the water—how joyful it is!" Huizi countered, "You are not a fish—how can you know they are happy?" Zhuangzi replied, "You are not me—how can you be certain that I don't know their joy?" Huizi pressed, "I am not you, so I cannot know your feelings. But you are certainly not a fish, so it is beyond doubt that you cannot know the fish's happiness."
Zhuangzi replied, "Let's start from the beginning. You asked me how I know the fish are happy, which shows you already understood that I know their joy. Now I tell you: I know it from my own experience. As we strolled by the Hao River, we were at ease and content. And these fish? They swim in the water, unhurried, watching us just as we watch them—so of course they share our delight."
Later, the idiom "calm and unhurried" came to describe someone who is composed and unflustered.
Source: *Zhuangzi*, "Autumn Floods"
Meaning of the Idiom: Later, the Chinese idiom "从容不迫" came to describe how someone is composed and unflustered.